If interfaces are only allowed to be public, how does this work

Question

This is the below code which compiles successfully

 import java.util.*;
    class Test
    {
        protected interface Yes
        {
            void show();
        }
    }
    
    class Testing extends Test implements Test.Yes
    {
        public void show()
        {
            System.out.println("show method of interface");
        }
    }
    
    class A
    {
        public static void main(String[] args)
        {
            Test.Yes obj;
            Testing t = new Testing();
            obj=t;
            obj.show();
        }
    }

Answer 1

Your understanding is incorrect - see the Java Language Specification, section 9.1.1.

The access modifiers protected and private pertain only to member interfaces whose declarations are directly enclosed by a class declaration (§8.5.1).

Answer 2

Interfaces itself can have any visibility that classes can have. For top-level interfaces that's limited to public and package-private, for nested interfaces it can be anything.

Until Java 9, everything inside interfaces had to be public, and everything without an explicit visibility modifier automatically was. Java 9 made it possible to add private methods to interfaces, which can be called by default method implementations.

Note that protected inside interfaces is still not allowed, and package-private cannot be used because members without an explicit public or private modifier are automatically public.