Interface in JDK8

New features added in interfaces in JDK 8

1. Prior to JDK 8, interface could not define implementation. We can now add default implementation for interface methods. This default implementation has special use and does not affect the intention behind interfaces.
   Suppose we need to add a new function in an existing interface. Obviously the old code will not work as the classes have not implemented those new functions. So with the help of default implementation, we will give a default body for the newly added functions. Then the old codes will still work.
   
   

   // An example to show that interfaces can // have methods from JDK 1.8 onwards interface in1 {     final int a = 10;     default void display()     {         System.out.println("hello");     } }    // A class that implements interface. class testClass implements in1 {     // Driver Code     public static void main (String[] args)     {         testClass t = new testClass();         t.display();     } }

   Output :

   hello
   

2. Another feature that was added in JDK 8 is that we can now define static methods in interfaces which can be called independently without an object. Note: these methods are not inherited.

   interface in1 {     final int a = 10;     static void display()     {         System.out.println("hello");     } }    // A class that implements interface. class testClass implements in1 {     // Driver Code     public static void main (String[] args)     {         in1.display();     } }

   Output :

   hello

Important points about interface or summary of article:

• We can’t create instance(interface can’t be instantiated) of interface but we can make reference of it that refers to the Object of its implementing class.
• A class can implement more than one interface.
• An interface can extends another interface or interfaces (more than one interface) .
• A class that implements interface must implements all the methods in interface.
• All the methods are public and abstract. And all the fields are public, static, and final.
• It is used to achieve multiple inheritance.
• It is used to achieve loose coupling.

New features added in interfaces in JDK 9 
From Java 9 onwards, interfaces can contain following also

1. Static methods
2. Private methods
3. Private Static methods