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Java 8 interface static and default methods

  • Java 8 introduced two new methods in interface they are
    1.default methods
    2.static methods
  • By this interfaces and abstract class are same but still having lot differences like abstract class can have a constructor etc will discuss more before that we need to know about these Java 8 features of interfaces.
  • Defaults methods are also called as defender methods can be implemented inside the interface
  • Like normal class now with java 8 we can declare static methods in side a interface.
  • Lets jump deep into Java 8 default and static methods 

  

1.Interface Default Methods in Java 8


  • Before Java 8 in interfaces we can and able to declare only abstract methods only.
  • If we declare a method without abstract that will be treated as abstract by default.
  • As we know all methods in interfaces are by default abstract methods.
  • These methods wont have body means implementations
  • The class which is implementing this interface need to provide body / implementation for this abstract methods.
  • Now with java 8 default methods we can add methods to interface without disturbing existing functionality.
  • So instead of overriding now we can inherit these default methods from interfaces

  • Defaults methods are also  known as defender methods or virtual extension methods
  • Default methods will help us to avoid utility classes.
  • We can define utility methods inside the interface and use it in all classes which is implementing.
  • One of the major reason to introduce this default methods in java 8 is to support lambda expressions in collections API and to enhance.


  1. package com.instanceofjava;
  2. interface Java8Interface{
  3.  
  4. abstract void show();
  5.   
  6. default void display(){
  7.  
  8. System.out.println("default method of interface");
  9.  
  10. }
  11.  
  12. }

  1. package com.instanceofjava;
  2. class Sample implements Java8Interface {
  3.  
  4. void show(){
  5. System.out.print("overridden method ")
  6.  }
  7. public static void main(String[] args){
  8.   
  9. Sample obj= new Sample();
  10.  
  11. obj.show(); // calling implemented method
  12. obj.display(); // calling inherited method
  13. Java8Interface.display(); calling using interface name
  14.  
  15. }
  16.  
  17. }



Output:

 

  1. overridden method
  2. default method of interface
  3. default method of interface


How to call default methods:

 

  • We can all these default methods by using interface name and also by using object of the class which is implementing.
  • From above example
  • obj.show(); // calling implemented method
  • obj.display(); // calling inherited method
  • Java8Interface.display(); calling using interface name

Can we override java 8 default method

  • As we discussed above default methods in interfaces are implemented methods with bodies
  • Yes we can override same method in class which is implementing this interface.
  • Lets see one sample program how to override and what happens if we override


  1. package com.instanceofjava;
  2. interface InterfaceWithDefault{
  3.  
  4. default void defMethod(){
  5.  
  6. System.out.println("default method of interface");
  7.  
  8. }
  9.  
  10. }

  1. package com.instanceofjava;
  2. class Demo implements InterfaceWithDefault{
  3.  
  4. void defMethod(){
  5.  
  6. System.out.print("overridden method in class Demo ") 
  7.  
  8.  }
  9. public static void main(String[] args){
  10.   
  11. Demo obj= new Demo();
  12.  
  13. obj.defMethod(); // calling overridden method
  14. Java8Interface.defMethod(); calling using interface name : interface defMethod will be called
  15.  
  16. }
  17.  
  18. }

Output:


  1. overridden method in class Demo
  2. default method of interface

What happens if we implement two interfaces having same default methods

  • Now lets see if a class implementing two interfaces which are having same default methods
  • Whatever the implementation in the two interfaces defined if we implementing two interfaces which are having a default method in both then compilation error will come if two methods have same signature. works fine if two methods have same name with different arguments.
  • Check the below example programs to understand more.


  1. package com.instanceofjava;
  2. interface A{
  3.  
  4. default void defMethod(){
  5.  
  6. System.out.println("default method of interface: A");
  7.  
  8. }
  9.  
  10. }

  1. package com.instanceofjava;
  2. interface B{
  3.  
  4. default void defMethod(){
  5.  
  6. System.out.println("default method of interface: B");
  7.  
  8. }
  9.  
  10. }

  1. package com.instanceofjava;
  2. class Demo implements A, B{ // compilation error will come
  3.  
  4. public static void main(String[] args){
  5.   
  6. Demo obj= new Demo();
  7.  
  8.  
  9. }
  10.  
  11. }


  • If we implement two interfaces which are having same method with same parameters then compilation error will occur.
  • Duplicate default methods named "defMethod" with the parameters () and () are inherited from the types A and B.
  • If we define two methods with different type of parameters then we can work with both interfaces.



  1. package com.instanceofjava;
  2. interface A{
  3.  
  4. default void defMethod(){
  5.  
  6. System.out.println("Default method of interface: A");
  7.  
  8. }
  9.  
  10. }

  1. package com.instanceofjava;
  2. interface B{
  3.  
  4. default void defMethod(String str){
  5.  
  6. System.out.println("Default method of interface: B");
  7. System.out.println(str);
  8.  
  9.  
  10. }
  11.  
  12. }

  1. package com.instanceofjava;
  2. class Demo implements A, B{ // compilation error will come
  3.  
  4. public static void main(String[] args){
  5.   
  6. Demo obj= new Demo();
  7. obj.defMethod();
  8. obj.defMethod("Java 8")
  9.  
  10.  
  11. }
  12.  
  13. }

Output:

  1. Default method of interface: A
  2. Default method of interface: B 
  3. Java 8


1.Interface Static Methods in Java 8

  • Another Java 8 interface method is static method.
  • Now we can define static methods inside interface but we can not override these static methods.
  • These static method will act as helper methods.
  • These methods are the parts of interface not belongs to implementation class objects.

  1. package com.instanceofjava;
  2. interface StaticInterface{
  3.  
  4. Static void print(String str){
  5.  
  6. System.out.println("Static method of interface:"+str);
  7.  
  8. }
  9. }

  1. package com.instanceofjava;
  2. class Demo implements StaticInterface{
  3.  
  4. public static void main(String[] args){
  5.   
  6.  StaticInterface.print("Java 8")
  7.  
  8. }
  9.  
  10. }

Output:

  1. Static method of interface: Java 8


Instance Of Java

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