Case expressions must be constant expressions for

问题:

I have a final class Ring defined as:

final class Ring {
    public static final int OUT = 3;
    public static final int MID = 2;
    public static final int IN  = 1;
}

I also have a public class MorrisBoard with the following code:

public class MorrisBoard {
    public static final Ring RING = new Ring();

    private boolean checkMillBy(int ring, int x, int y) {
    switch(ring) {
    case MorrisBoard.RING.OUT:
        //...
    case MorrisBoard.RING.MID: //etc.
        //...   
    }
    return false;
}

MorrisBoard.RING.OUT references a variable which is immutable for the lifetime of the program. All values are final.

However, I still get the following error: case expressions must be constant expressions. I'm confused by this - MorrisBoard.RING.OUT is a constant expression.

What is going on here?

回答1:

Replace

 case MorrisBoard.RING.OUT:

with

 case Ring.OUT:

So this will really be a constant as in "determined at compilation".

The specification precises that a "SwitchLabel" must be

• case followed by a constant expression
• case followed by the name of an enum value
• or default

What is considered a valid constant expression is described here in the specification. It's fairly limited.

回答2:

Simple solution for this problem is : Click on the switch and then press CTL+1, It will change your switch to if-else block statement, and will resolve your problem