Java Modifying key object inside map

Question

I am having a problem with JAVA map. I enter an object as a key in the map. Then I modify the key and the map does not consider the object as a key of the map any more. Even though the key inside the object has been modified accordingly.

I am working with the object CoreLabel from StanfordNLP but it applies to a general case I guess.

Map <CoreLabel, String> myMap = new HashMap...
CoreLabel key = someCreatedCoreLabel
myMap.put(key, someString)
myMap.get(key) != null ----> TRUE
key.setValue("someValue");
myMap.get(key) != null ----> FALSE

I hope I was clear enough. The question is why is the last statement false? I am not a very experienced programmer but I would expect it to be true. Maybe has something to do with the CoreLabel object?

I check if .equals() still holds, and it actually does

for(CoreLabel token: myMap.keySet()) {
     if(key.equals(token))
        System.out.println("OK");
}

Answer 1

The problem is that in modifying the value of the key, now the hash code of the key has changed as well. A HashMap will first use the hash code of the key to determine if it exists. The modified hash code didn't exist in the map, so it didn't even get to try the step of using the equals method. That's why it's a bad idea to change your key objects while they're in a HashMap.

Answer 2

This is explicitly documented in the Map Javadoc as dangerous and unlikely to work:

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.