What's the quickest way to remove an element from a Map by value in Java?

Question

What\'s the quickest way to remove an element from a Map by value in Java?

Currently I\'m using:

    DomainObj valueToRemove = new DomainObj();
    S         


        

Answer 1

If you have no way to figure out the key from the DomainObj, then I don't see how you can improve on that. There's no built in method to get the key from the value, so you have to iterate through the map.

If this is something you're doing all the time, you might maintain two maps (string->DomainObj and DomainObj->Key).

Answer 2

We know this situation arise rarely but is extremely helpful. I'll prefer BidiMap from org.apache.commons.collections .

Answer 3

map.values().removeAll(Collections.singleton(null));

reference to How to filter "Null" values from HashMap<String, String>?, we can do following for java 8:

map.values().removeIf(valueToRemove::equals);

Answer 4

i would use this

 Map x = new HashMap();
x.put(1, "value1");
x.put(2, "value2");
x.put(3, "value3");
x.put(4, "value4");
x.put(5, "value5");
x.put(6, "value6");

x.values().remove("value4");

edit: because objects are referenced by "pointer" not by value.

N

Answer 5

Without using a Bi-directional map (commons-collections and google collections have them), you're stuck with iterating the Map

Answer 6

You could always use the values collection, since any changes made to that collection will result in the change being reflected in the map. So if you were to call Map.values().remove(valueToRemove) that should work - though I'm not sure if you'll see performance better than what you have with that loop. One idea would be to extend or override the map class such that the backing collection then is always sorted by value - that would enable you to do a binary search on the value which may be faster.

Edit: This is essentially the same as Alcon's answer except I don't think his will work since the entrySet is still going to be ordered by key - in which case you can't call .remove() with the value.

This is also assuming that the value is supposed to be unique or that you would want to remove any duplicates from the Map as well.