CODEWITHSUNDEEP
javahashmapsethashset
Sempiternal
Sempiternal

Reputation: 127

Remove a key in hashmap when the value's hashset is Empty

I have a hashmap that maps strings keys to hashsets values, and I want to remove a key from the hashmap when the hashmaps's hashset value is empty. I'm having trouble approaching this. Here's what I've tried but I'm very stuck:

for(Map.Entry<String, HashSet<Integer>> entr : stringIDMap.entrySet()) 
{  

                String key = entr.getKey();  

                if (stringIDMap.get(key).isEmpty())
                {

                    stringIDMap.remove(key);
                    continue;
                }
     //few print statements...
}

Upvotes: 8

Views: 16010

Answers (3)

Ludov Dmitrii
Ludov Dmitrii

Reputation: 572

Since Java 8, there is an excelent short solution with lambda:

stringIDMap.entrySet().removeIf(ent -> ent.getValue().isEmpty());

or you can use more concise way by passing method reference

stringIDMap.values().removeIf(Set::isEmpty);

Upvotes: 3

Vahe Gharibyan
Vahe Gharibyan

Reputation: 5683

 Iterator<String> iterator = mMapFiles.keySet().iterator();
    while (iterator.hasNext()){
        if ( mMapFiles.get( iterator.next() ).size() < 1 )
            iterator.remove();
    }

Upvotes: 2

NPE
NPE

Reputation: 500327

In order to avoid ConcurrentModificationException, you need to use the Iterator interface directly:

Iterator<Map.Entry<String, HashSet<Integer>>> it = stringIDMap.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, HashSet<Integer>> e = it.next();
    String key = e.getKey();
    HashSet<Integer> value = e.getValue();
    if (value.isEmpty()) {
        it.remove();
    }
}

The reason your current code doesn't work is that you are attempting to remove elements from the map while iterating over it. When you call stringIDMap.remove(), this invalidates the iterator that the for-each loop uses under the cover, making further iteration impossible.

it.remove() solves this problem as it does not invalidate the iterator.

Upvotes: 19

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