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 /*

  * Copyright 2001-2007 Sun Microsystems, Inc.  All Rights Reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

  * CA 95054 USA or visit www.sun.com if you need additional information or

  * have any questions.

  */

 /*

  * @test

  * @bug 4607420

  * @summary A bug in the original JSR14 generics specification

  *          created a loophole in the type system.

  *

  * @compile/fail -source 1.5 Nonlinear.java

  */

 public class Nonlinear {

     // This is an example of lack of type safety for

     // the version of javac from jsr14_adding_generics-1_0-ea

     // It is a variant of the "classic" problem with polymorphic

     // references in SML, which resulted in the usual array of

     // fixes: notably value polymorphism.

     // This code compiles, but produces a ClassCastException

     // when executed, even though there are no explicit casts in

     // the program.

     public static void main (String [] args) {

         Integer x = new Integer (5);

         String y = castit (x);

         System.out.println (y);

     }

     static <A,B> A castit (B x) {

         // This method casts any type to any other type.

         // Oh dear.  This shouldn't type check, but does

         // because build () returns a type Ref<*>

         // which is a subtype of RWRef<A,B>.

         final RWRef<A,B> r = build ();

         r.set (x);

         return r.get ();

     }

     static <A> Ref<A> build () {

         return new Ref<A> ();

     }

     // Another way of doing this is a variant of the crackit

     // example discussed in the draft specification.

     //

     // The original duplicate was:

     //

     // static <A> Pair <A,A> duplicate (A x) {

     //     return new Pair<A,A> (x,x);

     // }

     //

     // which breaks the requirement that a type variable

     // instantiated by * only occurs once in the result type.

     //

     // However, we can achieve the same result with a different

     // type for duplicate, which uses its type variables linearly

     // in the result:

     static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {

         return new Pair<Ref<A>,B> (x,x);

     }

     // the cheat here is that A and B are used linearly in the result

     // type, but not in the polymorphic bounds.

     // We can use that to give an alternative implementation of

     // castit.

     static <A,B> A castit2 (B x) {

         Pair <Ref<A>, Ref<B>> p = duplicate (build ());

         p.snd.set (x);

         return p.fst.get ();

     }

 }

 interface RWRef<A,B> {

     public A get ();

     public void set (B x);

 }

 class Ref<A> implements RWRef <A,A> {

     A contents;

     public void set (A x) { contents = x; }

     public A get () { return contents; }

 }

 class Pair<A,B> {

     final A fst;

     final B snd;

     Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }

 }