Why interface variables need initialization?

Question

I want to create an interface that will force all classes that implement it to define a static final integer variable:

public interface FooInterface {
    static final int bar;
}

But the compiler says "Variable 'bar' might not have been initialized" . Why do I have to give it a value in the interface? I want every implementation to define its own value, so it seems illogical to me that I have to put there some arbitrary number that will never be used.

Answer 1

You can't do that with an interface. All variables in an interface are implicitly public final static .

You could define int getBar(); in the interface though, then all the implementing classes would need to return a value.

It would then be your responsibility to make sure that implementors are well behaved, but you can't prevent an implementation from returning different values, eg

public class Foo implements Bar {
    public int getBar() {
        return (int) System.currentTimeMillis();
    }
}

Answer 2

You're thinking about this from the wrong angle.

A static final cannot be overriden in an implementing class.

You probably want to do it like this:

public interface FooInterface {
    int getBar();
}

Answer 3

You can't do that. An interface can only force the classes that implement it to implement methods.

A static variable defined in the interface belongs to the interface. It doesn't force implementing classes to declare the same variable.

Answer 4

Every variables in an interface is static and final. final variables must be initialized on the first line or in the constructor. Because an interface doesn't have a constructor you must initialize final variable on the first line.

Answer 5

因为它们是最终的，所以最终变量需要初始化。

Answer 6

In a normal class, we have the option to define it like that and initialize its value in static initializer block:

public class FooClass {
    static final int bar;

    static {
        bar = 5;
    }
}

In case of an interface, static initialization block is not allowed that's why Java requires it to be set to a value at declaration.