What is a static interface in java?

Static inner interface and inner interface is the same, all access rules are the same as with inner static class. So inner interface can be accessible only if you have access to its parent class/interface. In case below you will have access to interface B only from package of interface A, because A has default access modifier. BTW: interface B could be static or not.

 interface A {
    void testA();
    public interface B {
        void testB();
    }
 } 

I'm curious about the case when it's not an inner interface.

The static modifier is only allowed on a nested classes or interfaces. In your example Entry is nested inside the Map interface.

For interfaces, the static modifier is actually optional. The distinction makes no sense for interfaces since they contain no code that could access the outer this anyway.