Tuesday, February 26, 2013

How to get Key from Value in Hashtable, HashMap or Map in Java


It's not easy to get key from value in Hashtable or HashMap, as compared to getting value from key, because Hash Map or Hashtable doesn't enforce one to one mapping between key and value inside Map in Java. infact Map allows same value to be mapped against multiple keys inside HashMap, Hashtable or any other Map implementation. What you have in your kitty is Hashtable.containsValue(String value) or Hashtable.containsKey(String key) to check whether key or value exists in Hashtable or not, but some time we want to retrieve value from Map corresponding to any key and there is no API method to do in Map. We can still do this, but it highly depends data in your Map because Hashtable and HashMap both allows duplicate values mapped to different key. In this Java tutorial, we will see example of how to get key from value in Hashtable in Java.

Java - Key from Value in Map

How to get key from value in Java HashMap example tutorialThere are essentially two ways to find key from values in Map, one is without using any third party library, and other is using third party library like Google collection or commons collections which provides bi directional Maps. Though most of the time projects already use  these utility libraries, so its better to use them if you already using it; but just because you need to find key  from value in Hashtable, adding new dependency doesn't make sense, especially if you can still do this by writing a function and iterating over Map in Java. In next section we will code example of retrieving value from key in hashtable as well as in hashmap.

Key from Value Hashtable HashMap Example

import java.util.HashMap;
import java.util.HashSet;
import java.util.Hashtable;
import java.util.Map;
import java.util.Set;

/**
 * Java program to get key from value in Java HashMap, Hashtable and Map.
 *
 * @author java67
 */
public class KeyFromValueExample {

public static void main(String args[]) {
       
        //how to get key from value in hashtable in Java
        Hashtable table = new Hashtable();
        table.put("Sony", "Bravia");
        table.put("Samsung", "Galaxy");
        table.put("Nokia", "Lumia");
      
        System.out.println("does hash table has Lumia as value : " + table.containsValue("Lumia"));
        System.out.println("does hash table Lumia as key : " + table.containsKey("Lumia"));
      
        //finding key corresponding to value in hashtable - one to one mapping
        String key= null;
        String value="Lumia";
        for(Map.Entry entry: table.entrySet()){
            if(value.equals(entry.getValue())){
                key = entry.getKey();
                break; //breaking because its one to one map
            }
        }
        System.out.println("got key from value in hashtable key:  "+ key +" value: " + value);
      
        //finding key corresponding to value in hashtable - one to many mapping
        table.put("HTC", "Lumia");
        Set keys = new HashSet();
      
        for(Map.Entry entry: table.entrySet()){
            if(value.equals(entry.getValue())){
                keys.add(entry.getKey()); //no break, looping entire hashtable
            }
        }
        System.out.println("keys : " + keys +" corresponding to value in hash table:  "+ value);
      
      
        //how to get from value in HashMap example - similar to Hashtable example
        HashMap map = new HashMap();
        map.put("one", 1);
        map.put("two", 2);
      
        //find key from value in HashMap Java - one to one mapping
        Integer intValue=2;
        String strKey = null;
        for(Map.Entry entry: map.entrySet()){
            if(intValue.equals(entry.getValue())){
                strKey = entry.getKey();
                break; //breaking because its one to one map
            }
        }
      
        System.out.println("key from value in hash table key:  "+ strKey +" value: " + intValue);
    }
}

Output
does hash table has Lumia as value : true
does hash table has Lumia as key : false
got key from value in hashtable key:  Nokia value: Lumia
keys : [Nokia, HTC] corresponding to value in hash talbe:  Lumia
key from value in hash table key:  two value: 2


If you are using bi-directional Map e.g. BiMap (http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/collect/BiMap.html ) from google collections you can do it in just one line :

bidirMap.inverse().get(value)

That's all on how to get key from value in Hashtable and HashMap in Java. Though using google collection for this task is much clear and concise, it’s not always best option. Using JDK to find key from value in HashMap is better, if you know your map is one to one or one to many. Let me know, if you come across any other way of finding key from value in Hashtable or HashMap in Java.

3 comments :

Anonymous said...

iterate throgh the map using MAP.Entry obtained for Map.entrySet() and find out the corresponding key for the value.

Jody said...

When I tried to use "Map.Entry entry: table.entrySet()", it prompted that "Type mismatch: cannot convert from element type Object to Map.Entry". Even if I copied the codes, it has no change. I don't know where has wrong?

satish said...

int arr[] = { 2, 4, 5, 8, 5, 6, 5, 9, 3, 3, 3 };
Map map = new HashMap();
for (int i : arr) {
Integer count = map.get(i);
map.put(i, count != null ? count + 1 : 0);
}
for (Map.Entry data : map.entrySet()) {
System.out.println("data : " + data.getKey() + " ,occurance :"
+ data.getValue());
}

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