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Private Methods in Interfaces

In Java 8 we were able to use method implementation in interfaces as default methods. But the private methods were missing in Java 8 when the default method is added.  So now that both default and private methods can exist within an interface. Before we are going to the details we can check the changes came for the interfaces in the previous versions

Java 7

In Java 7 we can provide only two things in interfaces
  • ·         Constant variables
  • ·         Abstract methods
If we need to use the method implementation, then we should go for the abstract class.

Example:
public interface SampleInterface_7 {
     
      String CONSTANT_1 = "CONSTANT_1";
     
      public void method1();

}

Java 8

In Java 8 we can provide following things in interfaces
  • ·         Constant variables
  • ·         Abstract methods
  • ·         Default methods
  • ·         Static methods

Default Methods

We need to use ‘default’ keyword with the signature to use the default method in interface. If we define a default method in interface, it is not mandatory to provide implementation for the default method.  If two interfaces have same default methods and if a class implementing both interfaces, then compiler will not be able to choose which method is to choose. Hence it’s mandatory to provide the implementation for the default method in this scenario.

Static Methods

Static methods are similar to default methods, but cannot override in the implementation classes.

Example:
public interface SampleInterface_8 {
     
      String CONSTANT_1 = "CONSTANT_1";
     
      public void method1();
     
      default void default_method() {
            System.out.println("This is default method");
      }

      public static void method2() {
            System.out.println("This is a public static method");
      }
}

Java 9

In Java 9 an interface can have six kinds of things
  • ·         Constant variables
  • ·         Abstract methods
  • ·         Default methods
  • ·         Static methods
  • ·         Private methods
  • ·         Private Static methods
‘abstract’ keyword cannot use with private methods. Private methods can be used only inside interfaces. And it can be used inside the static and non-static interface methods.
Example :
public interface SampleInterface_9 {
     
String CONSTANT_1 = "CONSTANT_1";
     
      public void method1();
     
      default void default_method() {
            System.out.println("This is default method");
            /**
             * Private method can be called from default method
             */
            method3();
      }

      public static void method2() {
            System.out.println("This is a public static method");
            /**
             * Static method can be called from static method
             */
            method4();
      }    
     
      private void method3() {
            System.out.println("This is private interface method");
      }
     
      private static void method4() {
            System.out.println("This is private static interface method");
      }

}
Sample Code is available at
https://github.com/sudeepcm/Java-9/tree/master/private-interface

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