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 */ 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 Nonlinear.java 31 */ 32 33 34 public 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 main(String [] args)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 castit(B x)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 build()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 duplicate(B x)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 castit2(B x)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 } 101 102 interface RWRef<A,B> { 103 get()104 public A get (); set(B x)105 public void set (B x); 106 107 } 108 109 class Ref<A> implements RWRef <A,A> { 110 111 A contents; 112 set(A x)113 public void set (A x) { contents = x; } get()114 public A get () { return contents; } 115 116 } 117 118 class Pair<A,B> { 119 120 final A fst; 121 final B snd; 122 Pair(A fst, B snd)123 Pair (A fst, B snd) { this.fst = fst; this.snd = snd; } 124 125 } 126