Get static field from instance

Question

I have a class with multiple subclasses:

class A {

    static int i = 5;
}

class B extends A{

    static int i = 6;
}

class C extends A {

    static int i = 7;
}

I'm trying to write a comparator that takes two A's and compares them based on their values of i. I'm stuck at:

public int compare(A a1, A a2) {

}

Neither a1.i nor a1.class.getField("i").getInt(null); work.

How can I get the value of the static field from the object?

Member variables are not polymorphic in Java (whether static or non-static). The best you could do is indeed use reflection, but there's probably a better way to achieve whatever the overall goal is here. — Oliver Charlesworth, May 26 '14 at 22:01
@AnubianNoob: You'd need to make it non-static, and override it in each derived class. — Oliver Charlesworth, May 26 '14 at 22:03
It will call the `static` method on A, not on the subclasses. — Luiggi Mendoza, May 26 '14 at 22:03
Could I add an abstract method `getI()` and use that? Or make `i` final and not static? — Anubian Noob, May 26 '14 at 22:04
Related: [Why doesn't Java allow overriding of static methods?](http://stackoverflow.com/q/2223386/1065197) — Luiggi Mendoza, May 26 '14 at 22:04
How about defining an Interface HasI with a getI() method and making A, B, and C implement HasI? Your compare will be public int comapre(HasI a, HasI b) — Alexandre Santos, May 26 '14 at 22:06
@AlexandreSantos In that case, I could just add and abstract `getI()` to `A` and have `B` and `C` override that, right? (Also that's worth an answer) — Anubian Noob, May 26 '14 at 22:13

score 2 · Answer 1 · answered May 26 '14 at 22:05

2

a1.i

Because a1 is declared a A, it is equivalent to A.i. The compiler should tell you about that with a warning. Most IDE will do that to and give a little message about what to do about it.

a1.class.getField("i").getInt(null);

Can't work because class is static.

You can use

a1.getClass().getDeclaredField("i").getInt(null);

getClass is an instance method to get the class of an object. getDeclaredField will return all fields, while getField will only return the public ones.

answered May 26 '14 at 22:05

njzk2

38,969
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*The compiler should complain about that* compiler won't complain about it, it will show a warning and sadly we programmers tend to ignore warnings because we only care for errors :) – Luiggi Mendoza May 26 '14 at 22:06
by `complain` I mean `show a warning`. I will edit to make it more explicit. – njzk2 May 26 '14 at 22:08
Compiler complain is usually known as a compiler error. – Luiggi Mendoza May 26 '14 at 22:11

Get static field from instance

1 Answers1