Question

static <V> void myMethod(Map<?, V> map)
{
 Iterator<Entry<?, V>> it = map.entrySet().iterator();
}

I’m seeing below compilation error:
Type mismatch: cannot convert from Iterator<Map.Entry<capture#5-of ?,V>> to Iterator<Map.Entry<?,V>>

Answer 1

Try

Iterator<? extends Entry<?, V>> it = map.entrySet().iterator();

The reason your attempt doesn’t work is a bit hard to see, particularly because Iterator<T> does not consume any T (i.e. it doesn’t have a method which takes a T as a parameter).

You can’t assign an Iterator<Entry<capture#5-of ?,V>> to an Iterator<Entry<?, V>> for the same reason that you can’t assign an Iterator<Entry<Integer, String>> to an Iterator<Entry<?, V>>. The capture#5 is just a name used to differentiate the specific ? found in your method’s parameter from other distinct wildcard instances. It could just as easily be a concrete type.

The reason this doesn’t work is more clear if instead of Iterator you think of a class like List.

List<Entry<Integer, String>> entries = new ArrayList<>();

//this is a compile error, but assume it is possible
List<Entry<?, String>> wildcardEntries = entries; 

//then since this is already possible
wilcardEntries.add(new Entry<String, String>("a", "b"));
Entry<Integer, String> entry1 = entries.get(0);

//this would result in a type error (ClassCastException)
Integer i = entry1.getKey();

By using ? extends Entry<?, V> you’re not exposing yourself to this, as you do not claim to know anything about the type of Entry your Iterator can consume, only what it can produce.

Edit

Though Jiman deleted his answer, he had a good point that using the for-each loop is a lot cleaner approach (assuming you’re just trying to iterate over the entries) that would avoid this issue entirely. This should work:

for ( Entry<?, V> entry : map.entrySet() ) {
   //...
}