Joint union in type parameter variance.

Test.java

class Test {
    interface Foo {
        int doFoo();
    }

    interface Bar {
        int doBar();
    }

    // here we use the union type parameter: Foo & Bar
    static class TypeUnion<T extends Foo & Bar> {
        public int test(final T thingi) {
            return thingi.doFoo() + thingi.doBar();
        }
    }

    static class Thingi implements Foo, Bar {
        public int doFoo() { return 23; }
        public int doBar() { return 42; }
    }

    public static void main(final String args[]) {
        final TypeUnion<Thingi> tu = new TypeUnion<Thingi>();
        final Thingi thingi = new Thingi();

        System.out.println("      foo: " + thingi.doFoo());
        System.out.println("      bar: " + thingi.doBar());
        System.out.println("foo + bar: " + tu.test(thingi));
    }
}

see http://stackoverflow.com/questions/15496/hidden-features-of-java/42686#42686