Java - get index of key in HashMap?

Question

In java if I am looping over the keySet() of a HashMap, how do I (inside the loop), get the numerical index of that key?

Basically, as I loop through the map, I want to be able to get 0,1,2...I figure this would be cleaner than declaring an int and incrementing with each iteration.

Thanks.

Answer 1

Simply put, hash-based collections aren't indexed so you have to do it manually.

Answer 2

The HashMap has no defined ordering of keys.

Answer 3

You can't - a set is unordered, so there's no index provided. You'll have to declare an int, as you say. Just remember that the next time you call keySet() you won't necessarily get the results in the same order.

Answer 4

Not sure if this is any "cleaner", but:

List keys = new ArrayList(map.keySet());
for (int i = 0; i < keys.size(); i++) {
    Object obj = keys.get(i);
    // do stuff here
}

Answer 5

If all you are trying to do is get the value out of the hashmap itself, you can do something like the following:

for (Object key : map.keySet()) {
    Object value = map.get(key);
    //TODO: this
}

Or, you can iterate over the entries of a map, if that is what you are interested in:

for (Map.Entry<Object, Object> entry : map.entrySet()) {
    Object key = entry.getKey();
    Object value = entry.getValue();
    //TODO: other cool stuff
}

As a community, we might be able to give you better/more appropriate answers if we had some idea why you needed the indexes or what you thought the indexes could do for you.