Overriding Constants in Java

Question

I have two classes that extend the same abstract class. They both need the same constant, but with different values. How can I do this? Some example code to show what I want to do.

abstract class A {
   public static int CONST;
}

public class B extends A {
   public static int CONST = 1;
}

public class C extends A {
   public static int CONST = 2;
}

public static void main(String[] args){
    A a = new B();
    System.out.println(a.CONST); // should print 1
}

The above code does not compile because CONST is not initialized int class A. How can I make it work? The value of CONST should be 1 for all instances of B, and 2 for all instances of C, and 1 or 2 for all instances of A. Is there any way to use statics for this?

score 25 · Accepted Answer · edited Dec 11 '11 at 21:19

25

You can't do that.

You can do this, however:

abstract class A {
   public abstract int getConst();
}

public class B extends A {
   @Override
   public int getConst() { return 1; }
}

public class C extends A {
   @Override
   public int getConst() { return 2; }
}

public static void main(String[] args){
    A a = new B();
    System.out.println(a.getConst());
}

edited Dec 11 '11 at 21:19

mre

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answered Dec 11 '11 at 21:13

Oliver Charlesworth

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Thank you! I just wish that I could use static, because the values do not differ from one instance of B to another. – Fractaly Dec 11 '11 at 21:15
@Fractaly: But they do vary from instance of `A` to another. – Oliver Charlesworth Dec 11 '11 at 21:16
1

shouldn't getConst() in A be abstract? – Fractaly Dec 11 '11 at 21:18
@Fractaly, I fixed it. I was thinking the same exact thing. :D – mre Dec 11 '11 at 21:19
@Fractly, Крысa: Yes, a typo on my part. – Oliver Charlesworth Dec 11 '11 at 21:22
1

@Fractaly: I don't know of a more concise way of doing this. – Oliver Charlesworth Dec 11 '11 at 22:00

score 6 · Answer 2 · answered Dec 11 '11 at 21:14

6

If a constant has a variable value, it's not a constant anymore. Static fields and methods are not polymorphic. You need to use a public method to do what you want.

answered Dec 11 '11 at 21:14

JB Nizet

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Overriding Constants in Java

2 Answers2