Delete an entry in hashmap while iterating

Question

I have a loop to iterate a hashmap. If the condition is satisfied I need to remove the key, value pair from the hashmap. I am unable to do it using the below code. How can I achieve this?

for(HashMap.Entry<Integer,Character> m:commons.entrySet()){
    while(i!=(int)m.getKey()){
        i++;
    }
    if(s2.charAt(i)!=(int)m.getKey()){
      commons.remove((int)m.getKey());
    }
}

could you show complete code please ? where does i variable come from ? — CodeScale, May 17 '20 at 19:55

score 1 · Answer 1 · answered May 17 '20 at 19:59

for (Iterator<Map.Entry<Integer, Character>> it = map.entrySet().iterator(); it.hasNext(); ) {
    Map.Entry<Integer, Character> entry = it.next();

    // casting to int is redundant in your code. I removed it.
    while (i != entry.getKey()) i++;

    if (s2.charAt(i) != entry.getKey()) {
        it.remove();
    }
}

You can use an Iterator to safely remove while iterating over a Map.

You can also look at .removeIf() (Java 8+), but with a loop inside your iteration, this is more readable, imo.

Thank you Harshal. It makes sense now – Nikitha Rao May 18 '20 at 21:05 — Nikitha Rao, May 18 '20 at 21:05

score 1 · Answer 2 · answered May 17 '20 at 20:34

The way you are trying to do it may cause ConcurrentModificationException. You should use an Iterator or Collection::removeIf which internally uses an Iterator.
Though you can compare a char with an int, probably you have mixed up key and value in the comparison. You have written
```
if(s2.charAt(i)!=(int)m.getKey())
```
in which you are comparing the s2.charAt(i) with the key which is an Integer. Syntactically, it is correct but probably you wanted to compare the value (which is a Characater) i.e. most likely you want to do
```
if(s2.charAt(i)!=(int)m.getValue())
```

Do it as follows:

for (Iterator<Entry<Integer, Character>> itr = commons.entrySet().iterator(); itr.hasNext();) {
    Entry<Integer, Character> entry = itr.next();

    while (i != entry.getKey()) {
        i++;
    }

    if (s2.charAt(i) != entry.getValue()) {
        itr.remove();
    }
}

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Delete an entry in hashmap while iterating

2 Answers2