Java Hashmap: How to get key from value?

Question

If I have the value \"foo\", and a HashMap ftw for which ftw.containsValue(\"foo\") returns true, how can I

Answer 1

for(int key: hm.keySet()) {
    if(hm.get(key).equals(value)) {
        System.out.println(key); 
    }
}

Answer 2

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);
    }
}

Answer 3

public static String getKey(Map<String, Integer> mapref, String value) {
    String key = "";
    for (Map.Entry<String, Integer> map : mapref.entrySet()) {
        if (map.getValue().toString().equals(value)) {
            key = map.getKey();
        }
    }
    return key;
}

Answer 4

1. If you want to get key from value, its best to use bidimap (bi-directional maps) , you can get key from value in O(1) time.

   But, the drawback with this is you can only use unique keyset and valueset.

2. There is a data structure called Table in java, which is nothing but map of maps like

   Table< A, B , C > == map < A , map < B, C > >

   Here you can get map<B,C> by querying T.row(a);, and you can also get map<A,C> by querying T.column(b);

In your special case, insert C as some constant.

So, it like < a1, b1, 1 > < a2, b2 , 1 > , ...

So, if you find via T.row(a1) ---> returns map of --> get keyset this returned map.

If you need to find key value then, T.column(b2) --> returns map of --> get keyset of returned map.

Advantages over the previous case :

1. Can use multiple values.
2. More efficient when using large data sets.

Answer 5

To find all the keys that map to that value, iterate through all the pairs in the hashmap, using map.entrySet().

Answer 6

Use a thin wrapper: HMap

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class HMap<K, V> {

   private final Map<K, Map<K, V>> map;

   public HMap() {
      map = new HashMap<K, Map<K, V>>();
   }

   public HMap(final int initialCapacity) {
      map = new HashMap<K, Map<K, V>>(initialCapacity);
   }

   public boolean containsKey(final Object key) {
      return map.containsKey(key);
   }

   public V get(final Object key) {
      final Map<K, V> entry = map.get(key);
      if (entry != null)
         return entry.values().iterator().next();
      return null;
   }

   public K getKey(final Object key) {
      final Map<K, V> entry = map.get(key);
      if (entry != null)
         return entry.keySet().iterator().next();
      return null;
   }

   public V put(final K key, final V value) {
      final Map<K, V> entry = map
            .put(key, Collections.singletonMap(key, value));
      if (entry != null)
         return entry.values().iterator().next();
      return null;
   }
}