Removing all items of a given value from a hashmap

Question

So I have a java hashmap like below:

hMap.put("1", "One");
hMap.put("2", "Two");
hMap.put("3", "Two");

I would like to remove ALL items where the value is "Two"

If I do something like:

hmap.values().remove("Two");

Only the first one is deleted, I want to remove them all, how can this be done?

Answer 1

for (Iterator<Map.Entry<String,String>> it = hMap.entrySet().iterator(); it.hasNext();) {
 Map.Entry<String,String> e = it.next();
 if ("Two".equals(e.getValue())) {
  it.remove();
 }
}

---

Answer 2

You have to iterate through the list, look at the value object, and conditionally do the remove. Note you'll get an exception if you try to remove an object while iterating over a HashMap. Would have to make a copy of the map or use ConcurrentHashMap.

Answer 3

You can use while( hmap.value().remove("Two") ); since the remove call returns true if the collection was changed as a result of the call.

Answer 4

hmap.values().removeAll(Collections.singleton("Two"));

EDIT: the (significant) disadvantage with this concise approach is that you are basically forced to comment it, saying something like

// remove("Two") would only remove the first one

otherwise, some well-meaning engineer will try to simplify it for you someday and break it. This happens... sometimes the well-intentioned do-gooder is even Future You!

Answer 5

(Updated solution for logging of removed values)

This solution uses the google-collections library [LINK]

import static com.google.common.collect.Maps.filterValues;
import static com.google.common.base.Predicates.equalTo;

...

Map<String, String> removedValues = filterValues(hMap, equalTo("Two"));      
System.out.println(removedValues); //Log Removed Values
removedValues.clear(); //Removes from original map, since this is a view.

Note - This solution takes advantage of the fact that the Map returned by the filterValues call is a view of the elements in the original HashMap. This allows us to examine them and log out the keys that were removed, and then remove them from the original map with a simple call to clear().

You may have reasons for not wanting to use the google-collections library in your project, but if you don't, I'd suggest checking it out.