duke@1: /*
ohair@554:  * Copyright (c) 2001, 2007, Oracle and/or its affiliates. All rights reserved.
duke@1:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
duke@1:  *
duke@1:  * This code is free software; you can redistribute it and/or modify it
duke@1:  * under the terms of the GNU General Public License version 2 only, as
duke@1:  * published by the Free Software Foundation.
duke@1:  *
duke@1:  * This code is distributed in the hope that it will be useful, but WITHOUT
duke@1:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
duke@1:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
duke@1:  * version 2 for more details (a copy is included in the LICENSE file that
duke@1:  * accompanied this code).
duke@1:  *
duke@1:  * You should have received a copy of the GNU General Public License version
duke@1:  * 2 along with this work; if not, write to the Free Software Foundation,
duke@1:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
duke@1:  *
ohair@554:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
ohair@554:  * or visit www.oracle.com if you need additional information or have any
ohair@554:  * questions.
duke@1:  */
duke@1: 
duke@1: /*
duke@1:  * @test
duke@1:  * @bug 4607420
duke@1:  * @summary A bug in the original JSR14 generics specification
duke@1:  *          created a loophole in the type system.
duke@1:  *
darcy@289:  * @compile/fail  Nonlinear.java
duke@1:  */
duke@1: 
duke@1: 
duke@1: public class Nonlinear {
duke@1: 
duke@1:     // This is an example of lack of type safety for
duke@1:     // the version of javac from jsr14_adding_generics-1_0-ea
duke@1: 
duke@1:     // It is a variant of the "classic" problem with polymorphic
duke@1:     // references in SML, which resulted in the usual array of
duke@1:     // fixes: notably value polymorphism.
duke@1: 
duke@1:     // This code compiles, but produces a ClassCastException
duke@1:     // when executed, even though there are no explicit casts in
duke@1:     // the program.
duke@1: 
duke@1:     public static void main (String [] args) {
duke@1:         Integer x = new Integer (5);
duke@1:         String y = castit (x);
duke@1:         System.out.println (y);
duke@1:     }
duke@1: 
duke@1:     static <A,B> A castit (B x) {
duke@1:         // This method casts any type to any other type.
duke@1:         // Oh dear.  This shouldn't type check, but does
duke@1:         // because build () returns a type Ref<*>
duke@1:         // which is a subtype of RWRef<A,B>.
duke@1:         final RWRef<A,B> r = build ();
duke@1:         r.set (x);
duke@1:         return r.get ();
duke@1:     }
duke@1: 
duke@1:     static <A> Ref<A> build () {
duke@1:         return new Ref<A> ();
duke@1:     }
duke@1: 
duke@1:     // Another way of doing this is a variant of the crackit
duke@1:     // example discussed in the draft specification.
duke@1:     //
duke@1:     // The original duplicate was:
duke@1:     //
duke@1:     // static <A> Pair <A,A> duplicate (A x) {
duke@1:     //     return new Pair<A,A> (x,x);
duke@1:     // }
duke@1:     //
duke@1:     // which breaks the requirement that a type variable
duke@1:     // instantiated by * only occurs once in the result type.
duke@1:     //
duke@1:     // However, we can achieve the same result with a different
duke@1:     // type for duplicate, which uses its type variables linearly
duke@1:     // in the result:
duke@1: 
duke@1:     static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
duke@1:         return new Pair<Ref<A>,B> (x,x);
duke@1:     }
duke@1: 
duke@1:     // the cheat here is that A and B are used linearly in the result
duke@1:     // type, but not in the polymorphic bounds.
duke@1: 
duke@1:     // We can use that to give an alternative implementation of
duke@1:     // castit.
duke@1: 
duke@1:     static <A,B> A castit2 (B x) {
duke@1:         Pair <Ref<A>, Ref<B>> p = duplicate (build ());
duke@1:         p.snd.set (x);
duke@1:         return p.fst.get ();
duke@1:     }
duke@1: 
duke@1: 
duke@1: }
duke@1: 
duke@1: interface RWRef<A,B> {
duke@1: 
duke@1:     public A get ();
duke@1:     public void set (B x);
duke@1: 
duke@1: }
duke@1: 
duke@1: class Ref<A> implements RWRef <A,A> {
duke@1: 
duke@1:     A contents;
duke@1: 
duke@1:     public void set (A x) { contents = x; }
duke@1:     public A get () { return contents; }
duke@1: 
duke@1: }
duke@1: 
duke@1: class Pair<A,B> {
duke@1: 
duke@1:     final A fst;
duke@1:     final B snd;
duke@1: 
duke@1:     Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
duke@1: 
duke@1: }