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  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 
  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 
 102 interface RWRef<A,B> {
 103 
 104     public A get ();
 105     public void set (B x);
 106 
 107 }
 108 
 109 class 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 
 118 class 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 }