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Interface Interview Questions

There are mainly three reasons to use interface. They are given below.

  • It is used to achieve fully abstraction.
  • By interface, we can support the functionality of multiple inheritance.
  • It can be used to achieve loose coupling.
In other words, Interface fields are public, static and final bydefault, and methods are public and abstract.
An interface in java is a blueprint of a class. It has static constants and abstract methods only.
The interface in java is a mechanism to achieve fully abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve fully abstraction and multiple inheritance in Java.
Java Interface also represents IS-A relationship.
It cannot be instantiated just like abstract class.
As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface.

In this example, Printable interface have only one method, its implementation is provided in the A class.


interface printable{
void print();
}

class A6 implements printable{
public void print(){System.out.println("Hello");}

public static void main(String args[]){
A6 obj = new A6();
obj.print();
}
}

Output:Hello

	

If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance




interface Printable{
void print();
}

interface Showable{
void show();
}

class A7 implements Printable,Showable{

public void print(){System.out.println("Hello");}
public void show(){System.out.println("Welcome");}

public static void main(String args[]){
A7 obj = new A7();
obj.print();
obj.show();
}
}

Output: Hello
Welcome
As we have explained in the inheritance chapter, multiple inheritance is not supported in case of class. But it is supported in case of interface because there is no ambiguity as implementation is provided by the implementation class.

For example:


interface Printable{
void print();
}
interface Showable{
void print();
}

class TestTnterface1 implements Printable,Showable{
public void print(){System.out.println("Hello");}
public static void main(String args[]){
TestTnterface1 obj = new TestTnterface1();
obj.print();
}
}
Output:Hello

	
As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity.
A class implements interface but one interface extends another interface .


interface Printable{
void print();
}
interface Showable extends Printable{
void show();
}
class Testinterface2 implements Showable{

public void print(){System.out.println("Hello");}
public void show(){System.out.println("Welcome");}
public static void main(String args[]){
Testinterface2 obj = new Testinterface2();
obj.print();
obj.show();
}
}

Output: Hello
Welcome
An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

//How Serializable interface is written?
public interface Serializable{
} 
An interface can have another interface i.e. known as nested interface. We will learn it in detail in the nested classes chapter.

For example:


interface printable{
void print();
interface MessagePrintable{
void msg();
}
}