Evolution of interface in Java

Prerequisite : Interfaces in Java
In Java SE 7 or earlier versions, an interface can have only:

• Constant variables
• Abstract methods

We can’t provide implementations of methods in interfaces.

public interface GFG{
      String a = "Geeksforgeeks is the best.";

      void hello(String a);
      void world(int x);
}

Java SE 8:
We can write method implementations in Interface from Java SE 8 and on-wards. We need to use “default” keyword to define them as shown below.
In Java SE 8 and later versions, an interface can have only four kinds of things:

• Constant variables
• Abstract methods
• Default methods
• Static methods

public interface GFG{
      String b = "Shubham is a brilliant coder.";

      default void hello(String a){
         System.out.println("Hello");
      }
      static void world(int x){
         System.out.println("World");
      }
      void bye();
   }

Java SE 9:
In Java SE 9 and on-wards, we can write private methods in Interfaces using ‘private’ access modifier as shown below (like other private methods).
In Java SE 9 and later versions, an interface can have:

• Constant variables
• Abstract methods
• Default methods
• Static methods
• Private methods
• Private Static methods

public interface GFG{
      String b = "Shubham is a brilliant coder.";

      default void hello(String a){
         System.out.println("Hello");
      }
      static void world(int x){
         System.out.println("World");
      }
      void bye();

      private void great(long v){

      }
   }