/*

 * @bug 4607420

 * @summary A bug in the original JSR14 generics specification

 *          created a loophole in the type system.

 *

 */

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; }

}