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

Use LinkedHashMap instead of HashMap It will always return keys in same order (as insertion) when calling keySet()

For more detail, see Class LinkedHashMap

Answer 2

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 3

HashMap 没有定义的键顺序。

Answer 4

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.

Answer 5

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 6

简而言之，基于散列的集合没有索引，因此您必须手动进行。

Answer 7

I was recently learning the concepts behind Hashmap and it was clear that there was no definite ordering of the keys. To iterate you can use:

Hashmap<String,Integer> hs=new Hashmap();
for(Map.Entry<String, Integer> entry : hs.entrySet()){
      String key=entry.getKey();
      int val=entry.getValue();
      //your code block  
  }

Answer 8

Posting this as an equally viable alternative to @Binil Thomas's answer - tried to add it as a comment, but was not convinced of the readability of it all.

int index = 0;

for (Object key : map.keySet()) {
   Object value = map.get(key);
   ++index;
}

Probably doesn't help the original question poster since this is the literal situation they were trying to avoid, but may aid others searching for an easy answer.

Answer 9

You can directly get the keys in Set<K> keySet() .

val teamMember = hashMapOf(
            "Coach" to "manager",
            "Player Coach" to "editor",
            "Player" to "member",
            "Supporter" to "readonly"
        )

teamMember.keys.forEachIndexed { index, key ->
   // Here is your key in string 
}

Answer 10

I don't know is this what you're looking for

List keys = new ArrayList(map.keySet());
int index = keys.indexOf(element);