Get keys from HashMap in Java

Question

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".

What do you expect `team1.getKey()` to return if: (1) the map is empty, or (2) if it contains multiple keys? — NPE, May 05 '12 at 14:47

score 386 · Accepted Answer · edited Mar 03 '16 at 06:34

386

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".

edited Mar 03 '16 at 06:34

Shashank Agrawal

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answered May 05 '12 at 14:34

Matteo

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But in this case i only have one key for each value. It is not possible to write anything like team1.getKey()? – masb May 05 '12 at 14:39
No you have a map with one element. But it's a map: a structure that can contain more than one element. – Matteo May 05 '12 at 14:40
13

What's the point of a Map with a single key? Create a class with a key field and a value field. – JB Nizet May 05 '12 at 14:41
9

If you want to store all keys in array list : `List keys = new ArrayList<>(mLoginMap.keySet());` – Pratik Butani Dec 24 '18 at 06:42

score 69 · Answer 2 · edited Dec 28 '18 at 19:20

69

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.

edited Dec 28 '18 at 19:20

FullStackDeveloper

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answered Jun 18 '15 at 18:17

james.garriss

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1

This is helpful in a unit testing scenario where you have full control over the contents of your `HashMap`. Good show. – risingTide Oct 16 '18 at 20:42
Nothing about knowing the index in the question. – user207421 Jun 16 '20 at 10:31
this is really slow and memory inefficient. probably not what you want to do for most cases, including the question – Will Kanga Nov 01 '20 at 18:39

Fabio Filippi · Answer 3 · 2020-12-31T09:48:03.203

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;
}

But what happens if several keys map to the same value? you should be returning a list of keys instead — Óscar López, May 05 '12 at 14:41

score 7 · Answer 4 · edited Nov 25 '13 at 23:54

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.

score 5 · Answer 5 · edited Sep 27 '21 at 11:45

5

Use functional operation for faster iteration.

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

edited Sep 27 '21 at 11:45

james.garriss

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answered Mar 02 '20 at 13:50

Emperor

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score 3 · Answer 6 · answered Sep 28 '21 at 09:57

3

foreach can be used too.

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

answered Sep 28 '21 at 09:57

sakirow

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score 2 · Answer 7 · answered May 05 '12 at 14:37

2

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).

answered May 05 '12 at 14:37

Mateusz Chromiński

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score 2 · Answer 8 · answered Sep 09 '13 at 17:37

2

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());
                }

answered Sep 09 '13 at 17:37

sachin

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Doesn't work. Clearly you haven't tried it. Calling `next()` twice in the loop means that you will print the odd-numbered keys along with the even-numbered values. – user207421 Jun 16 '20 at 10:28

score 1 · Answer 9 · answered Jul 24 '13 at 22:50

1

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") );

answered Jul 24 '13 at 22:50

tzg

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2

This method is totally redundant. – user207421 Jun 16 '20 at 10:28

score 1 · Answer 10 · edited Jun 16 '20 at 10:23

1

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]

edited Jun 16 '20 at 10:23

Steffen Harbich

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answered Jun 16 '20 at 08:15

ghanshyam singh

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`java,util.HashMap` is a class, not a package, and there is nothing here that wasn't here five years ago. – user207421 Jun 16 '20 at 10:30

score 0 · Answer 11 · answered Jun 10 '18 at 12:04

0

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];
}

answered Jun 10 '18 at 12:04

Orici

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Nothing about knowing the index in the question. – user207421 Jun 16 '20 at 10:31

score -1 · Answer 12 · edited Aug 05 '14 at 08:23

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);
   }    
}

score -1 · Answer 13 · answered May 04 '15 at 09:28

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);
        }
    }
}

Merely copies existing answers. -1 – james.garriss Nov 30 '15 at 12:11 — james.garriss, Nov 30 '15 at 12:11

score -2 · Answer 14 · answered Mar 22 '16 at 16:50

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 ;)

Emmanuel R · Answer 15 · 2017-05-31T09:31:39.683

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

Get keys from HashMap in Java

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