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SF/JavaOne, Day 3, JavaGenericsWhaaaa?!

  • Article

I'm at the Java Collections talk and they're bringing up the issue of
type safety with generics in java 1.5.  The speaker just told me
somethign that sent a shiver up my spine.  Let's say you compiled
the following code in java1.4:

class Old {

    static void addIntegerToList(List list) {

        list.Add(new Integer(42));

    }

}

then in java1.5 you write and compile the following code:

class New {

    static void Foo() {

        List<String> strings = new ArrayList<String>();

        Old.addIntegerToList(strings);

        Srting firstString = strings.getAt(0);

    }

}

Then the above code compiles fine without a single warning!  Ack!

Double-ack!

Ack*Ack*Ack!

The compiler will make you think that everything is typesafe and you
won't know that anything is wrong with your type system until it blows
up at runtime.

Comments

  • Anonymous
    June 28, 2005
    So does this apply to .NET 2, too?
  • Anonymous
    June 28, 2005
    Uwe: No, it does not. C# will not let this compile at all. This is a consequence of the fact that in java generics are forgotten (or "erased") so the compiler can't tell if your original code was:

    class Old {
    ....static void addIntegerToList(List list) {
    ........//stuff
    ....}
    }

    or if it was

    class Old {
    ....static void addIntegerToList(List<String> list) {
    ........//stuff
    ....}
    }

    the compiled form for both is identical. So it can only presume that things were performed in a typesafe manner. Which, of course, might be totally wrong.

    In C#/.Net this generic information is not lost. It is preserved in metadata so you can't ever have this kind of problem occurring.
  • Anonymous
    June 28, 2005
    Uh, duh. This is why I've pointed out in numerous places that Java doesn't have real generics....
  • Anonymous
    June 29, 2005
    The comment has been removed
  • Anonymous
    June 30, 2005
    The comment has been removed
  • Anonymous
    July 01, 2005
    The comment has been removed
  • Anonymous
    October 13, 2005
    The comment has been removed