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Java Hashmap: How to get key from value?


Question

If I have the value "foo", and a HashMap<String> ftw for which ftw.containsValue("foo") returns true, how can I get the corresponding key? Do I have to loop through the hashmap? What is the best way to do that?

2011/01/19
1
459
1/19/2011 12:20:58 AM

Accepted Answer

If you choose to use the Commons Collections library instead of the standard Java Collections API, you can achieve this with ease.

The BidiMap interface in the Collections library is a bi-directional map, allowing you to map a key to a value (like normal maps), and also to map a value to a key, thus allowing you to perform lookups in both directions. Obtaining a key for a value is supported by the getKey() method.

There is a caveat though, bidi maps cannot have multiple values mapped to keys, and hence unless your data set has 1:1 mappings between keys and values, you cannot use bidimaps.

Update

If you want to rely on the Java Collections API, you will have to ensure the 1:1 relationship between keys and values at the time of inserting the value into the map. This is easier said than done.

Once you can ensure that, use the entrySet() method to obtain the set of entries (mappings) in the Map. Once you have obtained the set whose type is Map.Entry, iterate through the entries, comparing the stored value against the expected, and obtain the corresponding key.

Update #2

Support for bidi maps with generics can be found in Google Guava and the refactored Commons-Collections libraries (the latter is not an Apache project). Thanks to Esko for pointing out the missing generic support in Apache Commons Collections. Using collections with generics makes more maintainable code.

2016/01/28
219
1/28/2016 9:14:45 PM


public class NewClass1 {

    public static void main(String[] args) {
       Map<Integer, String> testMap = new HashMap<Integer, String>();
        testMap.put(10, "a");
        testMap.put(20, "b");
        testMap.put(30, "c");
        testMap.put(40, "d");
        for (Entry<Integer, String> entry : testMap.entrySet()) {
            if (entry.getValue().equals("c")) {
                System.out.println(entry.getKey());
            }
        }
    }
}

Some additional info... May be useful to you

Above method may not be good if your hashmap is really big. If your hashmap contain unique key to unique value mapping, you can maintain one more hashmap that contain mapping from Value to Key.

That is you have to maintain two hashmaps

1. Key to value

2. Value to key 

In that case you can use second hashmap to get key.

2016/03/02

You could insert both the key,value pair and its inverse into your map structure

map.put("theKey", "theValue");
map.put("theValue", "theKey");

Using map.get("theValue") will then return "theKey".

It's a quick and dirty way that I've made constant maps, which will only work for a select few datasets:

  • Contains only 1 to 1 pairs
  • Set of values is disjoint from the set of keys (1->2, 2->3 breaks it)
2016/02/04

I think your choices are

  • Use a map implementation built for this, like the BiMap from google collections. Note that the google collections BiMap requires uniqueless of values, as well as keys, but it provides high performance in both directions performance
  • Manually maintain two maps - one for key -> value, and another map for value -> key
  • Iterate through the entrySet() and to find the keys which match the value. This is the slowest method, since it requires iterating through the entire collection, while the other two methods don't require that.
2016/01/28

Decorate map with your own implementation

class MyMap<K,V> extends HashMap<K, V>{

    Map<V,K> reverseMap = new HashMap<V,K>();

    @Override
    public V put(K key, V value) {
        // TODO Auto-generated method stub
        reverseMap.put(value, key);
        return super.put(key, value);
    }

    public K getKey(V value){
        return reverseMap.get(value);
    }
}
2016/05/23

There is no unambiguous answer, because multiple keys can map to the same value. If you are enforcing unique-ness with your own code, the best solution is to create a class that uses two Hashmaps to track the mappings in both directions.

2009/09/05