Incompatible generic wildcard captures

Question

in the following snippet:

package test;

import java.util.Collection;
import java.util.Iterator;
import java.util.Map;

public class WildcardsTest

        

Answer 1

Do it like this and it will work:

private final Iterator<? extends
    Map.Entry<K, ? extends Collection<V>>
> iterator;

You can still use the iterator like this:

public void foo(){
    while(iterator.hasNext()){
        Entry<K, ? extends Collection<V>> entry = iterator.next();
        Collection<V> value = entry.getValue();
    }
}

For reference, read the get and put principle (originally from Java Generics and Collections)

Answer 2

The assignment is incorrect, although the types seem to match exactly.

The two ?-wildcards can be bound to two different classes. Put it this way, it is quite obvious that there is a type mismatch:

private Iterator<Map.Entry<K, ArrayList<V>>> iterator;
public WildcardsTest(Map<K, HashSet<V>> map) {
    iterator = map.entrySet().iterator();
}

When you introduce the C, you "force them" to refer to the same class.