Generics parameter question

Question

I'm trying to create a binary tree that implements Map, or extends AbstractMap.package binarytree;
import java.util.Collection;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;
public abstract class MapTest<K, D> implements Map<K, D> {
     public Set<Entry<K, D>> entrySet() {
          // TODO Auto-generated method stub
          return null;
     public void putAll(Map<? extends K, ? extends D> m) {
          Set<Entry<? extends K, ? extends D>> s = m.entrySet();
          // Error on the above line.
                //Type mismatch: cannot convert from Set<Map.Entry<capture#1-of ? extends K,capture#2-of ? extends D>> to Set<Map.Entry<? extends K,? extends D>>     
}I'm using Eclipse. It wants me to change the line to :
Set<?> s = m.entrySet();I don't understand. EntrySet() is supposed to return Set<Entry<K, D>>, where K, D are the type parameters of the Map. The type of my Map parameter, m, is Map<? extends K, ? extends D>. Shouldn't m.entrySet() return Set<Entry<? extends K, ? extends D>>?
Why is does Eclipse want me to define the return value as a Set<?>?

Answer 1

Let me try an answer. First of all, generics in Java are not covariant, meaning that even though Integer is a subtype of Number, List<Integer> is not a subtype of List<Number>.
While someone using putAll could pass a Map<Apple,Pears> one day, here and now the compiler assigns the artificial type names capture#1 and capture#2. While these are subtypes of K and D respectively, Entry<capture#1,capture#2> is not a subtype of Entry<K,D> according to the rule above. Consequently the assignment between the two is illegal. Wrapping Set around it makes it even worse.
One mistake made easily goes as follows: Set<A extends Wine> is a set that in the future someone will use to store all kinds of Wine derivatives. THIS IS WRONG! The set will be used to store all kinds of A derivatives.
Consequently, Entry<capture#1,capture#2> will, in the future be some very specific type. And this type is certainly not compatible with Entry<K,D>. This is why the closest possible variable definition proposed by Eclipse is Set<?>.
A lot of in depth information about this stuff is provided by [Angelika Langer|http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html]. While trying to answer your question, I also put together this.
Harald.
Edited by: pifpafpuf on Dec 6, 2008 7:05 PM
Edited by: pifpafpuf on Dec 6, 2008 7:07 PM