using an array for a switch case statement in java

Question

I'm making a game, and i want the controls to be editable. well, i've got that part down, but they are being read and changed in a .txt file. that is the way i wanted it to work for now. the values are stored as the key value (ie. KeyEvent.VK_W is equal to 83, so the value for the line is 83). I also have it reading the values and saving them to a String array variable in my core class. In my key event class, the one that handles the pushing of the keys, i have it refering to the array to check if a command key was pushed. i'm continuously getting this error: case expressions must be constant expressions when i try it. here is the WRONG code:

switch(key){
  case Integer.parseInt(commands[1]):
      ...
      break;
}

and i get that error. the value of commands[1] is 83. it is the value for "W". here is my declaration of the variable:

for (int i = 0; i < commands.length; i++) {
                commands[i] = io.readSpecificLine(FILES.controlsFileFinalDir,
                        i + 1);
}

and if i have it print out every value, it does work. i've tried making the array final but that didnt work. i've run across the solution before, about 2 years ago, but i cant find it again. does anyone have any ideas on how to fix this? thanks in advance!

JB Nizet · Accepted Answer · 2013-01-01 16:54:52Z

4

As the compiler says, the case expressions must be constant expressions. You can't use an array element as a case expression. You can simply use an if/else if/else clause instead.

answered Jan 1, 2013 at 16:54

JB Nizet

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4 Comments

PulsePanda Over a year ago

i feel like the switch is so much easier and neater though. i was really looking for a way to fix the error, while keeping (hopefully) the array and for sure the switch :\

Russell Zahniser Over a year ago

See this link for a description of what counts as a compile-time constant.

Russell Zahniser Over a year ago

@mamdouh: You can use strings as switch labels in java 7, but they must still be constants. See this link

PulsePanda Over a year ago

i finally just went to the stupid if/else statements. i accepted it because thats the solution i used. thanks!

Ted Hopp · Accepted Answer · 2013-01-01 17:00:35Z

You can't use non-constant expressions in case statements. An alternative approach is to build a map from values to the actions. So instead of this (which doesn't actually make any sense to me):

switch (key) {
case Integer.parseInt(commands[1]):
    // action 1
    break;
// other cases...
default: 
    // default action
}

You can do something like this:

static Map<Integer, Runnable> keyMap = new HashMap<Integer, Runnable>();
static {
    keyMap.put(83, new Runnable() {
        @Override
        public void run() {
            // actions for code 83
        }
    });
    . . .
}

(If it makes more sense, this could also be done on a per-instance basis instead of as a static map.) Then later:

Runnable action = keyMap.get(Integer.parseInt(commands[1]));
if (action != null) {
    action.run();
} else {
    // default action
}

If you need to pass variables to your actions, you can define your own interface instead of using Runnable for the actions.

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