How to Remove a Key from Hashmap While Iterating Over It

How to remove a key from HashMap while iterating over it?

Try:

Iterator<Map.Entry<String,String>> iter = testMap.entrySet().iterator();
while (iter.hasNext()) {
    Map.Entry<String,String> entry = iter.next();
    if("Sample".equalsIgnoreCase(entry.getValue())){
        iter.remove();
    }
}

With Java 1.8 and onwards you can do the above in just one line:

testMap.entrySet().removeIf(entry -> "Sample".equalsIgnoreCase(entry.getValue()));

Delete elements from Hashmap while iterating over it

To remove a element whilst in the middle of iterating, use Iterator.remove().

iterating over and removing from a map

As of Java 8 you could do this as follows:

map.entrySet().removeIf(e -> <boolean expression>);

Oracle Docs: entrySet()

The set is backed by the map, so changes to the map are reflected in the set, and vice-versa

Can I use keySet to modify HashMap during iteration?

The HashMap.keySet() method states more precisely:

The set is backed by the map, so changes to the map are reflected in
the set, and vice-versa.

It means that the elements returned by keySet() and the keys of the Map refer to the same objects. So of course changing the state of any element of the Set (such as key.setFoo(new Foo());) will be reflected in the Map keys and reversely.

You should be cautious and prevent the map from being modified during the keyset() iteration :

If the map is modified while an iteration over the set is in progress
(except through the iterator's own remove operation), the results of
the iteration are undefined

You can remove entries of the map as :

The set supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove, Set.remove, removeAll,
retainAll, and clear operations.

But you cannot add entries in :

It does not support the add or addAll operations.

So in conclusion, during keySet() iterator use Set.remove() or more simply iterate with the Iterator of the keySet and invoke Iterator.remove() to remove elements from the map.

You can add new elements in a temporary Map that you will use after the iteration to populate the original Map.

For example :

Map<Key, Value> map = getMap(); // map generating is hidden

Map<Key, Value> tempMap = new HashMap<>();
for (Iterator<Key> keyIterator = map.keySet().iterator(); keyIterator.hasNext();) {
    Key key = keyIterator.next();
    if (isToRemove(key)) {
        keyIterator.remove();
    }
    else {
        tempMap.put(key, getNewValue());
    }
}

map.putAll(tempMap);

Edit :
Note that as you want to modify existing entries of the map, you should use an Map.EntrySet as explained in the Stuart Marks answer.

In other cases, using an intermediary Map or a Stream that creates a new Map is required.

Remove elements from hashset inside hashmap while iterating through java stream

you can use forEach:

placementByConcept.entrySet().forEach(e -> e.getValue().removeIf(s -> s.getBeatObjectiveId().equals("non-scored")));

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