Legal casting in generics

Question

I'm migrating from 1.4 to 1.5 and have a question about when casting is legal (and workable) in generics. After reading various literature, it seems that erasure removes type information at run time. Yet casting does seem to work. Here is the source of min() in Collections class -
    public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
        if (comp==null)
            return (T)min((Collection<SelfComparable>) (Collection) coll);
.....Obviously this code works. If erasure removes type info, how do the casts above work?
More generally, when is casting to generic types allowed and succeed in 1.5 and above?
And also I'm curious if the compiler generated any unchecked message on this piece of code (I don't see any suppresswarning tag in the source).
Thanks

Answer 1

Obviously this code works. If erasure removes type info, how do the casts above work?A generic call is replaced with explicit casts by the compiler:
public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
// becomes (generic type is erased)
public static Object min(Collection coll, Comparator comp) {
Number minimum = min(new ArrayList<Number>(), null);
// becomes (explicit cast added)
Number minimum = (Number) min(new ArrayList(), null);
More generally, when is casting to generic types allowed and succeed in 1.5 and above?It is allowed, but it is always an "unchecked" warning. Note that the generic cast is erased, so it can't succeed or fail.
The explicit cast that the compiler adds (in this case when the method is called) can fail if there are unchecked warnings.
Note that if a generic is bounded, the generic cast is erased to the bounds (if <T extends Number> then (T) will be (Number), but if that is an illegal cast you get an compiler error:
// eclipse compiler will also complain Double is final and cannot be extended.
public <T extends Double> T min(String arg, Number arg2) {
  // error: Cannot cast from String to T
  Double error = (T) arg;
  // warning: Type safety: The cast from Number to T is actually checking against the erased type Double
  // aka it might be an Integer
  Double warning = (T) arg2;
}