103 lines
2.5 KiB
Java
103 lines
2.5 KiB
Java
/*
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* @test /nodynamiccopyright/
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* @bug 4607420
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* @summary A bug in the original JSR14 generics specification
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* created a loophole in the type system.
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*
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* @compile/fail/ref=Nonlinear.out -XDrawDiagnostics Nonlinear.java
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*/
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public class Nonlinear {
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// This is an example of lack of type safety for
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// the version of javac from jsr14_adding_generics-1_0-ea
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// It is a variant of the "classic" problem with polymorphic
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// references in SML, which resulted in the usual array of
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// fixes: notably value polymorphism.
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// This code compiles, but produces a ClassCastException
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// when executed, even though there are no explicit casts in
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// the program.
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public static void meth() {
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Integer x = Integer.valueOf(5);
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String y = castit (x);
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System.out.println (y);
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}
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static <A,B> A castit (B x) {
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// This method casts any type to any other type.
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// Oh dear. This shouldn't type check, but does
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// because build () returns a type Ref<*>
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// which is a subtype of RWRef<A,B>.
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final RWRef<A,B> r = build ();
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r.set (x);
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return r.get ();
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}
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static <A> Ref<A> build () {
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return new Ref<A> ();
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}
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// Another way of doing this is a variant of the crackit
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// example discussed in the draft specification.
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//
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// The original duplicate was:
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//
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// static <A> Pair <A,A> duplicate (A x) {
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// return new Pair<A,A> (x,x);
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// }
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//
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// which breaks the requirement that a type variable
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// instantiated by * only occurs once in the result type.
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//
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// However, we can achieve the same result with a different
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// type for duplicate, which uses its type variables linearly
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// in the result:
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static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
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return new Pair<Ref<A>,B> (x,x);
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}
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// the cheat here is that A and B are used linearly in the result
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// type, but not in the polymorphic bounds.
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// We can use that to give an alternative implementation of
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// castit.
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static <A,B> A castit2 (B x) {
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Pair <Ref<A>, Ref<B>> p = duplicate (build ());
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p.snd.set (x);
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return p.fst.get ();
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}
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}
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interface RWRef<A,B> {
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public A get ();
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public void set (B x);
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}
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class Ref<A> implements RWRef <A,A> {
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A contents;
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public void set (A x) { contents = x; }
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public A get () { return contents; }
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}
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class Pair<A,B> {
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final A fst;
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final B snd;
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Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
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}
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