Fri, 28 Feb 2014 11:12:31 -0800
Merge
1 /*
2 * Copyright (c) 2001, 2007, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
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 Nonlinear.java
31 */
34 public class Nonlinear {
36 // This is an example of lack of type safety for
37 // the version of javac from jsr14_adding_generics-1_0-ea
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.
43 // This code compiles, but produces a ClassCastException
44 // when executed, even though there are no explicit casts in
45 // the program.
47 public static void main (String [] args) {
48 Integer x = new Integer (5);
49 String y = castit (x);
50 System.out.println (y);
51 }
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 }
63 static <A> Ref<A> build () {
64 return new Ref<A> ();
65 }
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:
83 static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
84 return new Pair<Ref<A>,B> (x,x);
85 }
87 // the cheat here is that A and B are used linearly in the result
88 // type, but not in the polymorphic bounds.
90 // We can use that to give an alternative implementation of
91 // castit.
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 }
100 }
102 interface RWRef<A,B> {
104 public A get ();
105 public void set (B x);
107 }
109 class Ref<A> implements RWRef <A,A> {
111 A contents;
113 public void set (A x) { contents = x; }
114 public A get () { return contents; }
116 }
118 class Pair<A,B> {
120 final A fst;
121 final B snd;
123 Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
125 }