aoqi@0: /*
aoqi@0:  * Copyright (c) 2001, 2007, Oracle and/or its affiliates. All rights reserved.
aoqi@0:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
aoqi@0:  *
aoqi@0:  * This code is free software; you can redistribute it and/or modify it
aoqi@0:  * under the terms of the GNU General Public License version 2 only, as
aoqi@0:  * published by the Free Software Foundation.
aoqi@0:  *
aoqi@0:  * This code is distributed in the hope that it will be useful, but WITHOUT
aoqi@0:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
aoqi@0:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
aoqi@0:  * version 2 for more details (a copy is included in the LICENSE file that
aoqi@0:  * accompanied this code).
aoqi@0:  *
aoqi@0:  * You should have received a copy of the GNU General Public License version
aoqi@0:  * 2 along with this work; if not, write to the Free Software Foundation,
aoqi@0:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
aoqi@0:  *
aoqi@0:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
aoqi@0:  * or visit www.oracle.com if you need additional information or have any
aoqi@0:  * questions.
aoqi@0:  */
aoqi@0: 
aoqi@0: /*
aoqi@0:  * @test
aoqi@0:  * @bug 4607420
aoqi@0:  * @summary A bug in the original JSR14 generics specification
aoqi@0:  *          created a loophole in the type system.
aoqi@0:  *
aoqi@0:  * @compile/fail  Nonlinear.java
aoqi@0:  */
aoqi@0: 
aoqi@0: 
aoqi@0: public class Nonlinear {
aoqi@0: 
aoqi@0:     // This is an example of lack of type safety for
aoqi@0:     // the version of javac from jsr14_adding_generics-1_0-ea
aoqi@0: 
aoqi@0:     // It is a variant of the "classic" problem with polymorphic
aoqi@0:     // references in SML, which resulted in the usual array of
aoqi@0:     // fixes: notably value polymorphism.
aoqi@0: 
aoqi@0:     // This code compiles, but produces a ClassCastException
aoqi@0:     // when executed, even though there are no explicit casts in
aoqi@0:     // the program.
aoqi@0: 
aoqi@0:     public static void main (String [] args) {
aoqi@0:         Integer x = new Integer (5);
aoqi@0:         String y = castit (x);
aoqi@0:         System.out.println (y);
aoqi@0:     }
aoqi@0: 
aoqi@0:     static <A,B> A castit (B x) {
aoqi@0:         // This method casts any type to any other type.
aoqi@0:         // Oh dear.  This shouldn't type check, but does
aoqi@0:         // because build () returns a type Ref<*>
aoqi@0:         // which is a subtype of RWRef<A,B>.
aoqi@0:         final RWRef<A,B> r = build ();
aoqi@0:         r.set (x);
aoqi@0:         return r.get ();
aoqi@0:     }
aoqi@0: 
aoqi@0:     static <A> Ref<A> build () {
aoqi@0:         return new Ref<A> ();
aoqi@0:     }
aoqi@0: 
aoqi@0:     // Another way of doing this is a variant of the crackit
aoqi@0:     // example discussed in the draft specification.
aoqi@0:     //
aoqi@0:     // The original duplicate was:
aoqi@0:     //
aoqi@0:     // static <A> Pair <A,A> duplicate (A x) {
aoqi@0:     //     return new Pair<A,A> (x,x);
aoqi@0:     // }
aoqi@0:     //
aoqi@0:     // which breaks the requirement that a type variable
aoqi@0:     // instantiated by * only occurs once in the result type.
aoqi@0:     //
aoqi@0:     // However, we can achieve the same result with a different
aoqi@0:     // type for duplicate, which uses its type variables linearly
aoqi@0:     // in the result:
aoqi@0: 
aoqi@0:     static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
aoqi@0:         return new Pair<Ref<A>,B> (x,x);
aoqi@0:     }
aoqi@0: 
aoqi@0:     // the cheat here is that A and B are used linearly in the result
aoqi@0:     // type, but not in the polymorphic bounds.
aoqi@0: 
aoqi@0:     // We can use that to give an alternative implementation of
aoqi@0:     // castit.
aoqi@0: 
aoqi@0:     static <A,B> A castit2 (B x) {
aoqi@0:         Pair <Ref<A>, Ref<B>> p = duplicate (build ());
aoqi@0:         p.snd.set (x);
aoqi@0:         return p.fst.get ();
aoqi@0:     }
aoqi@0: 
aoqi@0: 
aoqi@0: }
aoqi@0: 
aoqi@0: interface RWRef<A,B> {
aoqi@0: 
aoqi@0:     public A get ();
aoqi@0:     public void set (B x);
aoqi@0: 
aoqi@0: }
aoqi@0: 
aoqi@0: class Ref<A> implements RWRef <A,A> {
aoqi@0: 
aoqi@0:     A contents;
aoqi@0: 
aoqi@0:     public void set (A x) { contents = x; }
aoqi@0:     public A get () { return contents; }
aoqi@0: 
aoqi@0: }
aoqi@0: 
aoqi@0: class Pair<A,B> {
aoqi@0: 
aoqi@0:     final A fst;
aoqi@0:     final B snd;
aoqi@0: 
aoqi@0:     Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
aoqi@0: 
aoqi@0: }