duke@1: /*
ohair@554: * Copyright (c) 2001, 2007, Oracle and/or its affiliates. All rights reserved.
duke@1: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
duke@1: *
duke@1: * This code is free software; you can redistribute it and/or modify it
duke@1: * under the terms of the GNU General Public License version 2 only, as
duke@1: * published by the Free Software Foundation.
duke@1: *
duke@1: * This code is distributed in the hope that it will be useful, but WITHOUT
duke@1: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
duke@1: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
duke@1: * version 2 for more details (a copy is included in the LICENSE file that
duke@1: * accompanied this code).
duke@1: *
duke@1: * You should have received a copy of the GNU General Public License version
duke@1: * 2 along with this work; if not, write to the Free Software Foundation,
duke@1: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
duke@1: *
ohair@554: * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
ohair@554: * or visit www.oracle.com if you need additional information or have any
ohair@554: * questions.
duke@1: */
duke@1:
duke@1: /*
duke@1: * @test
duke@1: * @bug 4607420
duke@1: * @summary A bug in the original JSR14 generics specification
duke@1: * created a loophole in the type system.
duke@1: *
darcy@289: * @compile/fail Nonlinear.java
duke@1: */
duke@1:
duke@1:
duke@1: public class Nonlinear {
duke@1:
duke@1: // This is an example of lack of type safety for
duke@1: // the version of javac from jsr14_adding_generics-1_0-ea
duke@1:
duke@1: // It is a variant of the "classic" problem with polymorphic
duke@1: // references in SML, which resulted in the usual array of
duke@1: // fixes: notably value polymorphism.
duke@1:
duke@1: // This code compiles, but produces a ClassCastException
duke@1: // when executed, even though there are no explicit casts in
duke@1: // the program.
duke@1:
duke@1: public static void main (String [] args) {
duke@1: Integer x = new Integer (5);
duke@1: String y = castit (x);
duke@1: System.out.println (y);
duke@1: }
duke@1:
duke@1: static A castit (B x) {
duke@1: // This method casts any type to any other type.
duke@1: // Oh dear. This shouldn't type check, but does
duke@1: // because build () returns a type Ref<*>
duke@1: // which is a subtype of RWRef.
duke@1: final RWRef r = build ();
duke@1: r.set (x);
duke@1: return r.get ();
duke@1: }
duke@1:
duke@1: static Ref build () {
duke@1: return new Ref ();
duke@1: }
duke@1:
duke@1: // Another way of doing this is a variant of the crackit
duke@1: // example discussed in the draft specification.
duke@1: //
duke@1: // The original duplicate was:
duke@1: //
duke@1: // static Pair duplicate (A x) {
duke@1: // return new Pair (x,x);
duke@1: // }
duke@1: //
duke@1: // which breaks the requirement that a type variable
duke@1: // instantiated by * only occurs once in the result type.
duke@1: //
duke@1: // However, we can achieve the same result with a different
duke@1: // type for duplicate, which uses its type variables linearly
duke@1: // in the result:
duke@1:
duke@1: static > Pair[,B> duplicate (B x) {
duke@1: return new Pair][,B> (x,x);
duke@1: }
duke@1:
duke@1: // the cheat here is that A and B are used linearly in the result
duke@1: // type, but not in the polymorphic bounds.
duke@1:
duke@1: // We can use that to give an alternative implementation of
duke@1: // castit.
duke@1:
duke@1: static A castit2 (B x) {
duke@1: Pair ][, Ref> p = duplicate (build ());
duke@1: p.snd.set (x);
duke@1: return p.fst.get ();
duke@1: }
duke@1:
duke@1:
duke@1: }
duke@1:
duke@1: interface RWRef {
duke@1:
duke@1: public A get ();
duke@1: public void set (B x);
duke@1:
duke@1: }
duke@1:
duke@1: class Ref implements RWRef {
duke@1:
duke@1: A contents;
duke@1:
duke@1: public void set (A x) { contents = x; }
duke@1: public A get () { return contents; }
duke@1:
duke@1: }
duke@1:
duke@1: class Pair {
duke@1:
duke@1: final A fst;
duke@1: final B snd;
duke@1:
duke@1: Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
duke@1:
duke@1: }
]