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/*
* Copyright (c) 2001, 2007, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* @test
* @bug 4607420
* @summary A bug in the original JSR14 generics specification
* created a loophole in the type system.
*
* @compile/fail Nonlinear.java
*/
public class Nonlinear {
// This is an example of lack of type safety for
// the version of javac from jsr14_adding_generics-1_0-ea
// It is a variant of the "classic" problem with polymorphic
// references in SML, which resulted in the usual array of
// fixes: notably value polymorphism.
// This code compiles, but produces a ClassCastException
// when executed, even though there are no explicit casts in
// the program.
public static void main (String [] args) {
Integer x = new Integer (5);
String y = castit (x);
System.out.println (y);
}
static <A,B> A castit (B x) {
// This method casts any type to any other type.
// Oh dear. This shouldn't type check, but does
// because build () returns a type Ref<*>
// which is a subtype of RWRef<A,B>.
final RWRef<A,B> r = build ();
r.set (x);
return r.get ();
}
static <A> Ref<A> build () {
return new Ref<A> ();
}
// Another way of doing this is a variant of the crackit
// example discussed in the draft specification.
//
// The original duplicate was:
//
// static <A> Pair <A,A> duplicate (A x) {
// return new Pair<A,A> (x,x);
// }
//
// which breaks the requirement that a type variable
// instantiated by * only occurs once in the result type.
//
// However, we can achieve the same result with a different
// type for duplicate, which uses its type variables linearly
// in the result:
static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
return new Pair<Ref<A>,B> (x,x);
}
// the cheat here is that A and B are used linearly in the result
// type, but not in the polymorphic bounds.
// We can use that to give an alternative implementation of
// castit.
static <A,B> A castit2 (B x) {
Pair <Ref<A>, Ref<B>> p = duplicate (build ());
p.snd.set (x);
return p.fst.get ();
}
}
interface RWRef<A,B> {
public A get ();
public void set (B x);
}
class Ref<A> implements RWRef <A,A> {
A contents;
public void set (A x) { contents = x; }
public A get () { return contents; }
}
class Pair<A,B> {
final A fst;
final B snd;
Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
}
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