静态方法的受保护修饰符的目的是什么

Question

I found such an usage of protected modifier while searching for a solution for my other question: Ignoring case in strings with unitils ReflectionComparator

In the org.unitils.reflectionassert.ReflectionComparatorFactory class there is a method with the signature:

protected static List<Comparator> getComparatorChain(Set<ReflectionComparatorMode> modes)

But this is only a particular case.

After all we can always extend such any non-final class and "override" it's static protected method with the new public modifier. Say, we have a class A :

public class A {
    protected static void test() {
        // do some stuff
    }
}

and want to use it in another package:

public class UseA {
    private static class MyA extends A {
        public static void test() {
            A.test();
        }
    }

    void useA() {
        // A.test(); compile error, sure
        MyA.test();
    }
}

I concentrate my question on a general situation when some static method was declared as protected . I'm not asking about non-static fields or methods, because in some cases class can have a private constructor or a very complicated constructor with lots special params. But what is the purpose of such "hiding" static methods if entire class isn't final ? Is such usage an OOP mistake or just a very weak "protection"?

Answer 1

But what is the purpose of such "hiding" static methods if entire class isn't final?

A protected static method would allow you to provide "utility" type functionality to derived classes, without exposing them in the public API where they might not make sense on their own.

I don't know the implementation of the getComparatorChain method you reference, but I imagine it's such a method. It would be marked static if it's not tied to any specific instance, and marked protected so as not to be a part of the public API, but also to allow derived classes to re-use the utility method in it's own implementation.

Answer 2

Overriding a static method reference means hiding it's implementation. See: Java Documentation's Overriding and Hiding Methods

Static Methods

If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.

The distinction between hiding a static method and overriding an instance method has important implications:

• The version of the overridden instance method that gets invoked is the one in the subclass.
• The version of the hidden static method that gets invoked depends on whether it is invoked from the superclass or the subclass.