Accessing static field from Class<A> variable

Question

I have the following situation (I know it doesn't sound real but I simplified it for better understanding):

A class A, with a public static int f; declared
Other classes B, C, D, ... that extend A
A method foo (somewhere else, doesn't matter) with the following signature: int foo(Class< A> classz);

Now I want this method implementation to return the value if the static field f, from the class represented by classz; subclasses of A may have different values. I don't want to use reflection (and neither the new jdk7 invoke.* features). For now, I have this solution that works, but isn't good, since it instanciates an object (which should not be necessary) and it generate a warning by accessing a static field from an instance:

int foo(Class< A> classz) throws [don't matter] {
    return classz.newInstance().f;
}

Any suggestions? Thanks in advance?

PS: Don't know how to type Class< ?> without the space...

Jon Skeet · Accepted Answer · 2011-11-05 10:24:21Z

2

It's not really clear what you're trying to do or why you're passing in a class at all - it looks like you should just need:

int foo() {
    return A.f;
}

On success, that's what your current code will be returning anyway. Your code is equivalent to:

int foo(Class<A> classz) throws ... {
    A ignored = classz.newInstance();
    return A.f;
}

It sounds like you're trying to use polymorphism with static fields - that's simply not going to work. Static members aren't polymorphic, and neither are fields. If you're declaring extra f fields within each subclass, you would have to use reflection to get at them, but I advise you to redesign anyway. If you're not declaring extra f fields within each subclass, then you've only got one field anyway - B.f and C.f will basically resolve to A.f anyway...

edited Nov 5, 2011 at 10:24

answered Nov 5, 2011 at 10:17

Jon Skeet

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3 Comments

Fake Faker Over a year ago

The thing is classes B, C, D can have different values on f. The solution I posted works in the sense that it has this kind of 'polymorphism with static fields'; I mean when you access a static field from an instance it has this 'polymorphism'... (Well, mayble ill try to javap my solution to see how its done in bytecode level)

Jon Skeet Over a year ago

@FakeFaker: No, they can't have different values for f, unless they're declaring their own variables - in which case you're basically shadowing. It's not a good design and you will need to use reflection. And no, it doesn't have this sort of polymorphism. For example: A a = new B(); int x = a.f; will always use A.f, not B.f. It would be useful if you could clarify whether you really have declared f in the subclasses - because if you haven't, you've only got a single variable and I suspect you've fooled yourself into thinking you've got one per subclass.

Fake Faker Over a year ago

Hm...No I didn't declared it in subclasses and I have indeed fooled myself into thinking I have one per subclass :) that was stupid, I'll try to rethink my whole problem.. Thanks

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