final variable case in switch statement

Question

        final int a = 1;
        final int b;
        b = 2;
        final int x = 0;

        switch (x) {
            case a:break;     // ok
            case b:break;     // compiler error: Constant expression required

        }
        /* COMPILER RESULT:
                constant expression required
                case b:break;
                     ^
                1 error
        */

Why am I getting this sort of error? If I would have done final int b = 2, everything works.

Answer 1

The case in the switch statements should be constants at compile time. The command

final int b=2

assigns the value of 2 to b, right at the compile time. But the following command assigns the value of 2 to b at Runtime.

final int b;
b = 2;

Thus, the compiler complains, when it can't find a constant in one of the cases of the switch statement.

Answer 2

b may not have been initialized and it is possible to be assigned multiple values. In your example it is obviously initialized, but probably the compiler doesn't get to know that (and it can't). Imagine:

final int b;
if (something) {
   b = 1;
} else {
   b = 2;
}

The compiler needs a constant in the switch, but the value of b depends on some external variable.