Java Hashmap How to get key from value

Question

If I have the value  foo   and a HashMap lt String gt  ftw for which ftw containsValue  foo   returns true  how can I get the corresponding key  Do I have to loop through the hashmap  What is the best way to do that

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It sounds like the best way is for you to iterate over entries using map entrySet   since map containsValue   probably does this anyway

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If you choose to use the Commons Collections library instead of the standard Java Collections framework  you can achieve this with ease  The BidiMap interface in the Collections library is a bi-directional map  allowing you to map a key to a value  like normal maps   and also to map a value to a key  thus allowing you to perform lookups in both directions  Obtaining a key for a value is supported by the getKey   method  There is a caveat though  bidi maps cannot have multiple values mapped to keys  and hence unless your data set has 1 1 mappings between keys and values  you cannot use bidi maps   If you want to rely on the Java Collections API  you will have to ensure the 1 1 relationship between keys and values at the time of inserting the value into the map  This is easier said than done  Once you can ensure that  use the entrySet   method to obtain the set of entries  mappings  in the Map  Once you have obtained the set whose type is Map Entry  iterate through the entries  comparing the stored value against the expected  and obtain the corresponding key   Support for bidi maps with generics can be found in Google Guava and the refactored Commons-Collections libraries  the latter is not an Apache project   Thanks to Esko for pointing out the missing generic support in Apache Commons Collections  Using collections with generics makes more maintainable code   Since version 4 0 the official Apache Commons Collections    library supports generics  See the summary page of the  quot org apache commons collections4 bidimap quot  package for the list of available implementations of the BidiMap  OrderedBidiMap and SortedBidiMap interfaces that now support Java generics

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Let the value be maxValue   Set keySet   map keySet     keySet stream   filter x- gt map get x   maxValue  forEach x- gt  System out println x

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Use a thin wrapper  HMap  import java util Collections  import java util HashMap  import java util Map   public class HMap lt K  V gt        private final Map lt K  Map lt K  V gt  gt  map      public HMap           map   new HashMap lt K  Map lt K  V gt  gt              public HMap final int initialCapacity          map   new HashMap lt K  Map lt K  V gt  gt  initialCapacity            public boolean containsKey final Object key          return map containsKey key            public V get final Object key          final Map lt K  V gt  entry   map get key         if  entry    null           return entry values   iterator   next          return null           public K getKey final Object key          final Map lt K  V gt  entry   map get key         if  entry    null           return entry keySet   iterator   next          return null           public V put final K key  final V value          final Map lt K  V gt  entry   map              put key  Collections singletonMap key  value          if  entry    null           return entry values   iterator   next          return null

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You could insert both the key value pair and its inverse into your map structure  map put  theKey    theValue    map put  theValue    theKey      Using map get  theValue   will then return  theKey    It s a quick and dirty way that I ve made constant maps  which will only work for a select few datasets    Contains only 1 to 1 pairs Set of values is disjoint from the set of keys  1- 2  2- 3 breaks it

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I think this is best solution  original address  Java2s      import java util HashMap      import java util Map           public class Main              public static void main String   argv                Map lt String  String gt  map   new HashMap lt String  String gt                 map put  1   one                map put  2   two                map put  3   three                map put  4   four                 System out println getKeyFromValue map  three                      hm is the map you are trying to get value from it           public static Object getKeyFromValue Map hm  Object value                for  Object o   hm keySet                    if  hm get o  equals value                     return o                                            return null                          An easy usage   if you put all data in hasMap and you have item    Automobile   so you are looking its key in hashMap  that is good solution    getKeyFromValue hashMap  item   System out println  getKeyFromValue hashMap  item     getKeyFromValue hashMap  item

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Iterator lt Map Entry lt String String gt  gt  iterator   map entrySet   iterator    while  iterator hasNext          Map Entry lt String String gt  entry   iterator next        if  entry getValue   equals value you look for             String key you look for   entry getKey

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There is no unambiguous answer  because multiple keys can map to the same value   If you are enforcing unique-ness with your own code  the best solution is to create a class that uses two Hashmaps to track the mappings in both directions

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Using Java 8   ftw forEach  key  value  - gt        if  value equals  foo              System out print key

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My 2 cents   You can get the keys in an array and then loop through the array  This will affect performance of this code block if the map is pretty big   where in you are getting the keys in an array first which might consume some time and then you are looping  Otherwise for smaller maps it should be ok   String   keys    yourMap keySet   toArray new String 0     for int i   0   i  lt  keys length   i           This is your key         String key   keys i          This is your value     yourMap get key

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public static String getKey Map lt String  Integer gt  mapref  String value        String key           for  Map Entry lt String  Integer gt  map   mapref entrySet              if  map getValue   toString   equals value                 key   map getKey                        return key

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lambda w o use of external libraries creates the inverted map of values to keys can deal with multiple values for one key  in difference to the BidiMap  public static List lt String gt  getKeysByValue Map lt String String gt  map  String value      List lt String gt  list   map keySet   stream          collect groupingBy k - gt  map get  k     get  value      return  list    null   Collections emptyList     list       gets a list containing the key s  mapping value for an 1 1 mapping the returned list is empty or contains value

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try this   static String getKeyFromValue LinkedHashMap lt String  String gt  map String value        for  int x 0 x lt map size   x             if  String valueOf   new ArrayList lt String gt  map values     get x    equals value               return String valueOf  new ArrayList lt String gt  map keySet     get x              return null

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You can use the below   public class HashmapKeyExist       public static void main String   args            HashMap lt String  String gt  hmap   new HashMap lt String  String gt             hmap put  1    Bala            hmap put  2    Test             Boolean cantain   hmap containsValue  Bala            if hmap containsKey  2    amp  amp  hmap containsValue  Test                          System out println  Yes                      if cantain    true                        System out println  Yes                        Set setkeys   hmap keySet            Iterator it   setkeys iterator             while it hasNext                          String key    String  it next                if  hmap get key  equals  Bala                                  System out println key

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As far as I know keys and values of a HashMap are not mixed when you represent them as arrays   hashmap values   toArray     and  hashmap keySet   toArray     So the following code  since java 8  should work as expected   public Object getKeyByFirstValue Object value        int keyNumber    Arrays asList hashmap values   toArray    indexOf value       return hashmap keySet   toArray   keyNumber       However   WARNING   it works 2-3 times slower than iteration

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If you want to get key from value  its best to use bidimap  bi-directional maps    you can get key from value in O 1  time   But  the drawback with this is you can only use unique keyset and valueset  There is a data structure called Table in java  which is nothing but map of maps like  Table lt  A  B   C      map  lt  A   map  lt  B  C      Here you can get map lt B C gt  by querying T row a    and you can also get map lt A C gt  by querying T column b     In your special case  insert C as some constant   So  it like   lt  a1  b1  1       lt  a2  b2   1          So  if you find via T row a1  ---  returns map of  --  get keyset  this returned map   If you need to find key value then  T column b2  --  returns map of  --  get keyset of returned map   Advantages over the previous case      Can use multiple values  More efficient when using large data sets

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import java util HashMap  import java util HashSet  import java util Set   public class ValueKeysMap lt K  V gt  extends HashMap  lt K V gt       HashMap lt V  Set lt K gt  gt  ValueKeysMap   new HashMap lt V  Set lt K gt  gt           Override     public boolean containsValue Object value            return ValueKeysMap containsKey value               Override     public V put K key  V value            if  containsValue value                 Set lt K gt  keys   ValueKeysMap get value               keys add key             else               Set lt K gt  keys   new HashSet lt K gt                 keys add key               ValueKeysMap put value  keys                     return super put key  value               Override     public V remove Object key            V value   super remove key           Set lt K gt  keys   ValueKeysMap get value           keys remove key           if keys size      0               ValueKeysMap remove value                     return value             public Set lt K gt  getKeys4ThisValue V value           Set lt K gt  keys   ValueKeysMap get value           return keys             public boolean valueContainsThisKey K key  V value           if  containsValue value                 Set lt K gt  keys   ValueKeysMap get value               return keys contains key                     return false                       Take care of argument constructor and other api s like putAll

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If you build the map in your own code  try putting the key and value in the map together   public class KeyValue       public Object key      public Object value      public KeyValue Object key  Object value             map put key  new KeyValue key  value      Then when you have a value  you also have the key

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This method gets the Key for the given Value     param paramName     return     private String getKeyForValueFromMap String paramName        String keyForValue   null      if paramName  null             Set lt Entry lt String String gt  gt  entrySet   myMap   entrySet            if entrySet  null  amp  amp  entrySet size gt 0                for Entry lt String String gt  entry   entrySet                    if entry  null  amp  amp  paramName equalsIgnoreCase entry getValue                           keyForValue   entry getKey                                                        return keyForValue

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For Android development targeting API  lt  19  Vitalii Fedorenko one-to-one relationship solution doesn t work because Objects equals isn t implemented  Here s a simple alternative   public  lt K  V gt  K getKeyByValue Map lt K  V gt  map  V value        for  Map Entry lt K  V gt  entry   map entrySet                  if  value equals entry getValue                   return entry getKey                        return null

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If your data structure has many-to-one mapping between keys and values you should iterate over entries and pick all suitable keys   public static  lt T  E gt  Set lt T gt  getKeysByValue Map lt T  E gt  map  E value        Set lt T gt  keys   new HashSet lt T gt         for  Entry lt T  E gt  entry   map entrySet              if  Objects equals value  entry getValue                   keys add entry getKey                         return keys      In case of one-to-one relationship  you can return the first matched key   public static  lt T  E gt  T getKeyByValue Map lt T  E gt  map  E value        for  Entry lt T  E gt  entry   map entrySet              if  Objects equals value  entry getValue                   return entry getKey                        return null      In Java 8   public static  lt T  E gt  Set lt T gt  getKeysByValue Map lt T  E gt  map  E value        return map entrySet                  stream                  filter entry - gt  Objects equals entry getValue    value                  map Map Entry  getKey                 collect Collectors toSet         Also  for Guava users  BiMap may be useful  For example   BiMap lt Token  Character gt  tokenToChar        ImmutableBiMap of Token LEFT BRACKET       Token LEFT PARENTHESIS        Token token   tokenToChar inverse   get       Character c   tokenToChar get token

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You can get the key using values using following code    ArrayList valuesList   new ArrayList    Set keySet   initalMap keySet    ArrayList keyList   new ArrayList keySet    for int i   0   i  lt  keyList size     i           valuesList add initalMap get keyList get i        Collections sort valuesList   Map finalMap   new TreeMap    for int i   0   i  lt  valuesList size     i           String value    String  valuesList get i        for  int j   0   j  lt  keyList size     j               if initalMap get keyList get j   equals value                 finalMap put keyList get j  value                        System out println  fianl map ---------------------- gt       finalMap

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import java util ArrayList  import java util HashMap  import java util Iterator  import java util List  import java util Set   public class M  public static void main String   args             HashMap lt String  List lt String gt  gt  resultHashMap   new HashMap lt String  List lt String gt  gt              Set lt String gt  newKeyList   resultHashMap keySet              for  Iterator lt String gt  iterator   originalHashMap keySet   iterator    iterator hasNext                   String hashKey    String  iterator next                 if   newKeyList contains originalHashMap get hashKey                      List lt String gt  loArrayList   new ArrayList lt String gt                     loArrayList add hashKey                   resultHashMap put originalHashMap get hashKey   loArrayList                 else                   List lt String gt  loArrayList   resultHashMap get originalHashMap                          get hashKey                    loArrayList add hashKey                   resultHashMap put originalHashMap get hashKey   loArrayList                                    System out println  Original HashMap       originalHashMap           System out println  Result HashMap       resultHashMap

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While this does not directly answer the question  it is related   This way you don t need to keep creating iterating  Just create a reverse map once and get what you need          Both key and value types must define equals   and hashCode   for this to work     This takes into account that all keys are unique but all values may not be         param map     param  lt K gt      param  lt V gt      return     public static  lt K  V gt  Map lt V  List lt K gt  gt  reverseMap Map lt K V gt  map        if map    null  return null       Map lt V  List lt K gt  gt  reverseMap   new ArrayMap lt  gt          for Map Entry lt K V gt  entry   map entrySet              appendValueToMapList reverseMap  entry getValue    entry getKey                return reverseMap             Takes into account that the list may already have values          param map     param key     param value     param  lt K gt      param  lt V gt      return     public static  lt K  V gt  Map lt K  List lt V gt  gt  appendValueToMapList Map lt K  List lt V gt  gt  map  K key  V value        if map    null    key    null    value    null  return map       List lt V gt  list   map get key        if list    null            List lt V gt  newList   new ArrayList lt  gt             newList add value           map put key  newList             else           list add value              return map

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I m afraid you ll just have to iterate your map  Shortest I could come up with   Iterator lt Map Entry lt String String gt  gt  iter   map entrySet   iterator    while  iter hasNext          Map Entry lt String String gt  entry   iter next        if  entry getValue   equals value you look for             String key you look for   entry getKey

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It s important to note that since this question  Apache Collections supports Generic BidiMaps   So a few of the top voted answers are no longer accurate on that point   For a Serialized BidiMap that also supports duplicate values   1-to-many scenario   also consider MapDB org

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I think keySet   may be well to find the keys mapping to the value  and have a better coding style than entrySet     Ex   Suppose you have a HashMap map  ArrayList res  a value you want to find all the key mapping to   then store keys to the res   You can write code below       for  int key   map keySet              if  map get key     value                res add key                     rather than use entrySet   below       for  Map Entry s   map entrySet              if   int s getValue      value                res add  int s getKey                       Hope it helps

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I think your choices are   Use a map implementation built for this  like the BiMap from google collections   Note that the google collections BiMap requires uniqueless of values  as well as keys  but it provides high performance in both directions performance Manually maintain two maps - one for key -  value  and another map for value -  key Iterate through the entrySet   and to find the keys which match the value   This is the slowest method  since it requires iterating through the entire collection  while the other two methods don t require that

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To find all the keys that map to that value  iterate through all the pairs in the hashmap  using map entrySet

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for int key  hm keySet          if hm get key  equals value             System out println key

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Yes  you have to loop through the hashmap  unless you implement something along the lines of what these various answers suggest   Rather than fiddling with the entrySet  I d just get the keySet    iterate over that set  and keep the  first  key that gets you your matching value   If you need all the keys that match that value  obviously you have to do the whole thing   As Jonas suggests  this might already be what the containsValue method is doing  so you might just skip that test all-together  and just do the iteration every time  or maybe the compiler will already eliminate the redundancy  who knows    Also  relative to the other answers  if your reverse map looks like  Map lt Value  Set lt Key gt  gt    you can deal with non-unique key- value mappings  if you need that capability  untangling them aside    That would incorporate fine into any of the solutions people suggest here using two maps

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In java8  map entrySet   stream   filter entry - gt  entry getValue   equals value        forEach entry - gt  System out println entry getKey

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public static class SmartHashMap  lt T1 extends Object  T2 extends Object gt        public HashMap lt T1  T2 gt  keyValue      public HashMap lt T2  T1 gt  valueKey       public SmartHashMap            this keyValue   new HashMap lt T1  T2 gt             this valueKey   new HashMap lt T2  T1 gt                public void add T1 key  T2 value           this keyValue put key  value           this valueKey put value  key              public T2 getValue T1 key           return this keyValue get key              public T1 getKey T2 value           return this valueKey get value

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Decorate map with your own implementation  class MyMap lt K V gt  extends HashMap lt K  V gt        Map lt V K gt  reverseMap   new HashMap lt V K gt           Override     public V put K key  V value               TODO Auto-generated method stub         reverseMap put value  key           return super put key  value              public K getKey V value           return reverseMap get value

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public class NewClass1        public static void main String   args           Map lt Integer  String gt  testMap   new HashMap lt Integer  String gt             testMap put 10   a            testMap put 20   b            testMap put 30   c            testMap put 40   d            for  Entry lt Integer  String gt  entry   testMap entrySet                  if  entry getValue   equals  c                      System out println entry getKey                                       Some additional info    May be useful to you  Above method may not be good if your hashmap is really big  If your hashmap contain unique key to unique value mapping  you can maintain one more hashmap that contain mapping from Value to Key   That is you have to maintain two hashmaps  1  Key to value  2  Value to key    In that case you can use second hashmap to get key

[java] Java Hashmap: How to get key from value?

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