TreeMap iterator.remove() modifies the last Entry

Question

I have a Map.

Map<Integer,String> map = ...

The map has n elements (lets take for this example these 9)

  map.put(1,"one");
  map.put(2,"two");
  map.put(3,"three");
  map.put(4,"four");
  map.put(5,"five");
  map.put(6,"six");
  map.put(7,"seven");
  map.put(8,"eigth");
  map.put(9,"nine");

Now I want to iterate over this map, and remove the n-th element using the iterator.

private void remove(int num, final Map<Integer, String> map) {

  Iterator<Map.Entry<Integer,String>> it = map.entrySet().iterator();
  Map.Entry<Integer,String> entry;
  while(it.hasNext()){

     entry = it.next();

     if(Integer.valueOf(num).equals(entry.getKey())){
        it.remove();
        System.out.println(entry.getValue());
        // vs
        // System.out.println(entry.getValue());
        // it.remove();
     }
  }
}

From the javadoc, I'd assume, the semantics of the remove are well defined.

But depending on the implementation of the map - i.e. HashMap vs TreeMap there is a difference whether the it.remove() is done before or after entry.getValue().

for HashMaps map = new HashMap<>() the behavior is

...
remove(4, map); //output: four
//or
remove(5, map); //output: five

for TreeMap map = new TreeMap<>() the behavior is same, when I remove the current entry from the iterator after I have accessed it:

System.out.println(entry.getValue());
it.remove();

results in

remove(4, map); //output: four
//or
remove(5, map); //output: five

so far so good, but if I remove the element before I access the entry:

it.remove();
System.out.println(entry.getValue());

The output is unexpectedly

remove(4, map); //output: five !!!
//or
remove(5, map); //output: five ok

Apparently, the it.remove() of the TreeMap modifies the Entries, because the TreeMap is made up of Entries and the iterator actually returns the actual elements of the map. And depending on the current position in the tree, the internal references of the Entry point to the next or the current (removed) element.

But I'm not sure if this is a bug or if this is intentional. If the latter is the case, I wonder about the rationale behind?

Edit: Source code of TreeMap iterator.remove()

Answer 1

From the Map.Entry Javadoc:

the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the setValue operation on the map entry

And from the Map.Entry.getValue Javadoc:

If the mapping has been removed from the backing map (by the iterator's remove operation), the results of this call are undefined.

Calling entry.getValue() after it.remove() is forbidden. Java makes no promises about what will happen if you try it. You should retrieve the value before removing the entry.