Why `private static` field is not allowed in Java 8 interface?

Question

When I'm trying to compile the following code

public interface SomeInterface{
    private static Logger logger = Logger.getLogger();

    public default void someMethod(){
        logger.info("someMethod: default implementation");
    }
}

I get an error

Illegal modifier for the interface field SomeInterface.logger; only public, static & final are permitted

When I delete private modifier, code compiles, but I don't want other classes from the package to see this field.

Why Java doesn't allow me to do such thing when it actually does make sense?

Answer 1

In the pre-Java-8 view of the world, interfaces were purely for interface contracts, and private members exist purely for implementation, so this restriction was completely sensible.

In the post-Java-8 world, where interfaces can carry behavior (but not state), it starts to be reasonable to ask whether other features of classes should be applied to interfaces as well. (However, just because something might be "reasonable" doesn't mean it must be supported; there is often more than one reasonable way to construct the world.)

In Java 9, private methods in interfaces will be supported.