Get keys from HashMap in Java

问题:

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".

回答1:

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".

回答2:

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.

回答3:

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> getKey(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> getKeys(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> getKey(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 getKey(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> getKeys(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;
}

回答4:

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.

回答5:

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());
                }

回答6:

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).

回答7:

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") );

回答8:

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];
}

回答9:

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);
   }    
}

回答10:

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);
        }
    }
}

回答11:

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 ;)

回答12:

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