Remove multiple keys from Map in efficient way?

Question

I have a Map<String,String> with large number of key values pairs. Now I want to remove selected keys from that Map. Following code shows what I did to achieve that.

Set keySet = new HashSet(); //I added keys to keySet which I want to remove.

Then :

Iterator entriesIterator = keySet.iterator();
while (entriesIterator.hasNext()) {
   map.remove( entriesIterator.next().toString());
}

This is working fine. What would be the better approach?

score 319 · Accepted Answer · answered Jul 16 '13 at 11:55

319

Assuming your set contains the strings you want to remove, you can use the keySet method and map.keySet().removeAll(keySet);.

keySet returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.

Contrived example:

Map<String, String> map = new HashMap<>();
map.put("a", "");
map.put("b", "");
map.put("c", "");

Set<String> set = new HashSet<> ();
set.add("a");
set.add("b");

map.keySet().removeAll(set);

System.out.println(map); //only contains "c"

answered Jul 16 '13 at 11:55

assylias

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your suggestion is great. I guess removeAll(keySet) is doing what I did there – Ruchira Gayan Ranaweera Jul 16 '13 at 12:03
17

in terms of "efficiency" its probably just a for loop underneath, but in terms of cleaner code, nice win :) – rogerdpack Jan 25 '16 at 22:14
Obvious once you see it - thanks! Still baffles me why Map does have removeAll, and for that matter isnt Iterable over the entrySet directly – SteveR Dec 06 '20 at 14:08
Not working for me on Jenkins jvm – Barel elbaz Apr 03 '22 at 12:50

score 29 · Answer 2 · answered Nov 04 '20 at 14:24

For the sake of completion, and since Google brings you here when you look for a way to achieve this:

map.entrySet().removeIf(entry -> /* decide what you want to remove here */ );

This does not assume you have a predefined set of keys to remove but rather assumes you have a condition on which the keys should be removed. From the question, it is unclear if these keys are added manually or based on some sort of condition. In the latter case, this might be the cleaner code.

For the former case, this (untested) code might work as well:

map.entrySet().removeIf(entry -> keySet.contains(entry.getKey()) );

But obviously the answer provided by @assylias is much cleaner in this case!

score 11 · Answer 3 · answered Feb 14 '17 at 19:35

Just for the sake of completeness:

As guessed java.util.AbstractSet#removeAll really iterates over all entries, but with one little trick: It uses the iterator of the smaller collection:

if (size() <= collection.size()) {
    Iterator<?> it = iterator();
    while (it.hasNext()) {
        if (collection.contains(it.next())) {
            it.remove();
        }
    }
} else {
    Iterator<?> it = collection.iterator();
    while (it.hasNext()) {
        remove(it.next());
    }
}

score 4 · Answer 4 · answered Sep 25 '19 at 13:36

4

Using Java stream:

keySet.forEach(map::remove);

answered Sep 25 '19 at 13:36

Abdull

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This will not solve the actual request of removing multiple selected keys, as it doesn't have select parameter. – dildeepak Sep 30 '20 at 20:15
@dildeepak, what do you mean with "select parameter"? – Abdull Oct 01 '20 at 06:17

Remove multiple keys from Map in efficient way?

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