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