Get keys from HashMap in Java

Question

I have a Hashmap in Java like this   private Map lt String  Integer gt  team1   new HashMap lt String  Integer gt       Then I fill it like this   team1 put  United   5     How can I get the keys   Something like  team1 getKey   to return  United

User · Answer

To get keys in HashMap  We have keySet   method which is present in java util Hashmap package  ex     Map lt String String gt  map   new Hashmap lt String String gt     map put  key1   value1    map put  key2   value2        Now to get keys we can use keySet   on map object Set lt String gt  keys   map keySet      Now keys will have your all keys available in map  ex   key1 key2

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private Map lt String  Integer gt   map  new HashMap lt String  Integer gt     Iterator lt Map Entry lt String Integer gt  gt  itr    map entrySet   iterator                      please check                  while itr hasNext                                          System out println  key of     itr next   getKey     value of      Map  itr next   getValue

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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

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To get Key and its value     e g  private Map lt String  Integer gt  team1   new HashMap lt String  Integer gt       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

User · Answer

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

User · Answer

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 lt Object  Object gt  team1   new HashMap lt Object  Object gt      it s important that you use  lt Object  Object gt  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 lt String  Integer gt  team1   new HashMap lt String  Integer gt      private Map lt Integer  String gt  team1Keys   new HashMap lt Integer  String gt      and then  team1 put  United   5    team1Keys put 5   United     and remember  keep it simple

User · Answer

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

User · Answer

Use functional operation for faster iteration   team1 keySet   forEach  key  - gt          System out println key

User · Answer

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

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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

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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

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public class MyHashMapKeys        public static void main String a             HashMap lt String  String gt  hm   new HashMap lt String  String gt               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 lt String gt  keys   hm keySet            for String key  keys               System out println key

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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

User · Answer

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 lt String gt  findKey Integer value       return team1          entrySet            stream            filter e - gt  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 lt String gt  findKeys Integer value       return team1          entrySet            stream            filter e - gt  Objects equals e getValue    value            map Map Entry  getKey           collect Collectors toList        More  quot generic quot  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  lt K gt        Type of the key      param  lt T gt        Type of the value      return An optional containing a key  if found      public static  lt K  T gt  Optional lt K gt  findKey Map lt K  T gt  mapOrNull  T value        return Optional ofNullable mapOrNull  flatMap map - gt  map entrySet                stream                filter e - gt  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 lt String gt  findKeys Integer value      List lt String gt  keys   new ArrayList lt String gt        for String key   team1 keySet             if Objects equals team1 get key   value                 keys add key                   return keys

[java] Get keys from HashMap in Java

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