查找一个值的键，该值是其他哈希图中的键

Question

consider I have a hash map map(two, twooo) and other hash map pun(twooo, 3)

Now i need to get the key "two" from the value "3"

How can I do that?

Answer 1

Assuming that you have a different value for each key, you can do something like this:

private Map<String, Integer> team1 = new HashMap<String, Integer>();

  team1.put("1", 1);
  team1.put("2", 2)


private String getKey(Integer value){
   for(String key : team1.keySet()){
     if(team1.get(key).equals(value)){
        return key; //return the first found
     }
  }
  return null;
}

In same manner iterate through other hash map to find a key of value which is key in other hashMap.

Answer 2

If you're using HashMap , this will be very inefficient, you have to loop over the values in both maps to find the key you're looking for, something like:

public static <K1, VK, V2> K1 findKeyInFirstMap(
    Map<K1, VK> map1, Map<VK, V2> map2, V2 value) {
  VK valueKey = null;
  for (Entry<VK, V2> e : map2.entrySet()) {
    if (e.getValue().equals(value)) {
      valueKey = e.getKey();
      break;
    }
  }
  if (valueKey == null) {
    throw new IllegalArgumentException(value + " not found in map2");
  }
  for (Entry<K1, VK> e : map1.entrySet()) {
    if (e.getValue().equals(valueKey)) {
      return e.getKey();
    }
  }
  throw new IllegalArgumentException(valueKey + " not found in map1");
}

This assumes your maps don't have null keys or values, but why would you do that to yourself? :)

Much more efficient would be to use a bi-directional mapping, like Guava's BiMap . With a BiMap , it's much simpler, and O(1) instead of O(n):

map1.inverse().get(map2.inverse().get(value));

Or, with the same error semantics as the previous method:

public static <K1, VK, V2> K1 findKeyInFirstMap(
        BiMap<K1, VK> map1, BiMap<VK, V2> map2, V2 value) {
  VK valueKey = map2.inverse().get(value);
  if (valueKey == null) {
    throw new IllegalArgumentException(value + " not found in map2");
  }
  K1 key = map1.inverse().get(valueKey);
  if (key == null) {
    throw new IllegalArgumentException(valueKey + " not found in map1");
  }
  return key;
}