Function to return a list of sub-class elements

Question

Hi all
I'm trying to write a function using templates which takes a list and returns a sub list of a given type.something like shown below:
static <E, T> List<E> get_specific_elements(Collection<T> elements) { //assumes E extends T
          List<E> specific_elements= new ArrayList<E>();
          for (T element : elements)
line 4:               if (element instanceof E)
                    specific_elements.add((E)element);
          return specific_elements;
     }but it gives me at line 4
Cannot perform instanceof check against type parameter E. Use instead its erasure Object
      since generic type information will be erased at runtimeso is there a way to fix it or any other alternative to make it work.
thanks
Sree
Edited by: sreecs on Nov 5, 2008 10:03 PM
Edited by: sreecs on Nov 5, 2008 10:04 PM

Answer 1

In your code, there is no way for the runtime to know what E is. The solution is to tell it.
static <E extends T, T> List<E> get_specific_elements(Class<E> subtype, Collection<T> elements) {
    List<E> specificElements= new ArrayList<E>();
    for (T element : elements) {
        if (subtype.isInstance(element)) {
            specificElements.add(subtype.cast(element));
    return specificElements;
}Bruce