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

Question

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.

Answer 1

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!