Reverse HashMap keys and values in Java

Question

It s a simple question  I have a simple HashMap of which i want to reverse the keys and values   HashMap lt Character  String gt  myHashMap   new HashMap lt Character  String gt     myHashMap put  a    test one    myHashMap put  b    test two      and I want to create a new HashMap in which i put the opposites   HashMap lt String  Character gt  reversedHashMap   new HashMap lt String  Character gt     e g  Keys  test one   amp   test two  and values  a   amp   b

User · Answer

I wrote a simpler loop that works too  note that all my values are unique    HashMap lt Character  String gt  myHashMap   new HashMap lt Character  String gt     HashMap lt String  Character gt  reversedHashMap   new HashMap lt String  Character gt      for  char i   myHashMap keySet          reversedHashMap put myHashMap get i   i

User · Answer

private  lt A  B gt  Map lt B  A gt  invertMap Map lt A  B gt  map        Map lt B  A gt  reverseMap   new HashMap lt  gt         for  Map Entry lt A  B gt  entry   map entrySet              reverseMap put entry getValue    entry getKey               return reverseMap      It s important to remember that put replaces the value when called with the same key  So if you map has two keys with the same value only one of them will exist in the inverted map

User · Answer

Tested with below sample snippet  tried with MapUtils  and Java8 Stream feature  It worked with both cases   public static void main String   args        Map lt String  String gt  test   new HashMap lt String  String gt         test put  a    1        test put  d    1        test put  b    2        test put  c    3        test put  d    4        test put  d    41         System out println test        Map lt String  String gt  test1   MapUtils invertMap test        System out println test1        Map lt String  String gt  mapInversed                test entrySet                   stream                   collect Collectors toMap Map Entry  getValue  Map Entry  getKey         System out println mapInversed      Output   a 1  b 2  c 3  d 41   1 a  2 b  3 c  41 d   1 a  2 b  3 c  41 d

User · Answer

If the values are not unique  the safe way to inverse the map is by using java 8 s groupingBy function  Map lt String  Integer gt  map   new HashMap lt  gt     map put  a  1   map put  b  2    Map lt Integer  List lt String gt  gt  mapInversed    map entrySet       stream       collect Collectors groupingBy Map Entry  getValue  Collectors mapping Map Entry  getKey  Collectors toList

User · Answer

Iterate through the list of keys and values  then add them   HashMap lt String  Character gt  reversedHashMap   new HashMap lt String  Character gt     for  String key   myHashMap keySet         reversedHashMap put myHashMap get key   key

User · Answer

They all are unique  yes   If you re sure that your values are unique you can iterate over the entries of your old map    Map lt String  Character gt  myNewHashMap   new HashMap lt  gt     for Map Entry lt Character  String gt  entry   myHashMap entrySet         myNewHashMap put entry getValue    entry getKey         Alternatively  you can use a Bi-Directional map like Guava provides and use the inverse   method    BiMap lt Character  String gt  myBiMap   HashBiMap create    myBiMap put  a    test one    myBiMap put  b    test two     BiMap lt String  Character gt  myBiMapInversed   myBiMap inverse      As java-8 is out  you can also do it this way    Map lt String  Integer gt  map   new HashMap lt  gt     map put  a  1   map put  b  2    Map lt Integer  String gt  mapInversed        map entrySet           stream           collect Collectors toMap Map Entry  getValue  Map Entry  getKey     Finally  I added my contribution to the proton pack library  which contains utility methods for the Stream API  With that you could do it like this   Map lt Character  String gt  mapInversed   MapStream of map  inverseMapping   collect

User · Answer

To answer your question on how you can do it  you could get the entrySet from your map and then just put into the new map by using getValue as key and getKey as value    But remember that keys in a Map are unique  which means if you have one value with two different key in your original map  only the second key  in iteration order  will be kep as value in the new map

User · Answer

Apache commons collections library provides a utility method for inversing the map  You can use this if you are sure that the values of myHashMap are unique  org apache commons collections MapUtils invertMap java util Map map    Sample code  HashMap lt String  Character gt  reversedHashMap   MapUtils invertMap myHashMap

[java] Reverse HashMap keys and values in Java

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