Nonlinear.java revision 0:9a66ca7c79fa 
1/*
2 * Copyright 2001-2007 Sun Microsystems, Inc.  All Rights Reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
20 * CA 95054 USA or visit www.sun.com if you need additional information or
21 * have any questions.
22 */
23
24/*
25 * @test
26 * @bug 4607420
27 * @summary A bug in the original JSR14 generics specification
28 *          created a loophole in the type system.
29 *
30 * @compile/fail -source 1.5 Nonlinear.java
31 */
32
33
34public class Nonlinear {
35
36    // This is an example of lack of type safety for
37    // the version of javac from jsr14_adding_generics-1_0-ea
38
39    // It is a variant of the "classic" problem with polymorphic
40    // references in SML, which resulted in the usual array of
41    // fixes: notably value polymorphism.
42
43    // This code compiles, but produces a ClassCastException
44    // when executed, even though there are no explicit casts in
45    // the program.
46
47    public static void main (String [] args) {
48        Integer x = new Integer (5);
49        String y = castit (x);
50        System.out.println (y);
51    }
52
53    static <A,B> A castit (B x) {
54        // This method casts any type to any other type.
55        // Oh dear.  This shouldn't type check, but does
56        // because build () returns a type Ref<*>
57        // which is a subtype of RWRef<A,B>.
58        final RWRef<A,B> r = build ();
59        r.set (x);
60        return r.get ();
61    }
62
63    static <A> Ref<A> build () {
64        return new Ref<A> ();
65    }
66
67    // Another way of doing this is a variant of the crackit
68    // example discussed in the draft specification.
69    //
70    // The original duplicate was:
71    //
72    // static <A> Pair <A,A> duplicate (A x) {
73    //     return new Pair<A,A> (x,x);
74    // }
75    //
76    // which breaks the requirement that a type variable
77    // instantiated by * only occurs once in the result type.
78    //
79    // However, we can achieve the same result with a different
80    // type for duplicate, which uses its type variables linearly
81    // in the result:
82
83    static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
84        return new Pair<Ref<A>,B> (x,x);
85    }
86
87    // the cheat here is that A and B are used linearly in the result
88    // type, but not in the polymorphic bounds.
89
90    // We can use that to give an alternative implementation of
91    // castit.
92
93    static <A,B> A castit2 (B x) {
94        Pair <Ref<A>, Ref<B>> p = duplicate (build ());
95        p.snd.set (x);
96        return p.fst.get ();
97    }
98
99
100}
101
102interface RWRef<A,B> {
103
104    public A get ();
105    public void set (B x);
106
107}
108
109class Ref<A> implements RWRef <A,A> {
110
111    A contents;
112
113    public void set (A x) { contents = x; }
114    public A get () { return contents; }
115
116}
117
118class Pair<A,B> {
119
120    final A fst;
121    final B snd;
122
123    Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
124
125}
126