Can we have a private method or private static method in an interface in Java 9?

Yes, we can have private methods or private static methods in an interface in Java 9. We can use these methods to remove the code redundancy. Private methods can be useful or accessible only within that interface only. We can't access or inherit private methods from one interface to another interface or class.

Syntax

interface <interface-name> {
   private static void methodName() {
      // some statements
   }
   private void methodName() {
      // some statements
   }
}

Example

interface Java9Interface {
   public abstract void method1();
   public default void method2() {
      method4();
      method5();
      System.out.println("Inside default method");
   }
   public static void method3() {
      method5();    //  static method inside other static method
      System.out.println("Inside static method");
   }
   private void method4() {    // private method
      System.out.println("Inside private method");
   }
   private static void method5() {    // private static method
      System.out.println("Inside private static method");
   }
}
public class PrivateStaticMethodTest implements Java9Interface {
   @Override
   public void method1() {
       System.out.println("Inside abstract method");
   }
   public static void main(String args[]) {
      Java9Interface instance = new PrivateStaticMethodTest();
      instance.method1();
      instance.method2();
      Java9Interface.method3();
   }
}

Output

Inside abstract method
Inside private method
Inside private static method
Inside default method
Inside private static method
Inside static method