Generics with multiple interfaces

Question

I'm trying to figure out why the following is not valid:
public class MediaService<L extends Deque<MediaFile> & List<MediaFile>> {
        L x = new LinkedList<MediaFile>();
}Gives:
Type mismatch: cannot convert from LinkedList<MediaFile> to LSeeing as LinkedList implements both these interfaces, I would expect this to be legal. What else is the use of the & operator in Generics?

Answer 1

lhunath wrote:
Basically:
public static <T extends Deque<MediaFile> & List<MediaFile>> T newMediaList() {
return new LinkedList<MediaFile>();
It is possible to do this using covariant return types and a generic abstract factory, like this:
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
public class GT20 {
   public static void main(String[] args) {
      DequeListFactory<String, ?> factory = new LinkedListFactory<String>();
      Deque<String> d = factory.create();
      List<String> l = factory.create();
   private interface DequeListFactory<T, L extends Deque<T> & List<T>> {
      L create();
   private static class LinkedListFactory<T> implements DequeListFactory<T, LinkedList<T>> {
      @Override
      public LinkedList<T> create() {
         return new LinkedList<T>();
}However, I would seriously question the use of such a factory. If you really have a need for a union of interfaces (I would suggest a design that requires/allows users to use your objects as Lists and Deques, something is seriously wrong), I would create such a union and use that in your contract. Java doesn't allow anonymous unions as you're trying to do, and using generics is just a hack. Here's what I might consider:
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
public class GT21 {
   private interface ListDeque<T> extends List<T>, Deque<T> {}
   private interface MyInterface<T> {
      ListDeque<T> getListDeque();
   private static class MyClass<T> implements MyInterface<T>{
      @Override
      public ListDeque<T> getListDeque() {
         return new MyLinkedList<T>();
   private static class MyLinkedList<T> extends LinkedList<T>
      implements ListDeque<T> {
}