CODEWITHSUNDEEP
java
Jack Twain
Jack Twain

Reputation: 6372

Is it safe to change a HashMap key/value during iteration?

I have a HashMap. I loop through the map like this:

Map<Long, Integer> map = new HashMap<Long, Integer>();
for (Long key : map.keySet() ) {
   int value = map.get(key);
   value--;
   map.put(key, value);
}

Is the way I'm using to update the map safe? Safe in the sense that it doesn't damage the map because of the iteration.

Upvotes: 9

Views: 10249

Answers (3)

Kaushik Shankar
Kaushik Shankar

Reputation: 5619

As you can see in the HashMap source code, the put method only modifies the modCount when a new key is provided. modCount is used by the iterator to check for changes, and if such a change occurred between two calls to an iterator's next(), a ConcurrentModificationException would be thrown. This means that the way you are using put is safe.

Upvotes: 9

Steven Schlansker
Steven Schlansker

Reputation: 38526

You could consider writing your code more efficiently as:

Map<Long, Integer> map = new HashMap<Long, Integer>();
for (Entry<Long, Integer> entry : map.entrySet() ) {
    entry.setValue(entry.getValue() - 1);
}

This is a micro-optimization, but sometimes it matters, and you don't lose anything. It's shorter and clears up any ambiguity about the safety to boot!

Upvotes: 9

anubhava
anubhava

Reputation: 785128

It is perfectly safe operation that you're doing since you're just changing value of an existing key in the Map.

However if you are ever going to delete an entry from Map then remeber to use Iterator.

Upvotes: 3

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