Private and static private interface methods were introduced in Java 9. Being a private method, such a method cannot be accessed via implementing class or sub-interface. These methods were introduced to allow encapsulation where the implementation of certain methods will be kept in the interface only. It helps to reduce duplicity, increase maintainability, and to write clean code.

Interface Prior to Java 8

Prior to Java 8, interface can have only abstract method and constant variables. So implementing class has to implement the same. See the below example.

Example

package com.tutorialspoint;

interface util {
   public int sum(int a, int b);
}

public class Tester implements util {
   public static void main(String[] args) {
      Tester tester = new Tester();
      System.out.println(tester.sum(2, 3));
   }

   @Override
   public int sum(int a, int b) {		  
      return a + b;
   }
}

Output

Let us compile and run the above program, this will produce the following result −

5

In the above example, we can see that implementing class has to implement the method as it is implement the interface.

Default Method in Interface from Java 8

With Java 8, default methods were introduced where we can provide the default implementation of the method and implementing class is not needed to implement the same. This feature was introduced to facilitate lambda expression where existing collection framework can work with newly introduced functional interfaces without implementing all the methods of the interfaces. This helped in avoiding re-writing of collection framework. See the below example:

Example

package com.tutorialspoint;

interface util {
   public default int sum(int a, int b) {
      return a + b;
   }
}

public class Tester implements util {
   public static void main(String[] args) {
      Tester tester = new Tester();
      System.out.println(tester.sum(2, 3));
   }
}

Output

Let us compile and run the above program, this will produce the following result −

5

Private Method in Interface from Java 9

From java 9, interfaces are enhanced further to have private methods. Now Java 9 onwards, we can have private as well as private static methods in interface. This helps in encapsulating the functionality and helps to keep integrity of the method. As private method cannot be inherited, they can be used in public methods of the interface as shown below example:

Example

package com.tutorialspoint;

interface util {
   public default int operate(int a, int b) {
      return sum(a, b);
   }
   private int sum(int a, int b) {
      return a + b;
   } 
}

public class Tester implements util {
   public static void main(String[] args) {
      Tester tester = new Tester();
      System.out.println(tester.operate(2, 3));
   }
}

Output

Let us compile and run the above program, this will produce the following result −

5

Private Static Method in Interface from Java 9

Similarly, we can have private static method which can be called from static and non-static methods. See the example below:

Example

package com.tutorialspoint;

interface util {
   public default int operate(int a, int b) {
      return sum(a, b);
   }
   private static int sum(int a, int b) {
      return a + b;
   } 
}

public class Tester implements util {
   public static void main(String[] args) {
      Tester tester = new Tester();
      System.out.println(tester.operate(2, 3));
   }
}

Output

Let us compile and run the above program, this will produce the following result −

5

A private static method cannot call non-static method in an interface and it is not accessible outside the interface. Even implementing class cannot access the private static method.