Java language - Enum Switch

Today I want to look at the switch statement for enums and how they could be better validated.

enum switch

Lets consider a simple enum:

public enum Colour {
  RED, BLUE, GREEN
}

And how we might use it in a switch statement:

Colour col = ...
switch (col) {
  case RED:
    // do stuff with red
    break;
  case BLUE:
    // do stuff with blue
    break;
  case GREEN:
    // do stuff with green
    break;
}

But what happens if we add another colour, BLACK, to the enum?

public enum Colour {
  RED, BLUE, GREEN, BLACK
}

At this point our nice switch statement silently continues without doing anything. Which probably isn't what we want. So, we would need to search our entire application to find switch statements referencing Colour and update them.

So, could we do better? Well, what if we could mark the switch statement as 'must contain all the values of the enum':

Colour col = ...
@CompleteEnumSwitch
switch (col) {
  case RED:
    // do stuff with red
    break;
  case BLUE:
    // do stuff with blue
    break;
  case GREEN:
    // do stuff with green
    break;
}

Now, with the new annotation, this piece of code won't compile (as BLACK isn't handled). We would need to either add a case for BLACK, or a default clause.

Using an annotation here seems appropriate. It doesn't affect the bytecode, but does validate it. In other words, its very similar to @Override.

There is a downside to the annotation approach - its compile time only. If BLACK is added to the enum without recompiling the switch statement, then there will be no error either at compile time or runtime. But this seems a reasonable balance for the feature.

There's another problem though - an annotation can't be added at this point in the code at present. However, JSR 308 is working on this right now, so hopefully this restriction will disappear.

Moving beyond @CompleteEnumSwitch, I could also envisage an @NoFallThroughSwitch which would make falling through not compile in switch statements.

Any thoughts on this concept?

BTW, thanks to Ricky Clarkson for talking about complete switches which triggered me to write this.