Iterating through a HashMap while removing AND updating the collection-CodePudding

I have a count map where I keep track of the numbers of characters from a string. I want to iterate over that map, decrement the currently visited character count AND remove it if it reaches zero.

How can that be done in Java?

HashMap<Character, Integer> characterCount = new HashMap<>();
characterCount.put('a', 2);
characterCount.put('b', 1);
characterCount.put('c', 1);

Iterator<Map.Entry<Character, Integer>> iterator = characterCount.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry<Character, Integer> entry = iterator.next();

    // Decrement the chosen character from the map
    if (entry.getValue() == 1) {
        iterator.remove();
    } else {
        characterCount.put(entry.getKey(), entry.getValue() - 1);
    }

    // Call some logic the relies on the map with the remaining character count.
    // I want the characterCount.size() to return zero when there is no character with count > 0
    doSomeLogic(characterCount);

    // Restore the character to the map
    characterCount.put(entry.getKey(), entry.getValue());
}

The above code results in a ConcurrentModificationException.

CodePudding user response：

Since Map#entrySet returns a view of the mappings in the map, directly set the value of the Entry to update it.

if (entry.getValue() == 1) {
    iterator.remove();
} else {
    entry.setValue(entry.getValue() - 1);
}

CodePudding user response：

Here is one way. Map.computeIfPresent will remove the entry when it becomes null. Using the ternary operator either decrements the value or replaces with null if it is presently 1 (about to be decremented).

Map<Character, Integer> map = new HashMap<>();
map.put('a', 2);
map.put('b', 1);
map.put('c', 1);

map.computeIfPresent('c', (k,v)-> v == 1 ? null : v-1);
map.computeIfPresent('a', (k,v)-> v == 1 ? null : v-1);

System.out.println(map);

prints

{a=1, b=1}

So, here is how it would work for you, replacing your iterator and while loop.

for (char ch : characterCount.keySet()) {
      characterCount.computeIfPresent(ch, (k,v)-> v == 1 ? null : v-1);
}