PROPOSAL: Simplified Varargs Method Invocation (original) (raw)

Rémi Forax forax at univ-mlv.fr
Fri Mar 6 00:28:38 PST 2009

Reinier Zwitserloot a écrit :

Heh, that was the next proposal I was going to write.

(Apologies for a long and technically complicated post. I've rewritten it several times in an attempt to get to the point faster, but this is the best I can do). It really pains me to rain on your parade. I was doing some experimenting and found an admittedly very unlikely scenario that isn't backwards compatible. Then I realized, that this is actually (arguably) a bug, or at least a severe misfeature in javac right now! Therefore, this proposal should fix this problem as well. Of all proposals so far, I rate this one the highest, because it causes so much confusion and is closer to a bug in javac than a language feature, so I would be very happy if this proposal can be fully ironed out. Complete code showing the problem - copy and paste into Main.java, compile, and run: ------ import java.util.*; class Foo { public void foo(T... t) {} } class Bar extends Foo { //de-varargsing? Why is this legal? Because you want to be able to replace an array with a varargs in library without creating errors in code that use that library. public void foo(String[] t) { System.out.println(t.getClass().getComponentType()); } } public class Main { public static void main(String[] args) { Foo f = new Bar(); List l = Arrays.asList("a", "b", "c"); bar(f, l); } public static void bar(Foo f, List l) { f.foo(l.get(0), l.get(1), l.get(2)); } }

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The result is an error: Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; Right now, you get the usual varargs warning, but nothing else at compile time. At runtime, you get the above error. Warnings in Java are not like in C, it means something will go wrong :)

the call:

f.foo(l.get(0), l.get(1), l.get(2));

generate a warning because you are creating an arry of parameterized type (a Foo[]) which are "inherently unsafe" to quote gilad bracha.

The real bug here is the ability for any subclass to de-varargs a parameter. No

[...]

See:

Angelika Langer's Java Generics FAQ, "Why does the compiler sometimes issue an unchecked warning when I invoke a 'varargs' method?" ( http://tinyurl.com/8w2dk) Rémi

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