Get the map keys and not the map values

Question

I have some data like this:

Map<Integer, String> foo

Now I can get the corresponding String with foo.get(1)

But is it also possible to get all the Integers which have the String "abc"?

Like this pseudo code: Integer[] someMore = foo.getKeys("abc")

For that you need to create your own implementation to find keys with specific values... :) — Ketan Bhavsar, May 24 '13 at 11:58
you can loop through the map and check if the value for a certain key == the String you are looking for then add it to a arrayList — benst, May 24 '13 at 11:58
If that is the use case, then you have created the Map incorrectly, you need to swap the key and value. — Srinivas, May 24 '13 at 11:59
Have you had a look at this: http://stackoverflow.com/questions/4005816/map-how-to-get-all-keys-associated-with-a-value ? — Balázs Németh, May 24 '13 at 11:59
You have to iterate over the map and find all keys with a given value. This is nowhere near as efficient as the opposite though. See http://stackoverflow.com/a/46908/17713 — Matthias Meid, May 24 '13 at 11:59
http://www.rgagnon.com/javadetails/java-0564.html might helps — Alpesh Prajapati, May 24 '13 at 12:01

score 3 · Accepted Answer · answered May 24 '13 at 11:59

Try:

Set<Integer> myInts = new HashSet<Integer>();
for(Entry<Integer, String> entry : foo.entrySet()) { // go through the entries
    if(entry.getValue().equals("abc")) {             // check the value
        myInts.add(entry.getKey());                  // add the key
    }
}
// myInts now contains all the keys for which the value equals "abc"

score 1 · Answer 2 · answered May 24 '13 at 12:00

Map doesn't provide look-up by values. We need to do it by iterating over the Map entries

Set<Integer> matchingKeys =  new HashSet<Integer>();
for(Entry<Integer, String> e : map.entrySet()) {
    if(e.getValue().equals("abc")) {
          matchingKeys.add(e.getKey());
    }
}

score 0 · Answer 3 · edited May 23 '17 at 12:18

0

You can't get that with a regular map. You would have to call foo.entrySet() and create the array on your own.

Maybe you would be interested in using a bidirectional map. Here's a thread where you can read a bit about that. Bi-directional Map in Java?

edited May 23 '17 at 12:18

Community

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answered May 24 '13 at 12:02

Simon

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score 0 · Answer 4 · answered May 24 '13 at 12:04

Map<Integer, String> map = new Map<Integer, String>();
ArrayList<Integer> arraylist = new ArrayList<Integer>();
for (Entry<Integer, String> entry : map.entrySet()) {
    if (entry.getValue().equals("abc")) {
    arraylist.add(entry.getKey());
    }
}

Get the map keys and not the map values

4 Answers4