Re: Why Generics?

>>>>> "Chris" == Chris Uppal <chris.uppal@xxxxxxxxxxxxxxxxxxxxxxxxxxx> writes:

Chris> Phillip Lord wrote:

Chris> It was always possible, the new enums don't generate code
Chris> that couldn't be expressed in "normal" Java. Supporting
Chris> switch would require a fairly unpleasant amount of wordy
Chris> duplication, though. You'd have to have a public static
Chris> final int field corresponding the the 'ordinal' of each enum
Chris> object, which is obviously error prone unless the compiler
Chris> does it for you.
>>
>> I think, actually, you are wrong and it doesn't work. The problem
>> is that with switch case, the value of the cases must be a int or
>> char definable at compile time.

Chris> No, there's no problem except wordy, error-prone, repetition.

Chris> Suppose that we want an enumeration of the three primary
Chris> colours (this probably wouldn't compile but you'll be able to
Chris> see what I mean, I hope):

Chris> public final class Colour {
Chris> public static final int
Chris> __RED_ORDINAL = 0, __GREEN_ORDINAL = 1,
Chris> __BLUE_ORDINAL = 2;

Chris> and then a case statement can be written like:

Chris> public boolean isPutrid(Colour colour) {
Chris> switch (colour.ordinal()) { case
Chris> Colour.__RED_ORDINAL: case Colour.__BLUE_ORDINAL:
Chris> return false;
Chris> case Colour.__GREEN_ORDINAL:
Chris> return true;
Chris> default: // WTF ??
Chris> return false;
Chris> }
Chris> }

Chris> Which is almost exactly what the Java compiler does at the
Chris> bytecode level. Of course it would be a pain to maintain and
Chris> use (and not allow people to /ab/use) the __XXX_ORDINAL
Chris> numbers in parallel to the "real" enumeration objects --
Chris> that's what I meant when I called it wordy and error-prone,
Chris> but it isn't at all impossible.

Ah, okay, that would work.

The error prone bit is a serious problem.


Chris> Note that the implementation of switch is a /real/ switch (a
Chris> tableswitch bytecode instruction) which can be implemented as
Chris> a pure jump table. There is no cascade of if-thens, nor is
Chris> there any hashing going on.

Perhaps I have made a poor use of the word "hash".

I was sure that they implemented things with if-thens. Perhaps this is
what happened in an early version of the spec when I read it.

Chris> Actually, the bytecode generated by javac uses one more level
Chris> of indirection. The ordinal number is used to lookup an
Chris> integer value in a synthetic int[] array, and then the value
Chris> from the array is used in the generated switch statement. I'm
Chris> not sure why it does that. Possibly it's to allow switch
Chris> statements to continue to work if the enum definition is
Chris> changed after it has been compiled, though I don't really see
Chris> how that would work..


Hmm. That is strange.

Thanks for the information. Always good to learn more!

Phil
.

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