Interface in Java: An Interface Can Consist of Only Incomplete Methods or Abstract Methods in it.
Interface Example 1
public interface A {
public void test() { // Error
}
}
Interface Example 2
public interface A {
public void test(); // correct
}
Interface in Java Important Points
#01 When A class implements An Interface then it Means We Are Inheriting Incomplete Method From Interface to Class and Than in Class We Override and Complete that Method.
Example 1
Package p1
public interface A {
public void test();
}
Package p1
public class B implements A {
@Override
public void test() {
System.out.println(100);
}
public static void main ( String[] args ) {
B b1 = new B();
b1.test();
}
}
Output :
100
Example 2
Package p1
public interface A {
public void test1();
public void test2();
}
Package p1
public class B implements A {
@Override
public void test1(){
System.out.println(10);
}
@Override
public void test2(){
System.out.println(20);
}
public static void main(String[] args ) {
B b1 = new B();
b1.test1();
b1.test2();
}
}
Output :
10
20
You Also Learn –
- What is inheritance in Java
- What is Abstract Class in Java
- What is Lambda Expression in Java 8
- Types of Exception in Java
- What is Final Keyword in Java
- What is Exception Handling in Java
- What is Interface in Java
- What is Wrapper Class in Java
- Break Keyword in Java With Example
Note – Interface supports Multiple inheritances But Class Not Support Multiple inheritances.
Multiple Inheritance in Interface

Example 1
public interface A {
public void test1();
}
public interface B {
public void test2();
}
public interface C extends A,B {
}
public class D implements C {
@Override
public void test1() {
System.out.println( 100);
}
@Override
public void test2() {
System.out.println( 200);
}
public static void main(String[] args ) {
D d1 = new D();
d1.test1();
d1.test2();
}
}
Output :
100
200
Functional Interface in Java 8 With Example
Functional Interface in Java : – A functional interface can Consist of exactly one incomplete method in it.
Example 1
@FunctionalInterface
public interface A { // Correct
public void test();
}
Example 2
@FunctionalInterface
public interface A { // Error
public void test1();
public void test2();
}
Note – Functional Interface Was Introduced in Version 8 of java.
- Functional Interfaces Can Be Implemented by a Lambda Expression.
- We Cannot do Multiple inheritances on the Functional Interface.
- We Can Create Any No. of Complete Methods inside Functional Interface.
Interface in Java Program
Note 1 – All the variables created interface by default final and static.
public interface A {
int x=10; // final and static
}
public class B {
public static void main(String[] args) {
System.out.println(A.x);
}
}
Output: 10
Note 2 – An object of an interface can not be created.
public interface A {
int x=10;
}
public class B {
public static void main(String[] args) {
A a1 =new A(); // Error
System.out.println(A.x);
}
}
Note 3 – A reference variable of an interface can be created.
public interface A {
int x=10;
}
public class B {
public static void main(String[] args) {
A a1 =null;
System.out.println(a1);
}
}
Output:
null