CODEWITHSUNDEEP
javadata-structuresjava-6
masb
masb

Reputation: 2843

Get keys from HashMap in Java

I have a Hashmap in Java like this:

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

Then I fill it like this:

team1.put("United", 5);

How can I get the keys? Something like: team1.getKey() to return "United".

Upvotes: 223

Views: 989116

Answers (15)

sakirow
sakirow

Reputation: 229

foreach can be used too.

team1.forEach((key, value) -> System.out.println(key));

Upvotes: 3

Emperor
Emperor

Reputation: 91

Use functional operation for faster iteration.

team1.keySet().forEach((key) -> {
      System.out.println(key);
});

Upvotes: 6

Fabio Filippi
Fabio Filippi

Reputation: 1955

Check this.

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

(Use java.util.Objects.equals because HashMap can contain null)

Using JDK8+

/**
 * Find any key matching a value.
 *
 * @param value The value to be matched. Can be null.
 * @return Any key matching the value in the team.
 */
private Optional<String> findKey(Integer value){
    return team1
        .entrySet()
        .stream()
        .filter(e -> Objects.equals(e.getValue(), value))
        .map(Map.Entry::getKey)
        .findAny();
}

/**
 * Find all keys matching a value.
 *
 * @param value The value to be matched. Can be null.
 * @return all keys matching the value in the team.
 */
private List<String> findKeys(Integer value){
    return team1
        .entrySet()
        .stream()
        .filter(e -> Objects.equals(e.getValue(), value))
        .map(Map.Entry::getKey)
        .collect(Collectors.toList());
}

More "generic" and as safe as possible

/**
 * Find any key matching the value, in the given map.
 *
 * @param mapOrNull Any map, null is considered a valid value.
 * @param value     The value to be searched.
 * @param <K>       Type of the key.
 * @param <T>       Type of the value.
 * @return An optional containing a key, if found.
 */
public static <K, T> Optional<K> findKey(Map<K, T> mapOrNull, T value) {
    return Optional.ofNullable(mapOrNull).flatMap(map -> map.entrySet()
            .stream()
            .filter(e -> Objects.equals(e.getValue(), value))
            .map(Map.Entry::getKey)
            .findAny());
}

Or if you are on JDK7.

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

private List<String> findKeys(Integer value){
   List<String> keys = new ArrayList<String>();
   for(String key : team1.keySet()){
        if(Objects.equals(team1.get(key), value)){
             keys.add(key);
      }
   }
   return keys;
}

Upvotes: 30

ghanshyam singh
ghanshyam singh

Reputation: 67

To get keys in HashMap, We have keySet() method which is present in java.util.Hashmap package. ex : 

Map<String,String> map = new Hashmap<String,String>();
map.put("key1","value1");
map.put("key2","value2");

// Now to get keys we can use keySet() on map object
Set<String> keys = map.keySet();

Now keys will have your all keys available in map. ex: [key1,key2]

Upvotes: 1

james.garriss
james.garriss

Reputation: 13388

This is doable, at least in theory, if you know the index:

System.out.println(team1.keySet().toArray()[0]);

keySet() returns a set, so you convert the set to an array.

The problem, of course, is that a set doesn't promise to keep your order. If you only have one item in your HashMap, you're good, but if you have more than that, it's best to loop over the map, as other answers have done.

Upvotes: 75

Orici
Orici

Reputation: 431

A solution can be, if you know the key position, convert the keys into an String array and return the value in the position:

public String getKey(int pos, Map map) {
    String[] keys = (String[]) map.keySet().toArray(new String[0]);

    return keys[pos];
}

Upvotes: 0

Emmanuel R
Emmanuel R

Reputation: 65

To get Key and its value

e.g

private Map<String, Integer> team1 = new HashMap<String, Integer>();
  team1.put("United", 5);
  team1.put("Barcelona", 6);
    for (String key:team1.keySet()){
                     System.out.println("Key:" + key +" Value:" + team1.get(key)+" Count:"+Collections.frequency(team1, key));// Get Key and value and count
                }

Will print: Key: United Value:5 Key: Barcelona Value:6

Upvotes: -2

user3763927
user3763927

Reputation: 31

What I'll do which is very simple but waste memory is to map the values with a key and do the oposite to map the keys with a value making this:

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

it's important that you use <Object, Object> so you can map keys:Value and Value:Keys like this

team1.put("United", 5);

team1.put(5, "United");

So if you use team1.get("United") = 5 and team1.get(5) = "United"

But if you use some specific method on one of the objects in the pairs I'll be better if you make another map:

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

private Map<Integer, String> team1Keys = new HashMap<Integer, String>();

and then

team1.put("United", 5);

team1Keys.put(5, "United");

and remember, keep it simple ;)

Upvotes: -2

Matteo
Matteo

Reputation: 14930

A HashMap contains more than one key. You can use keySet() to get the set of all keys.

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

will store 1 with key "foo" and 2 with key "bar". To iterate over all the keys:

for ( String key : team1.keySet() ) {
    System.out.println( key );
}

will print "foo" and "bar".

Upvotes: 389

Abdul Rizwan
Abdul Rizwan

Reputation: 4108

public class MyHashMapKeys {

    public static void main(String a[]){
        HashMap<String, String> hm = new HashMap<String, String>();
        //add key-value pair to hashmap
        hm.put("first", "FIRST INSERTED");
        hm.put("second", "SECOND INSERTED");
        hm.put("third","THIRD INSERTED");
        System.out.println(hm);
        Set<String> keys = hm.keySet();
        for(String key: keys){
            System.out.println(key);
        }
    }
}

Upvotes: -1

raman rayat
raman rayat

Reputation: 414

Try this simple program:

public class HashMapGetKey {

public static void main(String args[]) {

      // create hash map

       HashMap map = new HashMap();

      // populate hash map

      map.put(1, "one");
      map.put(2, "two");
      map.put(3, "three");
      map.put(4, "four");

      // get keyset value from map

Set keyset=map.keySet();

      // check key set values

      System.out.println("Key set values are: " + keyset);
   }    
}

Upvotes: -1

&#211;scar L&#243;pez
&#211;scar L&#243;pez

Reputation: 235984

You can retrieve all of the Map's keys using the method keySet(). Now, if what you need is to get a key given its value, that's an entirely different matter and Map won't help you there; you'd need a specialized data structure, like BidiMap (a map that allows bidirectional lookup between key and values) from Apache's Commons Collections - also be aware that several different keys could be mapped to the same value.

Upvotes: 7

sachin
sachin

Reputation: 29

private Map<String, Integer> _map= new HashMap<String, Integer>();
Iterator<Map.Entry<String,Integer>> itr=  _map.entrySet().iterator();
                //please check 
                while(itr.hasNext())
                {
                    System.out.println("key of : "+itr.next().getKey()+" value of      Map"+itr.next().getValue());
                }

Upvotes: 2

tzg
tzg

Reputation: 666

If you just need something simple and more of a verification.

public String getKey(String key)
{
    if(map.containsKey(key)
    {
        return key;
    }
    return null;
}

Then you can search for any key.

System.out.println( "Does this key exist? : " + getKey("United") );

Upvotes: 1

Mateusz Chromiński
Mateusz Chromiński

Reputation: 2832

As you would like to get argument (United) for which value is given (5) you might also consider using bidirectional map (e.g. provided by Guava: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/BiMap.html).

Upvotes: 2

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