如何在Java中迭代HashMap值时替换它们

Question

I am using a Runnable to automatically subtract 20 from a players cooldown every second, but I have no idea how to replace the value of a value as I iterate through it. How can I have it update the value of each key?

public class CoolDownTimer implements Runnable {
    @Override
    public void run() {
        for (Long l : playerCooldowns.values()) {
            l = l - 20;
            playerCooldowns.put(Key???, l);
        }
    }

}

Answer 1

Using Java 8:

map.replaceAll((k, v) -> v - 20);

---

Using Java 7 or older:

You can iterate over the entries and update the values as follows:

for (Map.Entry<Key, Long> entry : playerCooldowns.entrySet()) {
    entry.setValue(entry.getValue() - 20);
}

Answer 2

Well, you can't do it by iterating over the set of values in the Map (as you are doing now), because if you do that then you have no reference to the keys, and if you have no reference to the keys, then you can't update the entries in the map, because you have no way of finding out which key was associated with the value you just updated.

When working with Maps, you have two options for updates like this, iterate through each Map.Entry<K,V> in the Map , or you can iterate through the key Set . There are methods on Map to do both of these things. Personally, I would iterate through each Map.Entry<K,V> .

for (Map.Entry<String, Long> entry : playerCooldowns.entrySet()) {
    entry.setValue(entry.getValue() - 20);
}

Answer 3

Why not iterate over the Map.Entry objects ? Each Entry will give you the key and value and you don't have to perform an additional get() on the Map to get a value.