jdk8-mips64-public/langtools: test/tools/javac/generics/Nonlinear.java@9a66ca7c79fa (annotated)

test/tools/javac/generics/Nonlinear.java@9a66ca7c79fa (annotated)

test/tools/javac/generics/Nonlinear.java

Sat, 01 Dec 2007 00:00:00 +0000

author: duke
date: Sat, 01 Dec 2007 00:00:00 +0000
changeset 1: 9a66ca7c79fa
child 289: 84061bd68019
permissions: -rw-r--r--

Initial load

 /*
  * 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; }
 }

Mercurial > jdk8-mips64-public > langtools / annotate

test/tools/javac/generics/Nonlinear.java@9a66ca7c79fa (annotated)

test/tools/javac/generics/Nonlinear.java