1 /*
2  * @test /nodynamiccopyright/
3  * @bug 4607420
4  * @summary A bug in the original JSR14 generics specification
5  *          created a loophole in the type system.
6  *
7  * @compile/fail/ref=Nonlinear.out -XDrawDiagnostics  Nonlinear.java
8  */
9 
10 
11 public class Nonlinear {
12 
13     // This is an example of lack of type safety for
14     // the version of javac from jsr14_adding_generics-1_0-ea
15 
16     // It is a variant of the "classic" problem with polymorphic
17     // references in SML, which resulted in the usual array of
18     // fixes: notably value polymorphism.
19 
20     // This code compiles, but produces a ClassCastException
21     // when executed, even though there are no explicit casts in
22     // the program.
23 
main(String [] args)24     public static void main (String [] args) {
25         Integer x = Integer.valueOf(5);
26         String y = castit (x);
27         System.out.println (y);
28     }
29 
castit(B x)30     static <A,B> A castit (B x) {
31         // This method casts any type to any other type.
32         // Oh dear.  This shouldn't type check, but does
33         // because build () returns a type Ref<*>
34         // which is a subtype of RWRef<A,B>.
35         final RWRef<A,B> r = build ();
36         r.set (x);
37         return r.get ();
38     }
39 
build()40     static <A> Ref<A> build () {
41         return new Ref<A> ();
42     }
43 
44     // Another way of doing this is a variant of the crackit
45     // example discussed in the draft specification.
46     //
47     // The original duplicate was:
48     //
49     // static <A> Pair <A,A> duplicate (A x) {
50     //     return new Pair<A,A> (x,x);
51     // }
52     //
53     // which breaks the requirement that a type variable
54     // instantiated by * only occurs once in the result type.
55     //
56     // However, we can achieve the same result with a different
57     // type for duplicate, which uses its type variables linearly
58     // in the result:
59 
duplicate(B x)60     static <A,B extends Ref<A>> Pair<Ref<A>,B> duplicate (B x) {
61         return new Pair<Ref<A>,B> (x,x);
62     }
63 
64     // the cheat here is that A and B are used linearly in the result
65     // type, but not in the polymorphic bounds.
66 
67     // We can use that to give an alternative implementation of
68     // castit.
69 
castit2(B x)70     static <A,B> A castit2 (B x) {
71         Pair <Ref<A>, Ref<B>> p = duplicate (build ());
72         p.snd.set (x);
73         return p.fst.get ();
74     }
75 
76 
77 }
78 
79 interface RWRef<A,B> {
80 
get()81     public A get ();
set(B x)82     public void set (B x);
83 
84 }
85 
86 class Ref<A> implements RWRef <A,A> {
87 
88     A contents;
89 
set(A x)90     public void set (A x) { contents = x; }
get()91     public A get () { return contents; }
92 
93 }
94 
95 class Pair<A,B> {
96 
97     final A fst;
98     final B snd;
99 
Pair(A fst, B snd)100     Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
101 
102 }
103