Increment all values in a Map by 1

Question

What's the neatest way to increment all values in a HashMap by 1? The map is <String, Integer>, but the key doesn't matter as every value will be incremented.

Is it "cleaner" to use lambdas with forEach/compute/etc. or just loop through the entries?

HashMap<String, Integer> map = mobCounter.get(mob);
for (Entry<String, Integer> e : map.entrySet()) {
    map.put(e.getKey(), e.getValue() + 1);
}

This doesn't look too messy to me but I'm wondering if people like seeing lambdas more.

Answer 1

You must not modify the keys of a HashMap while iterating over it. You are just lucky it worked.

Instead of the put(...), write e.setValue(e.getValue() + 1).

If you have some kind of Multiset available (e.g. when you are using Guava), you could replace your HashMap with a Multiset<String>, which will make your intention clearer.

Using lambdas, your code would look like:

map.replaceAll((k, v) -> v + 1);

This looks very nice to me. It cannot get any shorter.

Answer 2

The solution you've offered is a perfectly fine solution. While it is possible to do it with lambdas, I don't think it's strictly necessary.

The only things I would mention:

• map is a terrible name for a variable.
• Your map variable should have the interface type, not the implementation type.