Why doesn't Java allow private members in interface?

Question

Why doesn\'t Java allow private members in interface? Is there any particular reason?

Answer 1

Java allows private methods in an interface in Java 9. The default methods were introduced in Java 8. It is possible that multiple default methods want to share some code, then this code can be moved to a private method without exposing it to outer world. This bug has been fixed and starting in JDK 9 build 54, compiler support for private interface methods have been resurrected.

public interface IData{
   default void processData(int data) {
      validate(data);
      // do some work with it
   }
   default void consumeData(int data) {
      validate(data);
      // do some work with it
   }
   private void validate(int data) {
     // validate data
   }
}