Private Interfaces

Question

How can we use the methods of a private interface in our code?

Abstract classes are something which cannot be instantiated. So, if we need to use methods of abstract class, we can inherit them and use their methods.

But, when we talk about interfaces, we need to implement them to use their methods.

Answer 1

The private keyword means "anyone in the same class":

public class Foo {

   private interface X {...}
   private class X1 implements X {...}
}

This means all classes declared inside of Foo can use the interface Foo.X.

A common use case for this is the command pattern where Foo accepts, say, strings and converts them into internal command objects which all implement the same interface.

If you add a second class Bar to the file Foo.java, then it can't see Foo.X.