How can I combine two HashMap objects containing the same types

Question

I have two HashMap objects defined like so   HashMap lt String  Integer gt  map1   new HashMap lt String  Integer gt     HashMap lt String  Integer gt  map2   new HashMap lt String  Integer gt       I also have a third HashMap object   HashMap lt String  Integer gt  map3    How can I merge map1 and map2 together into map3

User · Answer

You could use Collection addAll   for other types  e g  List  Set  etc  For Map  you can use putAll

User · Answer

Below snippet takes more than one map and combine them    private static  lt K  V gt  Map lt K  V gt  combineMaps Map lt K  V gt     maps            if  maps    null    maps length    0                return Collections EMPTY MAP                     Map lt K  V gt  result   new HashMap lt  gt              for  Map lt K  V gt  map   maps                result putAll map                     return result          Demo example link

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You can use putAll function for Map as explained in the code below  HashMap lt String  Integer gt  map1   new HashMap lt String  Integer gt     map1 put  a   1   map1 put  b   2   map1 put  c   3   HashMap lt String  Integer gt  map2   new HashMap lt String  Integer gt     map1 put  aa   11   map1 put  bb   12   HashMap lt String  Integer gt  map3   new HashMap lt String  Integer gt     map3 putAll map1   map3 putAll map2   map3 keySet   stream   forEach System out  println   map3 values   stream   forEach System out  println

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Generic solution for combining two maps which can possibly share common keys   In-place   public static  lt K  V gt  void mergeInPlace Map lt K  V gt  map1  Map lt K  V gt  map2          BinaryOperator lt V gt  combiner        map2 forEach  k  v  - gt  map1 merge k  v  combiner  apply        Returning a new map   public static  lt K  V gt  Map lt K  V gt  merge Map lt K  V gt  map1  Map lt K  V gt  map2          BinaryOperator lt V gt  combiner        Map lt K  V gt  map3   new HashMap lt  gt  map1       map2 forEach  k  v  - gt  map3 merge k  v  combiner  apply        return map3

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If you don t need mutability for your final map  there is Guava s ImmutableMap with its Builder and putAll method which  in contrast to Java s Map interface method  can be chained   Example of use   Map lt String  Integer gt  mergeMyTwoMaps Map lt String  Integer gt  map1  Map lt String  Integer gt  map2      return ImmutableMap  lt String  Integer gt builder          putAll map1         putAll map2         build        Of course  this method can be more generic  use varargs and loop to putAll Maps from arguments etc  but I wanted to show a concept   Also  ImmutableMap and its Builder have few limitations  or maybe features      they are null hostile  throw NullPointerException - if any key or value in map is null  Builder don t accept duplicate keys  throws IllegalArgumentException if duplicate keys were added

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Method 1  Put maps in a List and then join public class Test15   public static void main String   args         Map lt String  List lt String gt  gt  map1   new HashMap lt  gt         map1 put  quot London quot   Arrays asList  quot A quot    quot B quot    quot C quot         map1 put  quot Wales quot   Arrays asList  quot P1 quot    quot P2 quot    quot P3 quot          Map lt String  List lt String gt  gt  map2   new HashMap lt  gt         map2 put  quot Calcutta quot   Arrays asList  quot Protijayi quot    quot Gina quot    quot Gini quot         map2 put  quot London quot   Arrays asList   quot P4 quot    quot P5 quot    quot P6 quot         map2 put  quot Wales quot   Arrays asList   quot P111 quot    quot P5555 quot    quot P677666 quot              System out println map1  System out println map2                         put the maps in an ArrayList          List lt Map lt String  List lt String gt  gt  gt  maplist   new ArrayList lt Map lt String List lt String gt  gt  gt         maplist add map1       maplist add map2           lt T K U gt  Collector lt T   Map lt K U gt  gt  toMap                                     Function lt   super T   extends K gt  keyMapper                                     Function lt   super T   extends U gt  valueMapper                                     BinaryOperator lt U gt  mergeFunction               Map lt String  List lt String gt  gt  collect   maplist stream        flatMap ch - gt  ch entrySet   stream         collect              Collectors toMap                             keyMapper                           Entry  getKey                             valueMapper             Entry  getValue                              mergeFunction       list a list b  - gt  Stream concat list a stream    list b stream    collect Collectors toList                                                    System out println  quot Final Result Map after join    gt   quot    collect               Wales  P1  P2  P3   London  A  B  C    Calcutta  Protijayi  Gina  Gini   Wales  P111  P5555  P677666   London  P4  P5  P6   Final Result Map after join    gt   Calcutta  Protijayi  Gina  Gini   Wales  P1  P2  P3  P111  P5555  P677666   London  A  B  C  P4  P5  P6              main      Method 2   Normal Map merge public class Test15   public static void main String   args         Map lt String  List lt String gt  gt  map1   new HashMap lt  gt         map1 put  quot London quot   Arrays asList  quot A quot    quot B quot    quot C quot         map1 put  quot Wales quot   Arrays asList  quot P1 quot    quot P2 quot    quot P3 quot          Map lt String  List lt String gt  gt  map2   new HashMap lt  gt         map2 put  quot Calcutta quot   Arrays asList  quot Protijayi quot    quot Gina quot    quot Gini quot         map2 put  quot London quot   Arrays asList   quot P4 quot    quot P5 quot    quot P6 quot         map2 put  quot Wales quot   Arrays asList   quot P111 quot    quot P5555 quot    quot P677666 quot              System out println map1  System out println map2                           lt T K U gt  Collector lt T   Map lt K U gt  gt  toMap                                     Function lt   super T   extends K gt  keyMapper                                     Function lt   super T   extends U gt  valueMapper                                     BinaryOperator lt U gt  mergeFunction                   Map lt String  List lt String gt  gt  collect   Stream of map1 map2       flatMap ch - gt  ch entrySet   stream         collect              Collectors toMap                             keyMapper                           Entry  getKey                             valueMapper             Entry  getValue                              mergeFunction       list a list b  - gt  Stream concat list a stream    list b stream    collect Collectors toList                                                    System out println  quot Final Result Map after join    gt   quot    collect               Wales  P1  P2  P3   London  A  B  C    Calcutta  Protijayi  Gina  Gini   Wales  P111  P5555  P677666   London  P4  P5  P6   Final Result Map after join    gt   Calcutta  Protijayi  Gina  Gini   Wales  P1  P2  P3  P111  P5555  P677666   London  A  B  C  P4  P5  P6               main           In Python  HashMap is called Dictionary and we can merge them very easily  x     Roopa   1   Tabu   2  y     Roopi   3   Soudipta   4    z      x   y  print z     Roopa   1   Tabu   2   Roopi   3   Soudipta   4

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One-liner using Java 8 Stream API   map3   Stream of map1  map2  flatMap m - gt  m entrySet   stream            collect Collectors toMap Entry  getKey  Entry  getValue     Among the benefits of this method is ability to pass a merge function  which will deal with values that have the same key  for example   map3   Stream of map1  map2  flatMap m - gt  m entrySet   stream            collect Collectors toMap Entry  getKey  Entry  getValue  Math  max

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you can use HashMap lt String  List lt Integer gt  gt  to merge both hashmaps and avoid losing elements paired with the same key   HashMap lt String  Integer gt  map1   new HashMap lt  gt     HashMap lt String  Integer gt  map2   new HashMap lt  gt     map1 put  key1   1   map1 put  key2   2   map1 put  key3   3   map2 put  key1   4   map2 put  key2   5   map2 put  key3   6   HashMap lt String  List lt Integer gt  gt  map3   new HashMap lt  gt     map1 forEach  str  num  - gt  map3 put str  new ArrayList lt  gt  Arrays asList num        checking for each key if its already in the map  and if so  you just add the integer to the list paired with this key for  Map Entry lt String  Integer gt  entry   map2 entrySet          Integer value   entry getValue        String key   entry getKey        if  map3 containsKey key             map3 get key  add value         else           map3 put key  new ArrayList lt  gt  Arrays asList value             map3 forEach  str  list  - gt  System out println       str          list            output    key1   1  4    key2   2  5    key3   3  6

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A small snippet I use very often to create map from other maps   static public  lt K  V gt  Map lt K  V gt  merge Map lt K  V gt     args        final Map lt K  V gt  buffer   new HashMap lt  gt          for  Map m   args            buffer putAll m              return buffer

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Very late but let me share what I did when I had the same issue   Map lt String  List lt String gt  gt  map1   new HashMap lt  gt     map1 put  India   Arrays asList  Virat    Mahi    Rohit     map1 put  NZ   Arrays asList  P1   P2   P3      Map lt String  List lt String gt  gt  map2   new HashMap lt  gt     map2 put  India   Arrays asList  Virat    Mahi    Rohit     map2 put  NZ   Arrays asList  P1   P2   P4      Map lt String  List lt String gt  gt  collect4   Stream of map1  map2                   flatMap map - gt  map entrySet   stream                     collect                          Collectors toMap                                  Map Entry  getKey                                  Map Entry  getValue                                   strings  strings2  - gt                                        List lt String gt  newList   new ArrayList lt  gt                                         newList addAll strings                                       newList addAll strings2                                       return newList                                                                                 collect4 forEach  s  strings  - gt  System out println s  - gt   strings      It gives the following output  NZ- gt  P1  P2  P3  P1  P2  P4  India- gt  Virat  Mahi  Rohit  Virat  Mahi  Rohit

User · Answer

you can use  - addAll method   http   download oracle com javase 6 docs api java util HashMap html  But there is always this issue that - if your two hash maps have any key same - then it will override the value of the key from first hash map with the value of the key from second hash map    For being on safer side - change the key values - you can use prefix or suffix on the keys -   different prefix suffix for first hash map and different prefix suffix for second hash map

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HashMap has a putAll method   http   download oracle com javase 6 docs api java util HashMap html

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map3   new HashMap lt  gt      map3 putAll map1   map3 putAll map2

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HashMap lt Integer String gt  hs1   new HashMap lt  gt         hs1 put 1  ram        hs1 put 2  sita        hs1 put 3  laxman        hs1 put 4  hanuman        hs1 put 5  geeta         HashMap lt Integer String gt  hs2   new HashMap lt  gt         hs2 put 5  rat        hs2 put 6  lion        hs2 put 7  tiger        hs2 put 8  fish        hs2 put 9  hen         HashMap lt Integer String gt  hs3   new HashMap lt  gt      Map is which we add      hs3 putAll hs1       hs3 putAll hs2        System out println   hs1       hs1       System out println   hs2       hs2       System out println   hs3       hs3     Duplicate items will not be added that is duplicate keys  as when we will print hs3 we will get only one value for key 5 which will be last value added and it will be rat     Set has a property of not allowing the duplicate key but values can be duplicate

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Java 8 alternative one-liner for merging two maps   defaultMap forEach  k  v  - gt  destMap putIfAbsent k  v      The same with method reference   defaultMap forEach destMap  putIfAbsent     Or idemponent for original maps solution with third map   Map lt String  Integer gt  map3   new HashMap lt String  Integer gt  map2   map1 forEach map3  putIfAbsent     And here is a way to merge two maps into fast immutable one with Guava that does least possible intermediate copy operations   ImmutableMap Builder lt String  Integer gt  builder   ImmutableMap  lt String  Integer gt builder    builder putAll map1   map2 forEach  k  v  - gt   if   map1 containsKey k   builder put k  v      ImmutableMap lt String  Integer gt  map3   builder build      See also Merge two maps with Java 8 for cases when values present in both maps need to be combined with mapping function

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If you know you don t have duplicate keys  or you want values in map2 to overwrite values from map1 for duplicate keys  you can just write  map3   new HashMap lt  gt  map1   map3 putAll map2     If you need more control over how values are combined  you can use Map merge  added in Java 8  which uses a user-provided BiFunction to merge values for duplicate keys   merge operates on individual keys and values  so you ll need to use a loop or Map forEach   Here we concatenate strings for duplicate keys   map3   new HashMap lt  gt  map1   for  Map Entry lt String  String gt  e   map2 entrySet        map3 merge e getKey    e getValue    String  concat     or instead of the above loop map2 forEach  k  v  - gt  map3 merge k  v  String  concat      If you know you don t have duplicate keys and want to enforce it  you can use a merge function that throws an AssertionError   map2 forEach  k  v  - gt      map3 merge k  v   v1  v2  - gt           throw new AssertionError  duplicate values for key    k         Taking a step back from this specific question  the Java 8 streams library provides toMap and groupingBy Collectors   If you re repeatedly merging maps in a loop  you may be able to restructure your computation to use streams  which can both clarify your code and enable easy parallelism using a parallel stream and concurrent collector

[java] How can I combine two HashMap objects containing the same types?

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