Get a key from value with a Map

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

public class Main {

  public static void main(String[] argv) {
    Map<String, String> map = new HashMap<String, String>();
    map.put("1","one");
    map.put("2","two");
    map.put("3","three");
    map.put("4","four");

    System.out.println(getKeyFromValue(map,"three"));
  }

  public static Object getKeyFromValue(Map hm, Object value) {
    for (Object o : hm.keySet()) {
      if (hm.get(o).equals(value)) {
        return o;
      }
    }
    return null;
  }
}