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
11public 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
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
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
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
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
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
79interface RWRef<A,B> {
80
81    public A get ();
82    public void set (B x);
83
84}
85
86class Ref<A> implements RWRef <A,A> {
87
88    A contents;
89
90    public void set (A x) { contents = x; }
91    public A get () { return contents; }
92
93}
94
95class Pair<A,B> {
96
97    final A fst;
98    final B snd;
99
100    Pair (A fst, B snd) { this.fst = fst; this.snd = snd; }
101
102}
103