Java-INterview-1

 

 

Basic Java Interview Questions

Q1. Explain JDK, JRE and JVM?

JDK vs JRE vs JVM

JDK	JRE	JVM
It stands for Java Development Kit.	It stands for Java Runtime Environment.	It stands for Java Virtual Machine.
It is the tool necessary to compile, document and package Java programs.	JRE refers to a runtime environment in which Java bytecode can be executed.	It is an abstract machine. It is a specification that provides a run-time environment in which Java bytecode can be executed.
It contains JRE + development tools.	It’s an implementation of the JVM which physically exists.	JVM follows three notations: Specification, Implementation, and Runtime Instance.

Q2. Explain public static void main(String args[]) in Java.

main() in Java is the entry point for any Java program. It is always written as public static void main(String[] args).

• public: Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class.
• static: It is a keyword in java which identifies it is class-based. main() is made static in Java so that it can be accessed without creating the instance of a Class. In case, main is not made static then the compiler will throw an error as main() is called by the JVM before any objects are made and only static methods can be directly invoked via the class. 
• void: It is the return type of the method. Void defines the method which will not return any value.
• main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.
• String args[]: It is the parameter passed to the main method.

Q3. Why Java is platform independent?

Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.

Q4. Why Java is not 100% Object-oriented?

Java is not 100% Object-oriented because it makes use of eight primitive data types such as boolean, byte, char, int, float, double, long, short which are not objects.

Q5. What are wrapper classes in Java?

Wrapper classes convert the Java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class. Refer to the below image which displays different primitive type, wrapper class and constructor argument.

Q6. What are constructors in Java?

In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

There are two types of constructors:

1. Default Constructor: In Java, a default constructor is the one which does not take any inputs. In other words, default constructors are the no argument constructors which will be created by default in case you no other constructor is defined by the user. Its main purpose is to initialize the instance variables with the default values. Also, it is majorly used for object creation. 
2. Parameterized Constructor: The parameterized constructor in Java, is the constructor which is capable of initializing the instance variables with the provided values. In other words, the constructors which take the arguments are called parameterized constructors.

Q7. What is singleton class in Java and how can we make a class singleton?

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Q8. What is the difference between Array list and vector in Java?

ArrayList	Vector
Array List is not synchronized.	Vector is synchronized.
Array List is fast as it’s non-synchronized.	Vector is slow as it is thread safe.
If an element is inserted into the Array List, it increases its Array size by 50%.	Vector defaults to doubling size of its array.
Array List does not define the increment size.	Vector defines the increment size.
Array List can only use Iterator for traversing an Array List.	Vector can use both Enumeration and Iterator for traversing.

Q9. What is the difference between equals() and == in Java?

Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.

“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

Q10. What are the differences between Heap and Stack Memory in Java?

The major difference between Heap and Stack memory are:

Features	Stack	Heap
Memory	Stack memory is used only by one thread of execution.	Heap memory is used by all the parts of the application.
Access	Stack memory can’t be accessed by other threads.	Objects stored in the heap are globally accessible.
Memory Management	Follows LIFO manner to free memory.	Memory management is based on the generation associated with each object.
Lifetime	Exists until the end of execution of the thread.	Heap memory lives from the start till the end of application execution.
Usage	Stack memory only contains local primitive and reference variables to objects in heap space.	Whenever an object is created, it’s always stored in the Heap space.

Q11. What is a package in Java? List down various advantages of packages.

Packages in Java, are the collection of related classes and interfaces which are bundled together. By using packages, developers can easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused. Below I have listed down a few of its advantages:

• Packages help in avoiding name clashes
• They provide easier access control on the code
• Packages can also contain hidden classes which are not visible to the outer classes and only used within the package
• Creates a proper hierarchical structure which makes it easier to locate the related classes

Q12. Why pointers are not used in Java?

Java doesn’t use pointers because they are unsafe and increases the complexity of the program. Since, Java is known for its simplicity of code, adding the concept of pointers will be contradicting. Moreover, since JVM is responsible for implicit memory allocation, thus in order to avoid direct access to memory by the user,  pointers are discouraged in Java.

Q13. What is JIT compiler in Java?

JIT stands for Just-In-Time compiler in Java. It is a program that helps in converting the Java bytecode into instructions that are sent directly to the processor. By default, the JIT compiler is enabled in Java and is activated whenever a Java method is invoked. The JIT compiler then compiles the bytecode of the invoked method into native machine code, compiling it “just in time” to execute. Once the method has been compiled, the JVM summons the compiled code of that method directly rather than interpreting it. This is why it is often responsible for the performance optimization of Java applications at the run time.

Q14. What are access modifiers in Java?

In Java, access modifiers are special keywords which are used to restrict the access of a class, constructor, data member and method in another class. Java supports four types of access modifiers:

1. Default
2. Private
3. Protected
4. Public

Modifier	Default	Private	Protected	Public
Same class	YES	YES	YES	YES
Same Package subclass	YES	NO	YES	YES
Same Package non-subclass	YES	NO	YES	YES
Different package subclass	NO	NO	YES	YES
Different package non-subclass	NO	NO	NO	YES

Q15. Define a Java Class.

A class in Java is a blueprint which includes all your data.  A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class.

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class Abc { member variables // class body methods}

Q16. What is an object in Java and how is it created?

An object is a real-world entity that has a state and behavior. An object has three characteristics:

1. State
2. Behavior
3. Identity

An object is created using the ‘new’ keyword. For example:

ClassName obj = new ClassName();

Q17. What is Object Oriented Programming?

Object-oriented programming or popularly known as OOPs is a programming model or approach where the programs are organized around objects rather than logic and functions. In other words, OOP mainly focuses on the objects that are required to be manipulated instead of logic. This approach is ideal for the programs large and complex codes and needs to be actively updated or maintained.

Q18. What are the main concepts of OOPs in Java?

Object-Oriented Programming or OOPs is a programming style that is associated with concepts like:

1. Inheritance: Inheritance is a process where one class acquires the properties of another.
2. Encapsulation: Encapsulation in Java is a mechanism of wrapping up the data and code together as a single unit.
3. Abstraction: Abstraction is the methodology of hiding the implementation details from the user and only providing the functionality to the users. 
4. Polymorphism: Polymorphism is the ability of a variable, function or object to take multiple forms.

Q19. What is the difference between a local variable and an instance variable?

In Java, a local variable is typically used inside a method, constructor, or a block and has only local scope. Thus, this variable can be used only within the scope of a block. The best benefit of having a local variable is that other methods in the class won’t be even aware of that variable.

Example

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if(x > 100) { String test = "Edureka"; }

 

Whereas, an instance variable in Java, is a variable which is bounded to its object itself. These variables are declared within a class, but outside a method. Every object of that class will create it’s own copy of the variable while using it. Thus, any changes made to the variable won’t reflect in any other instances of that class and will be bound to that particular instance only.

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class Test{ public String EmpName; public int empAge; }

Q20. Differentiate between the constructors and methods in Java?

Methods	Constructors
1. Used to represent the behavior of an object	1. Used to initialize the state of an object
2. Must have a return type	2. Do not have any return type
3. Needs to be invoked explicitly	3. Is invoked implicitly
4. No default method is provided by the compiler	4. A default constructor is provided by the compiler if the class has none
5. Method name may or may not be same as class name	5. Constructor name must always be the same as the class name

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Q21. What is final keyword in Java?

final is a special keyword in Java that is used as a non-access modifier. A final variable can be used in different contexts such as:

• final variable

When the final keyword is used with a variable then its value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.

• final method

When a method is declared final then it can’t be overridden by the inheriting class.

• final class

When a class is declared as final in Java, it can’t be extended by any subclass class but it can extend other class.

Q22. What is the difference between break and continue statements?

break	continue
1. Can be used in switch and loop (for, while, do while) statements	1. Can be only used with loop statements
2. It causes the switch or loop statements to terminate the moment it is executed	2. It doesn’t terminate the loop but causes the loop to jump to the next iteration
3. It terminates the innermost enclosing loop or switch immediately	3. A continue within a loop nested with a switch will cause the next loop iteration to execute

Example break:

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for (int i = 0; i < 5; i++) { if (i == 3) { break; } System.out.println(i); }

Example continue:

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for (int i = 0; i < 5; i++) { if(i == 2) { continue; } System.out.println(i); }

Q23.What is an infinite loop in Java? Explain with an example.

An infinite loop is an instruction sequence in Java that loops endlessly when a functional exit isn’t met. This type of loop can be the result of a programming error or may also be a deliberate action based on the application behavior. An infinite loop will terminate automatically once the application exits.

For example:

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public class InfiniteForLoopDemo { public static void main(String[] arg) { for(;;) System.out.println("Welcome to Edureka!"); // To terminate this program press ctrl + c in the console. } }

 

Q24. What is the difference between this() and super() in Java?

In Java, super() and this(), both are special keywords that are used to call the constructor. 

this()	super()
1. this() represents the current instance of a class	1. super() represents the current instance of a parent/base class
2. Used to call the default constructor of the same class	2. Used to call the default constructor of the parent/base class
3. Used to access methods of the current class	3. Used to access methods of the base class
4.  Used for pointing the current class instance	4. Used for pointing the superclass instance
5. Must be the first line of a block	5. Must be the first line of a block

Q25. What is Java String Pool?

Java String pool refers to a collection of Strings which are stored in heap memory. In this, whenever a new object is created, String pool first checks whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable else new object will be created in the String pool and the respective reference will be returned.

Q26. Differentiate between static and non-static methods in Java.

Static Method	Non-Static Method
1. The static keyword must be used before the method name	1. No need to use the static keyword before the method name
2. It is called using the class (className.methodName)	2. It is can be called like any general method
3. They can’t access any non-static instance variables or methods	3. It can access any static method and any static variable without creating an instance of the class

Q27. What is constructor chaining in Java?

In Java, constructor chaining is the process of calling one constructor from another with respect to the current object. Constructor chaining is possible only through legacy where a subclass constructor is responsible for invoking the superclass’ constructor first. There could be any number of classes in the constructor chain. Constructor chaining can be achieved in two ways:

1. Within the same class using this()
2. From base class using super()

Q28. Difference between String, StringBuilder, and StringBuffer.

Factor	String	StringBuilder	StringBuffer
Storage Area	Constant String Pool	Heap Area	Heap Area
Mutability	Immutable	Mutable	Mutable
Thread Safety	Yes	No	Yes
Performance	Fast	More efficient	Less efficient

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Q29. What is a classloader in Java?

The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:

1. Bootstrap ClassLoader
2. Extension ClassLoader
3. System/Application ClassLoader

Q30. Why Java Strings are immutable in nature?

In Java, string objects are immutable in nature which simply means once the String object is created its state cannot be modified. Whenever you try to update the value of that object instead of updating the values of that particular object, Java creates a new string object. Java String objects are immutable as String objects are generally cached in the String pool. Since String literals are usually shared between multiple clients, action from one client might affect the rest. It enhances security, caching, synchronization, and performance of the application. 

Q31. What is the difference between an array and an array list?

Array	ArrayList
Cannot contain values of different data types	Can contain values of different data types.
Size must be defined at the time of declaration	Size can be dynamically changed
Need to specify the index in order to add data	No need to specify the index
Arrays are not type parameterized	Arraylists are type
Arrays can contain primitive data types as well as objects	Arraylists can contain only objects, no primitive data types are allowed

Q32. What is a Map in Java?

In Java, Map is an interface of Util package which maps unique keys to values. The Map interface is not a subset of the main Collection interface and thus it behaves little different from the other collection types. Below are a few of the characteristics of Map interface: 

1. Map doesn’t contain duplicate keys.

2. Each key can map at max one value.

Q33. What is collection class in Java? List down its methods and interfaces.

In Java, the collection is a framework that acts as an architecture for storing and manipulating a group of objects. Using Collections you can perform various tasks like searching, sorting, insertion, manipulation, deletion, etc. Java collection framework includes the following:

• Interfaces
• Classes
• Methods

The below image shows the complete hierarchy of the Java Collection.

 

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OOPS Java Interview Questions

Q1. What is Polymorphism?

Polymorphism is briefly described as “one interface, many implementations”. Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form. There are two types of polymorphism:

1. Compile time polymorphism
2. Run time polymorphism

Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface.

Q2. What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. Let’s take a look at the example below to understand it better.

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class Car { void run() { System.out.println(“car is running”); } } class Audi extends Car { void run() { System.out.prinltn(“Audi is running safely with 100km”); } public static void main(String args[]) { Car b= new Audi();    //upcasting b.run(); } }

Q3. What is abstraction in Java?

Abstraction refers to the quality of dealing with ideas rather than events. It basically deals with hiding the details and showing the essential things to the user. Thus you can say that abstraction in Java is the process of hiding the implementation details from the user and revealing only the functionality to them. Abstraction can be achieved in two ways:

1. Abstract Classes (0-100% of abstraction can be achieved)
2. Interfaces (100% of abstraction can be achieved)

Q4. What do you mean by an interface in Java?

An interface in Java is a blueprint of a class or you can say it is a collection of abstract methods and static constants. In an interface, each method is public and abstract but it does not contain any constructor. Thus, interface basically is a group of related methods with empty bodies. Example:

public interface Animal {

  public void eat();

  public void sleep();

  public void run();

}

Q5. What is the difference between abstract classes and interfaces?

Abstract Class	Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden	An interface cannot provide any code at all, just the signature
In the case of an abstract class, a class may extend only one abstract class	A Class may implement several interfaces
An abstract class can have non-abstract methods	All methods of an Interface are abstract
An abstract class can have instance variables	An Interface cannot have instance variables
An abstract class can have any visibility: public, private, protected	An Interface visibility must be public (or) none
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method
An abstract class can contain constructors	An Interface cannot contain constructors
Abstract classes are fast	Interfaces are slow as it requires extra indirection to find the corresponding method in the actual class

Q6. What is inheritance in Java?

Inheritance in Java is the concept where the properties of one class can be inherited by the other. It helps to reuse the code and establish a relationship between different classes. Inheritance is performed between two types of classes:

1. Parent class (Super or Base class)
2. Child class (Subclass or Derived class)

A class which inherits the properties is known as Child Class whereas a class whose properties are inherited is known as Parent class.

Q7. What are the different types of inheritance in Java?

Java supports four types of inheritance which are:

1. Single Inheritance: In single inheritance, one class inherits the properties of another i.e there will be only one parent as well as one child class.
2. Multilevel Inheritance: When a class is derived from a class which is also derived from another class, i.e. a class having more than one parent class but at different levels, such type of inheritance is called Multilevel Inheritance.
3. Hierarchical Inheritance: When a class has more than one child classes (subclasses) or in other words, more than one child classes have the same parent class, then such kind of inheritance is known as hierarchical.
4. Hybrid Inheritance: Hybrid inheritance is a combination of two or more types of inheritance.

Q8. What is method overloading and method overriding?

Method Overloading :

• In Method Overloading, Methods of the same class shares the same name but each method must have a different number of parameters or parameters having different types and order.
• Method Overloading is to “add” or “extend” more to the method’s behavior.
• It is a compile-time polymorphism.
• The methods must have a different signature.
• It may or may not need inheritance in Method Overloading.

Let’s take a look at the example below to understand it better.

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class Adder { Static int add(int a, int b) { return a+b; } Static double add( double a, double b) { return a+b; } public static void main(String args[]) { System.out.println(Adder.add(11,11)); System.out.println(Adder.add(12.3,12.6)); }}

Method Overriding:  

• In Method Overriding, the subclass has the same method with the same name and exactly the same number and type of parameters and same return type as a superclass.
• Method Overriding is to “Change” existing behavior of the method.
• It is a run time polymorphism.
• The methods must have the same signature.
• It always requires inheritance in Method Overriding.

Let’s take a look at the example below to understand it better.

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• Assignments
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class Car { void run(){ System.out.println(“car is running”); } Class Audi extends Car{ void run() { System.out.prinltn("Audi is running safely with 100km"); } public static void main( String args[]) { Car b=new Audi(); b.run(); } }

Q9. Can you override a private or static method in Java?

You cannot override a private or static method in Java. If you create a similar method with the same return type and same method arguments in child class then it will hide the superclass method; this is known as method hiding. Similarly, you cannot override a private method in subclass because it’s not accessible there. What you can do is create another private method with the same name in the child class. Let’s take a look at the example below to understand it better.

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class Base { private static void display() { System.out.println("Static or class method from Base"); } public void print() { System.out.println("Non-static or instance method from Base"); } class Derived extends Base { private static void display() { System.out.println("Static or class method from Derived"); } public void print() { System.out.println("Non-static or instance method from Derived"); } public class test { public static void main(String args[]) { Base obj= new Derived(); obj1.display(); obj1.print(); } }

Q10. What is multiple inheritance? Is it supported by Java?

If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes.

The problem with multiple inheritance is that if multiple parent classes have the same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class.

Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred to as Diamond Problem.

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Q11. What is encapsulation in Java?

Encapsulation is a mechanism where you bind your data(variables) and code(methods) together as a single unit. Here, the data is hidden from the outer world and can be accessed only via current class methods. This helps in protecting the data from any unnecessary modification. We can achieve encapsulation in Java by:

• Declaring the variables of a class as private.
• Providing public setter and getter methods to modify and view the values of the variables.

Q12. What is an association?

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationships can be one to one, one to many, many to one and many to many.

Q13. What do you mean by aggregation?

An aggregation is a specialized form of Association where all object has their own lifecycle but there is ownership and child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belong to multiple departments, but if we delete the department teacher object will not destroy. 

Q14. What is composition in Java?

Composition is again a specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object does not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of a relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different houses if we delete the house room will automatically delete.

Q15. What is a marker interface?

A Marker interface can be defined as the interface having no data member and member functions. In simpler terms, an empty interface is called the Marker interface. The most common examples of Marker interface in Java are Serializable, Cloneable etc. The marker interface can be declared as follows.

1 2	public interface Serializable{ }

Q16. What is object cloning in Java?

Object cloning in Java is the process of creating an exact copy of an object. It basically means the ability to create an object with a similar state as the original object. To achieve this, Java provides a method clone() to make use of this functionality. This method creates a new instance of the class of the current object and then initializes all its fields with the exact same contents of corresponding fields. To object clone(), the marker interface java.lang.Cloneable must be implemented to avoid any runtime exceptions. One thing you must note is Object clone() is a protected method, thus you need to override it.

Q17. What is a copy constructor in Java?

Copy constructor is a member function that is used to initialize an object using another object of the same class. Though there is no need for copy constructor in Java since all objects are passed by reference. Moreover, Java does not even support automatic pass-by-value.

Q18. What is a constructor overloading in Java?

In Java, constructor overloading is a technique of adding any number of constructors to a class each having a different parameter list. The compiler uses the number of parameters and their types in the list to differentiate the overloaded constructors.

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class Demo { int i; public Demo(int a) { i=k; } public Demo(int a, int b) { //body } }

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Servlets – Java Interview Questions  

Q1. What is a servlet?

• Java Servlet is server-side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.
• The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets.
• All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services.
• Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image.

Q2. What are the differences between Get and Post methods?

Get	Post
Limited amount of data can be sent because data is sent in header.	Large amount of data can be sent because data is sent in body.
Not Secured because data is exposed in URL bar.	Secured because data is not exposed in URL bar.
Can be bookmarked	Cannot be bookmarked
Idempotent	Non-Idempotent
It is more efficient and used than Post	It is less efficient and used

Q3. What is Request Dispatcher?

RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response.

There are two methods defined in this interface:

1.void forward()

2.void include()

Q4. What are the differences between forward() method and sendRedirect() methods?

forward() method	SendRedirect() method
forward() sends the same request to another resource.	sendRedirect() method sends new request always because it uses the URL bar of the browser.
forward() method works at server side.	sendRedirect() method works at client side.
forward() method works within the server only.	sendRedirect() method works within and outside the server.

Q5. What is the life-cycle of a servlet?

There are 5 stages in the lifecycle of a servlet:

1. Servlet is loaded
2. Servlet is instantiated
3. Servlet is initialized
4. Service the request
5. Servlet is destroyed

Q6. How does cookies work in Servlets?

• Cookies are text data sent by server to the client and it gets saved at the client local machine.
• Servlet API provides cookies support through javax.servlet.http.Cookie class that implements Serializable and Cloneable interfaces.
• HttpServletRequest getCookies() method is provided to get the array of Cookies from request, since there is no point of adding Cookie to request, there are no methods to set or add cookie to request.
• Similarly HttpServletResponse addCookie(Cookie c) method is provided to attach cookie in response header, there are no getter methods for cookie.

Q7. What are the differences between ServletContext vs ServletConfig?

The difference between ServletContext and ServletConfig in Servlets JSP is in below tabular format.

ServletConfig	ServletContext
Servlet config object represent single servlet	It represent whole web application running on particular JVM and common for all the servlet
Its like local parameter associated with particular servlet	Its like global parameter associated with whole application
It’s a name value pair defined inside the servlet section of web.xml file so it has servlet wide scope	ServletContext has application wide scope so define outside of servlet tag in web.xml file.
getServletConfig() method is used to get the config object	getServletContext() method is  used to get the context object.
for example shopping cart of a user is a specific to particular user so here we can use servlet config	To get the MIME type of a file or application session related information is stored using servlet context object.

Q8. What are the different methods of session management in servlets?

Session is a conversational state between client and server and it can consists of multiple request and response between client and server. Since HTTP and Web Server both are stateless, the only way to maintain a session is when some unique information about the session (session id) is passed between server and client in every request and response.

Some of the common ways of session management in servlets are:

1. User Authentication
2. HTML Hidden Field
3. Cookies
4. URL Rewriting
5. Session Management API

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JDBC – Java Interview Questions 

1. What is JDBC Driver?

JDBC Driver is a software component that enables java application to interact with the database. There are 4 types of JDBC drivers:

1. JDBC-ODBC bridge driver
2. Native-API driver (partially java driver)
3. Network Protocol driver (fully java driver)
4. Thin driver (fully java driver)

2. What are the steps to connect to a database in java?

• Registering the driver class
• Creating connection
• Creating statement
• Executing queries
• Closing connection

3. What are the JDBC API components?

The java.sql package contains interfaces and classes for JDBC API.

Interfaces:

• Connection
• Statement
• PreparedStatement
• ResultSet
• ResultSetMetaData
• DatabaseMetaData
• CallableStatement etc.

Classes:

• DriverManager
• Blob
• Clob
• Types
• SQLException etc.

4. What is the role of JDBC DriverManager class?

The DriverManager class manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection.

5. What is JDBC Connection interface?

The Connection interface maintains a session with the database. It can be used for transaction management. It provides factory methods that returns the instance of Statement, PreparedStatement, CallableStatement and DatabaseMetaData.

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6.  What is the purpose of JDBC ResultSet interface?

The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database.

7. What is JDBC ResultSetMetaData interface?

The ResultSetMetaData interface returns the information of table such as total number of columns, column name, column type etc.

8. What is JDBC DatabaseMetaData interface?

The DatabaseMetaData interface returns the information of the database such as username, driver name, driver version, number of tables, number of views etc.

9. What do you mean by batch processing in JDBC?

Batch processing helps you to group related SQL statements into a batch and execute them instead of executing a single query. By using batch processing technique in JDBC, you can execute multiple queries which makes the performance faster.

10. What is the difference between execute, executeQuery, executeUpdate?

Statement execute(String query) is used to execute any SQL query and it returns TRUE if the result is an ResultSet such as running Select queries. The output is FALSE when there is no ResultSet object such as running Insert or Update queries. We can use getResultSet() to get the ResultSet and getUpdateCount() method to retrieve the update count.

Statement executeQuery(String query) is used to execute Select queries and returns the ResultSet. ResultSet returned is never null even if there are no records matching the query. When executing select queries we should use executeQuery method so that if someone tries to execute insert/update statement it will throw java.sql.SQLException with message “executeQuery method can not be used for update”.

Statement executeUpdate(String query) is used to execute Insert/Update/Delete (DML) statements or DDL statements that returns nothing. The output is int and equals to the row count for SQL Data Manipulation Language (DML) statements. For DDL statements, the output is 0.

You should use execute() method only when you are not sure about the type of statement else use executeQuery or executeUpdate method.

Q11. What do you understand by JDBC Statements?

JDBC statements are basically the statements which are used to send SQL commands to the database and retrieve data back from the database. Various methods like execute(), executeUpdate(), executeQuery, etc. are provided by JDBC to interact with the database.

JDBC supports 3 types of statements:

1. Statement: Used for general purpose access to the database and executes a static SQL query at runtime.
2. PreparedStatement: Used to provide input parameters to the query during execution.
3. CallableStatement: Used to access the database stored procedures and helps in accepting runtime parameters.

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Spring Framework – Java Interview Questions 

Q1. What is Spring?

Wikipedia defines the Spring framework as “an application framework and inversion of control container for the Java platform. The framework’s core features can be used by any Java application, but there are extensions for building web applications on top of the Java EE platform.” Spring is essentially a lightweight, integrated framework that can be used for developing enterprise applications in java.

Q2. Name the different modules of the Spring framework.

Some of the important Spring Framework modules are:

• Spring Context – for dependency injection.
• Spring AOP – for aspect oriented programming.
• Spring DAO – for database operations using DAO pattern
• Spring JDBC – for JDBC and DataSource support.
• Spring ORM – for ORM tools support such as Hibernate
• Spring Web Module – for creating web applications.
• Spring MVC – Model-View-Controller implementation for creating web applications, web services etc.

Q3. List some of the important annotations in annotation-based Spring configuration.

The important annotations are:

• @Required
• @Autowired
• @Qualifier
• @Resource
• @PostConstruct
• @PreDestroy

Q4. Explain Bean in Spring and List the different Scopes of Spring bean.

Beans are objects that form the backbone of a Spring application. They are managed by the Spring IoC container. In other words, a bean is an object that is instantiated, assembled, and managed by a Spring IoC container.

There are five Scopes defined in Spring beans.

• Singleton: Only one instance of the bean will be created for each container. This is the default scope for the spring beans. While using this scope, make sure spring bean doesn’t have shared instance variables otherwise it might lead to data inconsistency issues because it’s not thread-safe.
• Prototype: A new instance will be created every time the bean is requested.
• Request: This is same as prototype scope, however it’s meant to be used for web applications. A new instance of the bean will be created for each HTTP request.
• Session: A new bean will be created for each HTTP session by the container.
• Global-session: This is used to create global session beans for Portlet applications.

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Q5. Explain the role of DispatcherServlet and ContextLoaderListener.

DispatcherServlet is basically the front controller in the Spring MVC application as it loads the spring bean configuration file and initializes all the beans that have been configured. If annotations are enabled, it also scans the packages to configure any bean annotated with @Component, @Controller, @Repository or @Service annotations.

ContextLoaderListener, on the other hand, is the listener to start up and shut down the WebApplicationContext in Spring root. Some of its important functions includes tying up the lifecycle of Application Context to the lifecycle of the ServletContext and automating the creation of ApplicationContext.

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Q6. What are the differences between constructor injection and setter injection?

No.	Constructor Injection	Setter Injection
1)	No Partial Injection	Partial Injection
2)	Doesn’t override the setter property	Overrides the constructor property if both are defined.
3)	Creates a new instance if any modification occurs	Doesn’t create a new instance if you change the property value
4)	Better for too many properties	Better for a few properties.

Q7. What is autowiring in Spring? What are the autowiring modes?

Autowiring enables the programmer to inject the bean automatically. We don’t need to write explicit injection logic. Let’s see the code to inject bean using dependency injection.

1. <bean id=“emp” class=“com.javatpoint.Employee” autowire=“byName” />  

The autowiring modes are given below:

No.	Mode	Description
1)	no	this is the default mode, it means autowiring is not enabled.
2)	byName	Injects the bean based on the property name. It uses setter method.
3)	byType	Injects the bean based on the property type. It uses setter method.
4)	constructor	It injects the bean using constructor

Q8. How to handle exceptions in Spring MVC Framework?

Spring MVC Framework provides the following ways to help us achieving robust exception handling.

Controller Based:

We can define exception handler methods in our controller classes. All we need is to annotate these methods with @ExceptionHandler annotation.

Global Exception Handler:

Exception Handling is a cross-cutting concern and Spring provides @ControllerAdvice annotation that we can use with any class to define our global exception handler.

HandlerExceptionResolver implementation: 

For generic exceptions, most of the times we serve static pages. Spring Framework provides HandlerExceptionResolver interface that we can implement to create global exception handler. The reason behind this additional way to define global exception handler is that Spring framework also provides default implementation classes that we can define in our spring bean configuration file to get spring framework exception handling benefits.

Q9. What are some of the important Spring annotations which you have used?

Some of the Spring annotations that I have used in my project are:

@Controller – for controller classes in Spring MVC project.

@RequestMapping – for configuring URI mapping in controller handler methods. This is a very important annotation, so you should go through Spring MVC RequestMapping Annotation Examples

@ResponseBody – for sending Object as response, usually for sending XML or JSON data as response.

@PathVariable – for mapping dynamic values from the URI to handler method arguments.

@Autowired – for autowiring dependencies in spring beans.

@Qualifier – with @Autowired annotation to avoid confusion when multiple instances of bean type is present.

@Service – for service classes.

@Scope – for configuring the scope of the spring bean.

@Configuration, @ComponentScan and @Bean – for java based configurations.

AspectJ annotations for configuring aspects and advices , @Aspect, @Before, @After, @Around, @Pointcut, etc.

Q10. How to integrate Spring and Hibernate Frameworks?

We can use Spring ORM module to integrate Spring and Hibernate frameworks if you are using Hibernate 3+ where SessionFactory provides current session, then you should avoid using HibernateTemplate or HibernateDaoSupport classes and better to use DAO pattern with dependency injection for the integration.

Also, Spring ORM provides support for using Spring declarative transaction management, so you should utilize that rather than going for hibernate boiler-plate code for transaction management. 

Q11. Name the types of transaction management that Spring supports.

Two types of transaction management are supported by Spring. They are:

1. Programmatic transaction management: In this, the transaction is managed with the help of programming. It provides you extreme flexibility, but it is very difficult to maintain.
2. Declarative transaction management: In this, transaction management is separated from the business code. Only annotations or XML based configurations are used to manage the transactions.

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Hibernate – Java Interview Questions

1. What is Hibernate Framework?

Object-relational mapping or ORM is the programming technique to map application domain model objects to the relational database tables. Hibernate is Java-based ORM tool that provides a framework for mapping application domain objects to the relational database tables and vice versa.

Hibernate provides a reference implementation of Java Persistence API, that makes it a great choice as ORM tool with benefits of loose coupling. We can use the Hibernate persistence API for CRUD operations. Hibernate framework provide option to map plain old java objects to traditional database tables with the use of JPA annotations as well as XML based configuration.

Similarly, hibernate configurations are flexible and can be done from XML configuration file as well as programmatically.

2. What are the important benefits of using Hibernate Framework?

Some of the important benefits of using hibernate framework are:

1. Hibernate eliminates all the boiler-plate code that comes with JDBC and takes care of managing resources, so we can focus on business logic.
2. Hibernate framework provides support for XML as well as JPA annotations, that makes our code implementation independent.
3. Hibernate provides a powerful query language (HQL) that is similar to SQL. However, HQL is fully object-oriented and understands concepts like inheritance, polymorphism, and association.
4. Hibernate is an open source project from Red Hat Community and used worldwide. This makes it a better choice than others because learning curve is small and there are tons of online documentation and help is easily available in forums.
5. Hibernate is easy to integrate with other Java EE frameworks, it’s so popular that Spring Framework provides built-in support for integrating hibernate with Spring applications.
6. Hibernate supports lazy initialization using proxy objects and perform actual database queries only when it’s required.
7. Hibernate cache helps us in getting better performance.
8. For database vendor specific feature, hibernate is suitable because we can also execute native sql queries.

Overall hibernate is the best choice in current market for ORM tool, it contains all the features that you will ever need in an ORM tool.

3. Explain Hibernate architecture.

Hibernate has a layered architecture which helps the user to operate without having to know the underlying APIs. Hibernate makes use of the database and configuration data to provide persistence services (and persistent objects) to the application. It includes many objects such as persistent object, session factory, transaction factory, connection factory, session, transaction etc.

The Hibernate architecture is categorized in four layers.

• Java application layer
• Hibernate framework layer
• Backhand API layer
• Database layer

4. What are the differences between get and load methods?

The differences between get() and load() methods are given below.

No.	get()	load()
1)	Returns null if object is not found.	Throws ObjectNotFoundException if an object is not found.
2)	get() method always hit the database.	load() method doesn’t hit the database.
3)	It returns a real object, not a proxy.	It returns a proxy object.
4)	It should be used if you are not sure about the existence of instance.	It should be used if you are sure that the instance exists.

5. What are the advantages of Hibernate over JDBC?

Some of the important advantages of Hibernate framework over JDBC are:

1. Hibernate removes a lot of boiler-plate code that comes with JDBC API, the code looks cleaner and readable.
2. Hibernate supports inheritance, associations, and collections. These features are not present with JDBC API.
3. Hibernate implicitly provides transaction management, in fact, most of the queries can’t be executed outside transaction. In JDBC API, we need to write code for transaction management using commit and rollback. 
4. JDBC API throws SQLException that is a checked exception, so we need to write a lot of try-catch block code. Most of the times it’s redundant in every JDBC call and used for transaction management. Hibernate wraps JDBC exceptions and throw JDBCException or HibernateException un-checked exception, so we don’t need to write code to handle it. Hibernate built-in transaction management removes the usage of try-catch blocks.
5. Hibernate Query Language (HQL) is more object-oriented and close to Java programming language. For JDBC, we need to write native SQL queries.
6. Hibernate supports caching that is better for performance, JDBC queries are not cached hence performance is low.
7. Hibernate provides option through which we can create database tables too, for JDBC tables must exist in the database.
8. Hibernate configuration helps us in using JDBC like connection as well as JNDI DataSource for the connection pool. This is a very important feature in enterprise application and completely missing in JDBC API.
9. Hibernate supports JPA annotations, so the code is independent of the implementation and easily replaceable with other ORM tools. JDBC code is very tightly coupled with the application.

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JSP – Java Interview Questions

1. What are the life-cycle methods for a jsp?

Methods	Description
public void jspInit()	It is invoked only once, same as init method of servlet.
public void _jspService(ServletRequest request,ServletResponse)throws ServletException,IOException	It is invoked at each request, same as service() method of servlet.
public void jspDestroy()	It is invoked only once, same as destroy() method of servlet.

2. What are the JSP implicit objects?

JSP provides 9 implicit objects by default. They are as follows:

Object	Type
1) out	JspWriter
2) request	HttpServletRequest
3) response	HttpServletResponse
4) config	ServletConfig
5) session	HttpSession
6) application	ServletContext
7) pageContext	PageContext
8) page	Object
9) exception	Throwable

3. What are the differences between include directive and include action?

include directive	include action
The include directive includes the content at page translation time.	The include action includes the content at request time.
The include directive includes the original content of the page so page size increases at runtime.	The include action doesn’t include the original content rather invokes the include() method of Vendor provided class.
It’s better for static pages.	It’s better for dynamic pages.

4. How to disable caching on back button of the browser?

<%
response.setHeader(“Cache-Control”,”no-store”);
response.setHeader(“Pragma”,”no-cache”);
response.setHeader (“Expires”, “0”);                    //prevents caching at the proxy server
%>   

5. What are the different tags provided in JSTL?

There are 5 type of JSTL tags.

1. core tags
2. sql tags
3. xml tags
4. internationalization tags
5. functions tags

6. How to disable session in JSP?

1. <%@ page session=“false” %>   

7.  How to delete a Cookie in a JSP?

The following code explains how to delete a Cookie in a JSP :

1 2 3 4 5 6 7 8 9 10 11

Cookie mycook = new Cookie("name1","value1");   response.addCookie(mycook1);   Cookie killmycook = new Cookie("mycook1","value1");   killmycook . set MaxAge ( 0 );   killmycook . set Path ("/");   killmycook . addCookie ( killmycook 1 );

8. Explain the jspDestroy() method.

jspDestry() method is invoked from javax.servlet.jsp.JspPage interface whenever a JSP page is about to be destroyed. Servlets destroy methods can be easily overridden to perform cleanup, like when closing a database connection.

9.  How is JSP better than Servlet technology?

JSP is a technology on the server’s side to make content generation simple. They are document-centric, whereas servlets are programs. A Java server page can contain fragments of Java program, which execute and instantiate Java classes. However, they occur inside an HTML template file. It provides the framework for the development of a Web Application.

10. Why should we not configure JSP standard tags in web.xml?

We don’t need to configure JSP standard tags in web.xml because when container loads the web application and find TLD files, it automatically configures them to be used directly in the application JSP pages. We just need to include it in the JSP page using taglib directive.

11. How will you use JSP EL in order to get the HTTP method name?

Using pageContext JSP EL implicit object you can get the request object reference and make use of the dot operator to retrieve the HTTP method name in the JSP page. The JSP EL code for this purpose will look like ${pageContext.request.method}.

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Exception and Thread Java Interview Questions

Q1. What is the difference between Error and Exception?

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors you cannot repair them at runtime. Though error can be caught in the catch block but the execution of application will come to a halt and is not recoverable.

While exceptions are conditions that occur because of bad input or human error etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving the user feedback for entering proper values etc.

Q2. How can you handle Java exceptions?

There are five keywords used to handle exceptions in Java: 

1. try
2. catch
3. finally
4. throw
5. throws

Q3. What are the differences between Checked Exception and Unchecked Exception?

Checked Exception

• The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions. 
• Checked exceptions are checked at compile-time.
• Example: IOException, SQLException etc.

Unchecked Exception

• The classes that extend RuntimeException are known as unchecked exceptions. 
• Unchecked exceptions are not checked at compile-time.
• Example: ArithmeticException, NullPointerException etc.

Q4. What purpose do the keywords final, finally, and finalize fulfill? 

Final:

Final is used to apply restrictions on class, method, and variable. A final class can’t be inherited, final method can’t be overridden and final variable value can’t be changed. Let’s take a look at the example below to understand it better.

1 2 3 4 5 6

class FinalVarExample { public static void main( String args[]) { final int a=10;   // Final variable a=50;             //Error as value can't be changed }

Finally

Finally is used to place important code, it will be executed whether the exception is handled or not. Let’s take a look at the example below to understand it better.

1 2 3 4 5 6 7 8 9 10 11 12

class FinallyExample { public static void main(String args[]){ try { int x=100; } catch(Exception e) { System.out.println(e); } finally { System.out.println("finally block is executing");} }} }

Finalize

Finalize is used to perform clean up processing just before the object is garbage collected. Let’s take a look at the example below to understand it better.

1 2 3 4 5 6 7 8 9 10 11 12 13

class FinalizeExample { public void finalize() { System.out.println("Finalize is called"); } public static void main(String args[]) { FinalizeExample f1=new FinalizeExample(); FinalizeExample f2=new FinalizeExample(); f1= NULL; f2=NULL; System.gc(); } }

 Q5. What are the differences between throw and throws? 

throw keyword	throws keyword
Throw is used to explicitly throw an exception.	Throws is used to declare an exception.
Checked exceptions can not be propagated with throw only.	Checked exception can be propagated with throws.
Throw is followed by an instance.	Throws is followed by class.
Throw is used within the method.	Throws is used with the method signature.
You cannot throw multiple exception	You can declare multiple exception e.g. public void method()throws IOException,SQLException.

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Q6. What is exception hierarchy in java?

The hierarchy is as follows:

Throwable is a parent class of all Exception classes. There are two types of Exceptions: Checked exceptions and UncheckedExceptions or RunTimeExceptions. Both type of exceptions extends Exception class whereas errors are further classified into Virtual Machine error and Assertion error.

Q7. How to create a custom Exception?

To create you own exception extend the Exception class or any of its subclasses.

• class New1Exception extends Exception { }               // this will create Checked Exception
• class NewException extends IOException { }             // this will create Checked exception
• class NewException extends NullPonterExcpetion { }  // this will create UnChecked exception

Q8. What are the important methods of Java Exception Class?

Exception and all of it’s subclasses doesn’t provide any specific methods and all of the methods are defined in the base class Throwable.

1. String getMessage() – This method returns the message String of Throwable and the message can be provided while creating the exception through it’s constructor.
2. String getLocalizedMessage() – This method is provided so that subclasses can override it to provide locale specific message to the calling program. Throwable class implementation of this method simply use getMessage() method to return the exception message.
3. Synchronized Throwable getCause() – This method returns the cause of the exception or null id the cause is unknown.
4. String toString() – This method returns the information about Throwable in String format, the returned String contains the name of Throwable class and localized message.
5. void printStackTrace() – This method prints the stack trace information to the standard error stream, this method is overloaded and we can pass PrintStream or PrintWriter as an argument to write the stack trace information to the file or stream.

Q9. What are the differences between processes and threads?

	Process	Thread
Definition	An executing instance of a program is called a process.	A thread is a subset of the process.
Communication	Processes must use inter-process communication to communicate with sibling processes.	Threads can directly communicate with other threads of its process.
Control	Processes can only exercise control over child processes.	Threads can exercise considerable control over threads of the same process.
Changes	Any change in the parent process does not affect child processes.	Any change in the main thread may affect the behavior of the other threads of the process.
Memory	Run in separate memory spaces.	Run in shared memory spaces.
Controlled by	Process is controlled by the operating system.	Threads are controlled by programmer in a program.
Dependence	Processes are independent.	Threads are dependent.

Q10. What is a finally block? Is there a case when finally will not execute?

Finally block is a block which always executes a set of statements. It is always associated with a try block regardless of any exception that occurs or not. 
Yes, finally will not be executed if the program exits either by calling System.exit() or by causing a fatal error that causes the process to abort.

Q11. What is synchronization?

Synchronization refers to multi-threading. A synchronized block of code can be executed by only one thread at a time. As Java supports execution of multiple threads, two or more threads may access the same fields or objects. Synchronization is a process which keeps all concurrent threads in execution to be in sync. Synchronization avoids memory consistency errors caused due to inconsistent view of shared memory. When a method is declared as synchronized the thread holds the monitor for that method’s object. If another thread is executing the synchronized method the thread is blocked until that thread releases the monitor. 

 Q12. Can we write multiple catch blocks under single try block? 

Yes we can have multiple catch blocks under single try block but the approach should be from specific to general. Let’s understand this with a programmatic example.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

public class Example { public static void main(String args[]) { try { int a[]= new int[10]; a[10]= 10/0; } catch(ArithmeticException e) { System.out.println("Arithmetic exception in first catch block"); } catch(ArrayIndexOutOfBoundsException e) { System.out.println("Array index out of bounds in second catch block"); } catch(Exception e) { System.out.println("Any exception in third catch block"); } }

Q13. What are the important methods of Java Exception Class?

Methods are defined in the base class Throwable. Some of the important methods of Java exception class are stated below. 

1. String getMessage() – This method returns the message String about the exception. The message can be provided through its constructor.
2. public StackTraceElement[] getStackTrace() – This method returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack whereas the last element in the array represents the method at the bottom of the call stack.

3. Synchronized Throwable getCause() – This method returns the cause of the exception or null id as represented by a Throwable object.

4. String toString() – This method returns the information in String format. The returned String contains the name of Throwable class and localized message.
5. void printStackTrace() – This method prints the stack trace information to the standard error stream. 

Q14. What is OutOfMemoryError in Java?

OutOfMemoryError is the subclass of java.lang.Error which generally occurs when our JVM runs out of memory.

Q15. What is a Thread?

A thread is the smallest piece of programmed instructions which can be executed independently by a scheduler. In Java, all the programs will have at least one thread which is known as the main thread. This main thread is created by the JVM when the program starts its execution. The main thread is used to invoke the main() of the program.

Q16. What are the two ways to create a thread?

In Java, threads can be created in the following two ways:- 

• By implementing the Runnable interface.
• By extending the Thread

Q17. What are the different types of garbage collectors in Java?

Garbage collection in Java a program which helps in implicit memory management. Since in Java, using the new keyword you can create objects dynamically, which once created will consume some memory. Once the job is done and there are no more references left to the object, Java using garbage collection destroys the object and relieves the memory occupied by it. Java provides four types of garbage collectors:

• Serial Garbage Collector
• Parallel Garbage Collector
• CMS Garbage Collector
• G1 Garbage Collector

 ////

 

 

3) List the features of Java Programming language.

There are the following features in Java Programming Language.

• Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it.


• Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior.


• Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.


• Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed.


• Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured.


• Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.


• Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture (32 bit or 64 bit) which doesn't exist in Java.


• Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.


• High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++).


• Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.


• Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.


• Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

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4) What do you understand by Java virtual machine?

Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.

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5) What is the difference between JDK, JRE, and JVM?

JVM

JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification which specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE.

JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance which is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.

JRE

JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

JDK

JDK is an acronym for Java Development Kit. It is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:

• Standard Edition Java Platform
• Enterprise Edition Java Platform
• Micro Edition Java Platform

More Details.

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6) How many types of memory areas are allocated by JVM?

Many types:

1. Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
2. Heap: It is the runtime data area in which the memory is allocated to the objects
3. Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
4. Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
5. Native Method Stack: It contains all the native methods used in the application.

More Details.

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7) What is JIT compiler?

Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

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8) What is the platform?

A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.

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9) What are the main differences between the Java platform and other platforms?

There are the following differences between the Java platform and other platforms.

• Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
• Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.

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10) What gives Java its 'write once and run anywhere' nature?

The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.

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11) What is classloader?

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

1. Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes, etc.
2. Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.
3. System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using "-cp" or "-classpath" switch. It is also known as Application classloader.

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12) Is Empty .java file name a valid source file name?

Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:

1. //save by .java only  
2. class A{  
3. public static void main(String args[]){  
4. System.out.println("Hello java");  
5. }  
6. }  
7. //compile by javac .java  
8. //run by     java A  

compile it by javac .java

run it by java A

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13) Is delete, next, main, exit or null keyword in java?

No.

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14) If I don't provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?

It is empty, but not null.

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15) What if I write static public void instead of public static void?

The program compiles and runs correctly because the order of specifiers doesn't matter in Java.

---

16) What is the default value of the local variables?

The local variables are not initialized to any default value, neither primitives nor object references.

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17) What are the various access specifiers in Java?

In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.

• Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
• Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
• Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.
• Private The private class, methods, or variables defined as private can be accessed within the class only.

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18) What is the purpose of static methods and variables?

The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.

For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.

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19) What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

• Packages avoid the name clashes.
• The Package provides easier access control.
• We can also have the hidden classes that are not visible outside and used by the package.
• It is easier to locate the related classes.

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20) What is the output of the following Java program?

1. class Test   
2. {  
3.     public static void main (String args[])   
4.     {  
5.         System.out.println(10 + 20 + "Javatpoint");   
6.         System.out.println("Javatpoint" + 10 + 20);  
7.     }  
8. }  

The output of the above code will be

30Javatpoint
Javatpoint1020

Explanation

In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint.

In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.

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21) What is the output of the following Java program?

1. class Test   
2. {  
3.     public static void main (String args[])   
4.     {  
5.         System.out.println(10 * 20 + "Javatpoint");   
6.         System.out.println("Javatpoint" + 10 * 20);  
7.     }  
8. }  

The output of the above code will be

200Javatpoint
Javatpoint200

Explanation

In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint.

In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.

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22) What is the output of the following Java program?

1. class Test   
2. {  
3.     public static void main (String args[])   
4.     {  
5.         for(int i=0; 0; i++)   
6.         {  
7.             System.out.println("Hello Javatpoint");  
8.         }  
9.     }  
10. }  

The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.

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Core Java - OOPs Concepts: Initial OOPs Interview Questions

There is given more than 50 OOPs (Object-Oriented Programming and System) interview questions. However, they have been categorized in many sections such as constructor interview questions, static interview questions, Inheritance Interview questions, Abstraction interview question, Polymorphism interview questions, etc. for better understanding.

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23) What is object-oriented paradigm?

It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.

• Follows the bottom-up approach in program design.
• Focus on data with methods to operate upon the object's data
• Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality.
• Implements the real-time approach like inheritance, abstraction, etc.
• The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.

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24) What is an object?

The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.

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25) What is the difference between an object-oriented programming language and object-based programming language?

There are the following basic differences between the object-oriented language and object-based language.

• Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn't follow all the concepts of OOPs like inheritance and polymorphism.
• Object-oriented languages do not have the inbuilt objects whereas Object-based languages have the inbuilt objects, for example, JavaScript has window object.
• Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.

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26) What will be the initial value of an object reference which is defined as an instance variable?

All object references are initialized to null in Java.

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Core Java - OOPs Concepts: Constructor Interview Questions

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27) What is the constructor?

The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.

More Details.

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28) How many types of constructors are used in Java?

Based on the parameters passed in the constructors, there are two types of constructors in Java.

• Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.
• Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.

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29) What is the purpose of a default constructor?

The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.

1. class Student3{  
2. int id;  
3. String name;  
4.   
5. void display(){System.out.println(id+" "+name);}  
6.   
7. public static void main(String args[]){  
8. Student3 s1=new Student3();  
9. Student3 s2=new Student3();  
10. s1.display();  
11. s2.display();  
12. }  
13. }  

Test it Now

Output:

0 null
0 null

Explanation: In the above class, you are not creating any constructor, so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.

More Details.

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30) Does constructor return any value?

Ans: yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor). More Details.

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31)Is constructor inherited?

No, The constructor is not inherited.

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32) Can you make a constructor final?

No, the constructor can't be final.

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33) Can we overload the constructors?

Yes, the constructors can be overloaded by changing the number of arguments accepted by the constructor or by changing the data type of the parameters. Consider the following example.

1. class Test   
2. {  
3.     int i;   
4.     public Test(int k)  
5.     {  
6.         i=k;  
7.     }  
8.     public Test(int k, int m)  
9.     {  
10.         System.out.println("Hi I am assigning the value max(k, m) to i");  
11.         if(k>m)  
12.         {  
13.             i=k;   
14.         }  
15.         else   
16.         {  
17.             i=m;  
18.         }  
19.     }  
20. }  
21. public class Main   
22. {  
23.     public static void main (String args[])   
24.     {  
25.         Test test1 = new Test(10);  
26.         Test test2 = new Test(12, 15);  
27.         System.out.println(test1.i);  
28.         System.out.println(test2.i);  
29.     }  
30. }  
31.       

In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.

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34) What do you understand by copy constructor in Java?

There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++.

There are many ways to copy the values of one object into another in java. They are:

• By constructor
• By assigning the values of one object into another
• By clone() method of Object class

In this example, we are going to copy the values of one object into another using java constructor.

1. //Java program to initialize the values from one object to another  
2. class Student6{  
3.     int id;  
4.     String name;  
5.     //constructor to initialize integer and string  
6.     Student6(int i,String n){  
7.     id = i;  
8.     name = n;  
9.     }  
10.     //constructor to initialize another object  
11.     Student6(Student6 s){  
12.     id = s.id;  
13.     name =s.name;  
14.     }  
15.     void display(){System.out.println(id+" "+name);}  
16.    
17.     public static void main(String args[]){  
18.     Student6 s1 = new Student6(111,"Karan");  
19.     Student6 s2 = new Student6(s1);  
20.     s1.display();  
21.     s2.display();  
22.    }  
23. }  

Test it Now

Output:

111 Karan
111 Karan

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35) What are the differences between the constructors and methods?

There are many differences between constructors and methods. They are given below.

Java Constructor	Java Method
A constructor is used to initialize the state of an object.	A method is used to expose the behavior of an object.
A constructor must not have a return type.	A method must have a return type.
The constructor is invoked implicitly.	The method is invoked explicitly.
The Java compiler provides a default constructor if you don't have any constructor in a class.	The method is not provided by the compiler in any case.
The constructor name must be same as the class name.	The method name may or may not be same as class name.

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36) What is the output of the following Java program?

1. public class Test   
2. {  
3.     Test(int a, int b)  
4.     {  
5.         System.out.println("a = "+a+" b = "+b);  
6.     }  
7.     Test(int a, float b)  
8.     {  
9.         System.out.println("a = "+a+" b = "+b);  
10.     }  
11.     public static void main (String args[])  
12.     {  
13.         byte a = 10;   
14.         byte b = 15;  
15.         Test test = new Test(a,b);  
16.     }  
17. }  

The output of the following program is:

a = 10 b = 15

Here, the data type of the variables a and b, i.e., byte gets promoted to int, and the first parameterized constructor with the two integer parameters is called.

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37) What is the output of the following Java program?

1. class Test   
2. {  
3.     int i;   
4. }  
5. public class Main   
6. {  
7.     public static void main (String args[])   
8.     {  
9.         Test test = new Test();   
10.         System.out.println(test.i);  
11.     }  
12. }  

The output of the program is 0 because the variable i is initialized to 0 internally. As we know that a default constructor is invoked implicitly if there is no constructor in the class, the variable i is initialized to 0 since there is no constructor in the class.

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38) What is the output of the following Java program?

1. class Test   
2. {  
3.     int test_a, test_b;  
4.     Test(int a, int b)   
5.     {  
6.     test_a = a;   
7.     test_b = b;   
8.     }  
9.     public static void main (String args[])   
10.     {  
11.         Test test = new Test();   
12.         System.out.println(test.test_a+" "+test.test_b);  
13.     }  
14. }  

There is a compiler error in the program because there is a call to the default constructor in the main method which is not present in the class. However, there is only one parameterized constructor in the class Test. Therefore, no default constructor is invoked by the constructor implicitly.

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Core Java - OOPs Concepts: static keyword Interview Questions

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39) What is the static variable?

The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.

1. //Program of static variable  
2.   
3. class Student8{  
4.    int rollno;  
5.    String name;  
6.    static String college ="ITS";  
7.      
8.    Student8(int r,String n){  
9.    rollno = r;  
10.    name = n;  
11.    }  
12.  void display (){System.out.println(rollno+" "+name+" "+college);}  
13.   
14.  public static void main(String args[]){  
15.  Student8 s1 = new Student8(111,"Karan");  
16.  Student8 s2 = new Student8(222,"Aryan");  
17.    
18.  s1.display();  
19.  s2.display();  
20.  }  
21. }  

Test it Now

Output:111 Karan ITS
       222 Aryan ITS

More Details.

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40) What is the static method?

• A static method belongs to the class rather than the object.
• There is no need to create the object to call the static methods.
• A static method can access and change the value of the static variable.

More Details.

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41) What are the restrictions that are applied to the Java static methods?

Two main restrictions are applied to the static methods.

• The static method can not use non-static data member or call the non-static method directly.
• this and super cannot be used in static context as they are non-static.

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42) Why is the main method static?

Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call main() method which will lead to the extra memory allocation. More Details.

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43) Can we override the static methods?

No, we can't override static methods.

44) What is the static block?

Static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.

1. class A2{  
2.   static{System.out.println("static block is invoked");}  
3.   public static void main(String args[]){  
4.    System.out.println("Hello main");  
5.   }  
6. }  

Test it Now

Output: static block is invoked
       Hello main

More Details.

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45) Can we execute a program without main() method?

Ans) No, It was possible before JDK 1.7 using the static block. Since JDK 1.7, it is not possible. More Details.

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46) What if the static modifier is removed from the signature of the main method?

Program compiles. However, at runtime, It throws an error "NoSuchMethodError."

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47) What is the difference between static (class) method and instance method?

static or class method	instance method
1)A method that is declared as static is known as the static method.	A method that is not declared as static is known as the instance method.
2)We don't need to create the objects to call the static methods.	The object is required to call the instance methods.
3)Non-static (instance) members cannot be accessed in the static context (static method, static block, and static nested class) directly.	Static and non-static variables both can be accessed in instance methods.
4)For example: public static int cube(int n){ return n*n*n;}	For example: public void msg(){...}.

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48) Can we make constructors static?

As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.

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49) Can we make the abstract methods static in Java?

In Java, if we make the abstract methods static, It will become the part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.

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50) Can we declare the static variables and methods in an abstract class?

Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.

1. abstract class Test  
2. {  
3.     static int i = 102;  
4.     static void TestMethod()  
5.     {  
6.         System.out.println("hi !! I am good !!");  
7.     }  
8. }  
9. public class TestClass extends Test   
10. {  
11.     public static void main (String args[])  
12.     {  
13.         Test.TestMethod();  
14.         System.out.println("i = "+Test.i);  
15.     }  
16. }  

Output

hi !! I am good !!
i = 102

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Core Java - OOPs Concepts: Inheritance Interview Questions

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51) What is this keyword in java?

The this keyword is a reference variable that refers to the current object. There are the various uses of this keyword in Java. It can be used to refer to current class properties such as instance methods, variable, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.

More Details.

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52) What are the main uses of this keyword?

There are the following uses of this keyword.

• this can be used to refer to the current class instance variable.
• this can be used to invoke current class method (implicitly)
• this() can be used to invoke the current class constructor.
• this can be passed as an argument in the method call.
• this can be passed as an argument in the constructor call.
• this can be used to return the current class instance from the method.

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53) Can we assign the reference to this variable?

No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.

1. public class Test  
2. {  
3.     public Test()  
4.     {  
5.         this = null;   
6.         System.out.println("Test class constructor called");  
7.     }  
8.     public static void main (String args[])  
9.     {  
10.         Test t = new Test();  
11.     }  
12. }  

Output

Test.java:5: error: cannot assign a value to final variable this
        this = null; 
        ^
1 error

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54) Can this keyword be used to refer static members?

Yes, It is possible to use this keyword to refer static members because this is just a reference variable which refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.

1. public class Test   
2. {  
3.     static int i = 10;   
4.     public Test ()  
5.     {  
6.         System.out.println(this.i);      
7.     }  
8.     public static void main (String args[])  
9.     {  
10.         Test t = new Test();  
11.     }  
12. }  

Output

10

---

55) How can constructor chaining be done using this keyword?

Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.

1. public class Employee  
2. {  
3.     int id,age;   
4.     String name, address;  
5.     public Employee (int age)  
6.     {  
7.         this.age = age;  
8.     }  
9.     public Employee(int id, int age)  
10.     {  
11.         this(age);  
12.         this.id = id;  
13.     }  
14.     public Employee(int id, int age, String name, String address)  
15.     {  
16.         this(id, age);  
17.         this.name = name;   
18.         this.address = address;   
19.     }  
20.     public static void main (String args[])  
21.     {  
22.         Employee emp = new Employee(105, 22, "Vikas", "Delhi");  
23.         System.out.println("ID: "+emp.id+" Name:"+emp.name+" age:"+emp.age+" address: "+emp.address);  
24.     }  
25.       
26. }  

Output

ID: 105 Name:Vikas age:22 address: Delhi

---

56) What are the advantages of passing this into a method instead of the current class object itself?

As we know, that this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object.

• this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
• this can be used in the synchronized block.

---

57) What is the Inheritance?

Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

There are five types of inheritance in Java.

• Single-level inheritance
• Multi-level inheritance
• Multiple Inheritance
• Hierarchical Inheritance
• Hybrid Inheritance

Multiple inheritance is not supported in Java through class.

More Details.

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58) Why is Inheritance used in Java?

There are various advantages of using inheritance in Java that is given below.

• Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
• Runtime polymorphism cannot be achieved without using inheritance.
• We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
• Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
• Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic method contained by the base class.

---

59) Which class is the superclass for all the classes?

The object class is the superclass of all other classes in Java.

---

60) Why is multiple inheritance not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.

Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile time error.

1. class A{  
2. void msg(){System.out.println("Hello");}  
3. }  
4. class B{  
5. void msg(){System.out.println("Welcome");}  
6. }  
7. class C extends A,B{//suppose if it were  
8.    
9.  Public Static void main(String args[]){  
10.    C obj=new C();  
11.    obj.msg();//Now which msg() method would be invoked?  
12. }  
13. }  

Test it Now

 Compile Time Error

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61) What is aggregation?

Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.

Address.java

1. public class Address {  
2. String city,state,country;  
3.   
4. public Address(String city, String state, String country) {  
5.     this.city = city;  
6.     this.state = state;  
7.     this.country = country;  
8. }  
9.   
10. }  

Employee.java

1. public class Emp {  
2. int id;  
3. String name;  
4. Address address;  
5.   
6. public Emp(int id, String name,Address address) {  
7.     this.id = id;  
8.     this.name = name;  
9.     this.address=address;  
10. }  
11.   
12. void display(){  
13. System.out.println(id+" "+name);  
14. System.out.println(address.city+" "+address.state+" "+address.country);  
15. }  
16.   
17. public static void main(String[] args) {  
18. Address address1=new Address("gzb","UP","india");  
19. Address address2=new Address("gno","UP","india");  
20.   
21. Emp e=new Emp(111,"varun",address1);  
22. Emp e2=new Emp(112,"arun",address2);  
23.       
24. e.display();  
25. e2.display();  
26.       
27. }  
28. }  

Output

111 varun
gzb UP india
112 arun
gno UP india 

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62) What is composition?

Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

---

63) What is the difference between aggregation and composition?

Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).

---

64) Why does Java not support pointers?

The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.

---

65) What is super in java?

The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.

1. class Animal{  
2. Animal(){System.out.println("animal is created");}  
3. }  
4. class Dog extends Animal{  
5. Dog(){  
6. System.out.println("dog is created");  
7. }  
8. }  
9. class TestSuper4{  
10. public static void main(String args[]){  
11. Dog d=new Dog();  
12. }  
13. }  

Test it Now

Output:

animal is created
dog is created

More Details.

---

66) How can constructor chaining be done by using the super keyword?

1. class Person  
2. {  
3.     String name,address;   
4.     int age;  
5.     public Person(int age, String name, String address)  
6.     {  
7.         this.age = age;  
8.         this.name = name;  
9.         this.address = address;  
10.     }  
11. }  
12. class Employee extends Person   
13. {  
14.     float salary;  
15.     public Employee(int age, String name, String address, float salary)  
16.     {  
17.         super(age,name,address);  
18.         this.salary = salary;  
19.     }  
20. }  
21. public class Test   
22. {  
23.     public static void main (String args[])  
24.     {  
25.         Employee e = new Employee(22, "Mukesh", "Delhi", 90000);  
26.         System.out.println("Name: "+e.name+" Salary: "+e.salary+" Age: "+e.age+" Address: "+e.address);  
27.     }  
28. }  

Output

Name: Mukesh Salary: 90000.0 Age: 22 Address: Delhi

---

67) What are the main uses of the super keyword?

There are the following uses of super keyword.

• super can be used to refer to the immediate parent class instance variable.
• super can be used to invoke the immediate parent class method.
• super() can be used to invoke immediate parent class constructor.

---

68) What are the differences between this and super keyword?

There are the following differences between this and super keyword.

• The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
• The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
• The super and this must be the first statement inside constructor otherwise the compiler will throw an error.

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69) What is the output of the following Java program?

1. class Person   
2. {  
3.     public Person()   
4.     {  
5.         System.out.println("Person class constructor called");  
6.     }  
7. }  
8. public class Employee extends Person   
9. {  
10.     public Employee()   
11.     {  
12.         System.out.println("Employee class constructor called");  
13.     }  
14.     public static void main (String args[])  
15.     {  
16.         Employee e = new Employee();  
17.     }  
18. }  

Output

Person class constructor called
Employee class constructor called

Explanation

The super() is implicitly invoked by the compiler if no super() or this() is included explicitly within the derived class constructor. Therefore, in this case, The Person class constructor is called first and then the Employee class constructor is called.

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70) Can you use this() and super() both in a constructor?

No, because this() and super() must be the first statement in the class constructor.

Example:

1. public class Test{  
2.     Test()  
3.      {  
4.          super();   
5.          this();  
6.          System.out.println("Test class object is created");  
7.      }  
8.      public static void main(String []args){  
9.      Test t = new Test();  
10.      }  
11. }  

Output:

Test.java:5: error: call to this must be first statement in constructor

---

71)What is object cloning?

The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.

1. protected Object clone() throws CloneNotSupportedException    
2.       

More Details.

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Core Java - OOPs Concepts: Method Overloading Interview Questions

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72) What is method overloading?

Method overloading is the polymorphism technique which allows us to create multiple methods with the same name but different signature. We can achieve method overloading in two ways.

• By Changing the number of arguments
• By Changing the data type of arguments

Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.

More Details.

---

73) Why is method overloading not possible by changing the return type in java?

In Java, method overloading is not possible by changing the return type of the program due to avoid the ambiguity.

1. class Adder{  
2. static int add(int a,int b){return a+b;}  
3. static double add(int a,int b){return a+b;}  
4. }  
5. class TestOverloading3{  
6. public static void main(String[] args){  
7. System.out.println(Adder.add(11,11));//ambiguity  
8. }}  

Test it Now

Output:

Compile Time Error: method add(int, int) is already defined in class Adder

More Details.

---

74) Can we overload the methods by making them static?

No, We cannot overload the methods by just applying the static keyword to them(number of parameters and types are the same). Consider the following example.

1. public class Animal  
2. {  
3.     void consume(int a)  
4.     {  
5.         System.out.println(a+" consumed!!");  
6.     }  
7.     static void consume(int a)  
8.     {  
9.         System.out.println("consumed static "+a);  
10.     }  
11.     public static void main (String args[])  
12.     {  
13.         Animal a = new Animal();  
14.         a.consume(10);  
15.         Animal.consume(20);  
16.     }  
17. }    

Output

Animal.java:7: error: method consume(int) is already defined in class Animal
    static void consume(int a)
                ^
Animal.java:15: error: non-static method consume(int) cannot be referenced from a static context
        Animal.consume(20);
              ^
2 errors

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75) Can we overload the main() method?

Yes, we can have any number of main methods in a Java program by using method overloading.

More Details.

---

76) What is method overloading with type promotion?

By Type promotion is method overloading, we mean that one data type can be promoted to another implicitly if no exact matching is found.

As displayed in the above diagram, the byte can be promoted to short, int, long, float or double. The short datatype can be promoted to int, long, float or double. The char datatype can be promoted to int, long, float or double and so on. Consider the following example.

1. class OverloadingCalculation1{  
2.   void sum(int a,long b){System.out.println(a+b);}  
3.   void sum(int a,int b,int c){System.out.println(a+b+c);}  
4.   
5.   public static void main(String args[]){  
6.   OverloadingCalculation1 obj=new OverloadingCalculation1();  
7.   obj.sum(20,20);//now second int literal will be promoted to long  
8.   obj.sum(20,20,20);  
9.   }  
10. }  

Test it Now

Output

40
60

---

77) What is the output of the following Java program?

1. class OverloadingCalculation3{    
2.   void sum(int a,long b){System.out.println("a method invoked");}    
3.   void sum(long a,int b){System.out.println("b method invoked");}    
4.     
5.   public static void main(String args[]){    
6.   OverloadingCalculation3 obj=new OverloadingCalculation3();    
7.   obj.sum(20,20);//now ambiguity    
8.   }    
9. }    

Output

OverloadingCalculation3.java:7: error: reference to sum is ambiguous
obj.sum(20,20);//now ambiguity  
     ^ 
      both method sum(int,long) in OverloadingCalculation3 
      and method sum(long,int) in OverloadingCalculation3 match
1 error

Explanation

There are two methods defined with the same name, i.e., sum. The first method accepts the integer and long type whereas the second method accepts long and the integer type. The parameter passed that are a = 20, b = 20. We can not tell that which method will be called as there is no clear differentiation mentioned between integer literal and long literal. This is the case of ambiguity. Therefore, the compiler will throw an error.

---

Core Java - OOPs Concepts: Method Overriding Interview Questions

---

78) What is method overriding:

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to implement the interface methods.

Rules for Method overriding

• The method must have the same name as in the parent class.
• The method must have the same signature as in the parent class.
• Two classes must have an IS-A relationship between them.

More Details.

---

79) Can we override the static method?

No, you can't override the static method because they are the part of the class, not the object.

---

80) Why can we not override static method?

It is because the static method is the part of the class, and it is bound with class whereas instance method is bound with the object, and static gets memory in class area, and instance gets memory in a heap.

---

81) Can we override the overloaded method?

Yes.

---

82) Difference between method Overloading and Overriding.

Method Overloading	Method Overriding
1) Method overloading increases the readability of the program.	Method overriding provides the specific implementation of the method that is already provided by its superclass.
2) Method overloading occurs within the class.	Method overriding occurs in two classes that have IS-A relationship between them.
3) In this case, the parameters must be different.	In this case, the parameters must be the same.

---

83) Can we override the private methods?

No, we cannot override the private methods because the scope of private methods is limited to the class and we cannot access them outside of the class.

---

84) Can we change the scope of the overridden method in the subclass?

Yes, we can change the scope of the overridden method in the subclass. However, we must notice that we cannot decrease the accessibility of the method. The following point must be taken care of while changing the accessibility of the method.

• The private can be changed to protected, public, or default.
• The protected can be changed to public or default.
• The default can be changed to public.
• The public will always remain public.

---

85) Can we modify the throws clause of the superclass method while overriding it in the subclass?

Yes, we can modify the throws clause of the superclass method while overriding it in the subclass. However, there are some rules which are to be followed while overriding in case of exception handling.

• If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception, but it can declare the unchecked exception.
• If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.

---

86) What is the output of the following Java program?

1. class Base  
2. {  
3.     void method(int a)  
4.     {  
5.         System.out.println("Base class method called with integer a = "+a);  
6.     }  
7.        
8.     void method(double d)  
9.     {  
10.         System.out.println("Base class method called with double d ="+d);  
11.     }  
12. }  
13.    
14. class Derived extends Base  
15. {  
16.     @Override  
17.     void method(double d)  
18.     {  
19.         System.out.println("Derived class method called with double d ="+d);  
20.     }  
21. }  
22.    
23. public class Main  
24. {      
25.     public static void main(String[] args)  
26.     {  
27.         new Derived().method(10);  
28.     }  
29. }  

Output

Base class method called with integer a = 10

Explanation

The method() is overloaded in class Base whereas it is derived in class Derived with the double type as the parameter. In the method call, the integer is passed.

87) Can you have virtual functions in Java?

Yes, all functions in Java are virtual by default.

---

88) What is covariant return type?

Now, since java5, it is possible to override any method by changing the return type if the return type of the subclass overriding method is subclass type. It is known as covariant return type. The covariant return type specifies that the return type may vary in the same direction as the subclass.

1. class A{  
2. A get(){return this;}  
3. }  
4.   
5. class B1 extends A{  
6. B1 get(){return this;}  
7. void message(){System.out.println("welcome to covariant return type");}  
8.   
9. public static void main(String args[]){  
10. new B1().get().message();  
11. }  
12. }  

Test it Now

Output: welcome to covariant return type

More Details.

---

89) What is the output of the following Java program?

1. class Base   
2. {  
3.     public void baseMethod()  
4.     {  
5.         System.out.println("BaseMethod called ...");  
6.     }  
7. }  
8. class Derived extends Base   
9. {  
10.     public void baseMethod()  
11.     {  
12.         System.out.println("Derived method called ...");  
13.     }  
14. }  
15. public class Test   
16. {  
17.     public static void main (String args[])  
18.     {  
19.         Base b = new Derived();  
20.         b.baseMethod();  
21.     }  
22. }  

---

Output

Derived method called ...

Explanation

The method of Base class, i.e., baseMethod() is overridden in Derived class. In Test class, the reference variable b (of type Base class) refers to the instance of the Derived class. Here, Runtime polymorphism is achieved between class Base and Derived. At compile time, the presence of method baseMethod checked in Base class, If it presence then the program compiled otherwise the compiler error will be shown. In this case, baseMethod is present in Base class; therefore, it is compiled successfully. However, at runtime, It checks whether the baseMethod has been overridden by Derived class, if so then the Derived class method is called otherwise Base class method is called. In this case, the Derived class overrides the baseMethod; therefore, the Derived class method is called.

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Core Java - OOPs Concepts: final keyword Interview Questions

---

90) What is the final variable?

In Java, the final variable is used to restrict the user from updating it. If we initialize the final variable, we can't change its value. In other words, we can say that the final variable once assigned to a value, can never be changed after that. The final variable which is not assigned to any value can only be assigned through the class constructor.

1. class Bike9{  
2.  final int speedlimit=90;//final variable  
3.  void run(){  
4.   speedlimit=400;  
5.  }  
6.  public static void main(String args[]){  
7.  Bike9 obj=new  Bike9();  
8.  obj.run();  
9.  }  
10. }//end of class  

Test it Now

Output:Compile Time Error

More Details.

---

91) What is the final method?

If we change any method to a final method, we can't override it. More Details.

1. class Bike{  
2.   final void run(){System.out.println("running");}  
3. }  
4.      
5. class Honda extends Bike{  
6.    void run(){System.out.println("running safely with 100kmph");}  
7.      
8.    public static void main(String args[]){  
9.    Honda honda= new Honda();  
10.    honda.run();  
11.    }  
12. }  

Test it Now

Output:Compile Time Error

---

92) What is the final class?

If we make any class final, we can't inherit it into any of the subclasses.

1. final class Bike{}  
2.   
3. class Honda1 extends Bike{  
4.   void run(){System.out.println("running safely with 100kmph");}  
5.     
6.   public static void main(String args[]){  
7.   Honda1 honda= new Honda1();  
8.   honda.run();  
9.   }  
10. }  

Test it Now

Output:Compile Time Error

More Details.

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93) What is the final blank variable?

A final variable, not initialized at the time of declaration, is known as the final blank variable. We can't initialize the final blank variable directly. Instead, we have to initialize it by using the class constructor. It is useful in the case when the user has some data which must not be changed by others, for example, PAN Number. Consider the following example:

1. class Student{  
2. int id;  
3. String name;  
4. final String PAN_CARD_NUMBER;  
5. ...  
6. }  

More Details.

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94) Can we initialize the final blank variable?

Yes, if it is not static, we can initialize it in the constructor. If it is static blank final variable, it can be initialized only in the static block. More Details.

---

95) Can you declare the main method as final?

Yes, We can declare the main method as public static final void main(String[] args){}.

---

96) What is the output of the following Java program?

1. class Main {  
2.  public static void main(String args[]){  
3.    final int i;  
4.    i = 20;  
5.    System.out.println(i);  
6.  }  
7. }  

Output

20

Explanation

Since i is the blank final variable. It can be initialized only once. We have initialized it to 20. Therefore, 20 will be printed.

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97) What is the output of the following Java program?

1. class Base   
2. {  
3.     protected final void getInfo()  
4.     {  
5.         System.out.println("method of Base class");  
6.     }  
7. }  
8.    
9. public class Derived extends Base  
10. {  
11.     protected final void getInfo()  
12.     {  
13.         System.out.println("method of Derived class");  
14.     }  
15.     public static void main(String[] args)  
16.     {  
17.         Base obj = new Base();  
18.         obj.getInfo();  
19.     }  
20. }  

Output

	Derived.java:11: error: getInfo() in Derived cannot override getInfo() in Base
    protected final void getInfo()
                         ^
  overridden method is final
1 error

Explanation

The getDetails() method is final; therefore it can not be overridden in the subclass.

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98) Can we declare a constructor as final?

The constructor can never be declared as final because it is never inherited. Constructors are not ordinary methods; therefore, there is no sense to declare constructors as final. However, if you try to do so, The compiler will throw an error.

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99) Can we declare an interface as final?

No, we cannot declare an interface as final because the interface must be implemented by some class to provide its definition. Therefore, there is no sense to make an interface final. However, if you try to do so, the compiler will show an error.

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100) What is the difference between the final method and abstract method?

The main difference between the final method and abstract method is that the abstract method cannot be final as we need to override them in the subclass to give its definition.