Why the iterator on map.vaules can be used to remove HashMap#Entry?

Question

I'm trying to remove all the entry of which the value is null. The code is:

Map<String, String> map = new HashMap<>();
map.put("one", null);
map.put("two", null);
map.put("three", "THREE");

Iterator iterator = map.values().iterator();
while (iterator.hasNext())
{
    if (iterator.next() == null) {
        iterator.remove();
    }
}

for (Map.Entry<String, String> e : map.entrySet()) {
    System.out.println(e.getKey() + ":" + e.getValue());
}

My question is iterator is bind to map.values, why it can remove the whole entry?

Answer 1

It is possible because Map#values returns a view of the values that is backed by the map.

From the official Java-Doc of Map#values:

Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. [...] The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

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Note that the AbstractMap class, from which most map implementations extend, has an extra field transient volatile Collection<V> values and that is exactly what you will get there. As you see the collection is internally used by the Map and thus changes on it are also reflected on the Map itself. See also: Source code of AbstractMap

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If you like to go in detail, take a look at the AbstractMap#values method in the source code. There they create the values-collection as a wrapper that operates on the original map. For example its next method iterates on the entries Entry<K, V> of the Map but only return their value with Entry#getValue and so on.
Also the remove method, as you can see, gets passed over to the iterator of Entry<K, V>, thus the remove will in the end be executed on the original map again.

Answer 2

Explanations have been given by Zabuza, but because there is proper way to remove your elements, I write them :

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To remove Entry with null value you can use Streams :

map = map.entrySet()
         .stream()
         .filter(entry -> entry.getValue()!=null)
         .collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));

Or in one - line : map.entrySet().removeIf(e -> e.getValue()==null);

Or : map.values().removeIf(v -> v == null)