Java How to convert List to Map

Question

Recently I have conversation with a colleague about what would be the optimal way to convert List to Map in Java and if there any specific benefits of doing so   I want to know optimal conversion approach and would really appreciate if any one can guide me    Is this good approach   List lt Object   gt  results  Map lt Integer  String gt  resultsMap   new HashMap lt Integer  String gt     for  Object   o   results        resultsMap put  Integer  o 0    String  o 1

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Just in case this question isn t closed as a duplicate  the right answer is to use Google Collections   Map lt String Role gt  mappedRoles   Maps uniqueIndex yourList  new Function lt Role String gt        public String apply Role from        return from getName       or something else

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Without java-8  you ll be able to do this in one line Commons collections  and the Closure class  List lt Item gt  list   SuppressWarnings  unchecked   Map lt Key  Item gt  map    new HashMap lt Key  Item gt  gt          CollectionUtils forAllDo list  new Closure              Override         public void execute Object input                Item item    Item  input              put i getKey    item

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You can leverage the streams API of Java 8   public class ListToMap      public static void main String   args        List lt User gt  items   Arrays asList new User  One    new User  Two    new User  Three          Map lt String  User gt  map   createHashMap items       for String key   map keySet            System out println key        map get key                 public static Map lt String  User gt  createHashMap List lt User gt  items        Map lt String  User gt  map   items stream   collect Collectors toMap User  getId  Function identity          return map          For more details visit  http   codecramp com java-8-streams-api-convert-list-map

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Alexis has already posted an answer in Java 8 using method toMap keyMapper  valueMapper   As per doc for this method implementation      There are no guarantees on the type  mutability  serializability  or   thread-safety of the Map returned    So in case we are interested in a specific implementation of Map interface e g  HashMap then we can use the overloaded form as   Map lt String  Item gt  map2                   itemList stream   collect Collectors toMap Item  getKey    key for map                         Function identity          value for map                          o n  - gt  o                 merge function in case of conflict with keys                         HashMap  new               map factory - we want HashMap and not any Map implementation   Though using either Function identity   or i- gt i is fine but it seems Function identity   instead of i - gt  i might save some memory as per this related answer

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Since Java 8  the answer by  ZouZou using the Collectors toMap collector is certainly the idiomatic way to solve this problem   And as this is such a common task  we can make it into a static utility   That way the solution truly becomes a one-liner          Returns a map where each entry is an item of   code list  mapped by the    key produced by applying   code mapper  to the item         param list the list to map     param mapper the function to produce the key from a list item     return the resulting map     throws IllegalStateException on duplicate key     public static  lt K  T gt  Map lt K  T gt  toMapBy List lt T gt  list          Function lt   super T    extends K gt  mapper        return list stream   collect Collectors toMap mapper  Function identity          And here s how you would use it on a List lt Student gt    Map lt Long  Student gt  studentsById   toMapBy students  Student  getId

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With java-8  you ll be able to do this in one line using streams  and the Collectors class   Map lt String  Item gt  map        list stream   collect Collectors toMap Item  getKey  item - gt  item      Short demo   import java util Arrays  import java util List  import java util Map  import java util stream Collectors   public class Test      public static void main  String    args           List lt Item gt  list   IntStream rangeClosed 1  4                                      mapToObj Item  new                                      collect Collectors toList        Item  i 1   Item  i 2   Item  i 3   Item  i 4            Map lt String  Item gt  map                list stream   collect Collectors toMap Item  getKey  item - gt  item             map forEach  k  v  - gt  System out println k       gt      v            class Item        private final int i       public Item int i           this i   i             public String getKey            return  Key-  i              Override     public String toString             return  Item  i     i                  Output   Key-1   gt  Item  i 1  Key-2   gt  Item  i 2  Key-3   gt  Item  i 3  Key-4   gt  Item  i 4    As noted in comments  you can use Function identity   instead of item - gt  item  although I find i - gt  i rather explicit   And to be complete note that you can use a binary operator if your function is not bijective  For example let s consider this List and the mapping function that for an int value  compute the result of it modulo 3   List lt Integer gt  intList   Arrays asList 1  2  3  4  5  6   Map lt String  Integer gt  map        intList stream   collect toMap i - gt  String valueOf i   3   i - gt  i      When running this code  you ll get an error saying java lang IllegalStateException  Duplicate key 1  This is because 1   3 is the same as 4   3 and hence have the same key value given the key mapping function  In this case you can provide a merge operator   Here s one that sum the values   i1  i2  - gt  i1   i2  that can be replaced with the method reference Integer  sum   Map lt String  Integer gt  map        intList stream   collect toMap i - gt  String valueOf i   3                                       i - gt  i                                      Integer  sum      which now outputs   0   gt  9  i e 3   6  1   gt  5  i e 1   4  2   gt  7  i e 2   5    Hope it helps

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like already said  in java-8 we have the concise solution by Collectors     list stream   collect           groupingBy Item  getKey              and also  you can nest multiple group passing an other groupingBy method as second parameter     list stream   collect           groupingBy Item  getKey  groupingBy Item  getOtherKey               In this way  we ll have multi level map  like this  Map lt key  Map lt key  List lt Item gt  gt  gt

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Here s a little method I wrote for exactly this purpose   It uses Validate from Apache Commons   Feel free to use it          Converts a  lt code gt List lt  code gt  to a map  One of the methods of the list is called to retrive    the value of the key to be used and the object itself from the list entry is used as the    objct  An empty  lt code gt Map lt  code gt  is returned upon null input     Reflection is used to retrieve the key from the object instance and method name passed in         param  lt K gt  The type of the key to be used in the map     param  lt V gt  The type of value to be used in the map and the type of the elements in the               collection     param coll The collection to be converted      param keyType The class of key     param valueType The class of the value     param keyMethodName The method name to call on each instance in the collection to retrieve               the key     return A map of key to value instances     throws IllegalArgumentException if any of the other paremeters are invalid      public static  lt K  V gt  Map lt K  V gt  asMap final java util Collection lt V gt  coll          final Class lt K gt  keyType          final Class lt V gt  valueType          final String keyMethodName         final HashMap lt K  V gt  map   new HashMap lt K  V gt         Method method   null       if  isEmpty coll   return map      notNull keyType  Messages getString KEY TYPE NOT NULL        notNull valueType  Messages getString VALUE TYPE NOT NULL        notEmpty keyMethodName  Messages getString KEY METHOD NAME NOT NULL         try              return the Method to invoke to get the key for the map         method   valueType getMethod keyMethodName             catch  final NoSuchMethodException e            final String message               String format                      Messages getString METHOD NOT FOUND                       keyMethodName                      valueType           e fillInStackTrace            logger error message  e           throw new IllegalArgumentException message  e             try           for  final V value   coll                 Object object              object   method invoke value                SuppressWarnings  unchecked               final K key    K  object              map put key  value                       catch  final Exception e            final String message               String format                      Messages getString METHOD CALL FAILED                       method                      valueType           e fillInStackTrace            logger error message  e           throw new IllegalArgumentException message  e             return map

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I like Kango V s answer  but I think it s too complex   I think this is simpler - maybe too simple   If inclined  you could replace String with a Generic marker  and make it work for any Key type   public static  lt E gt  Map lt String  E gt  convertListToMap Collection lt E gt  sourceList  ListToMapConverterInterface lt E gt  converterInterface        Map lt String  E gt  newMap   new HashMap lt String  E gt         for  E item   sourceList             newMap put  converterInterface getKeyForItem  item    item              return newMap     public interface ListToMapConverterInterface lt E gt        public String getKeyForItem E item       Used like this           Map lt String  PricingPlanAttribute gt  pricingPlanAttributeMap   convertListToMap  pricingPlanAttributeList                  new ListToMapConverterInterface lt PricingPlanAttribute gt                            Override                     public String getKeyForItem PricingPlanAttribute item                            return item getFullName

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List lt Item gt  list  Map lt Key Item gt  map   new HashMap lt Key Item gt     for  Item i   list  map put i getKey   i     Assuming of course that each Item has a getKey   method that returns a key of the proper type

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A List and Map are conceptually different  A List is an ordered collection of items  The items can contain duplicates  and an item might not have any concept of a unique identifier  key   A Map has values mapped to keys  Each key can only point to one value   Therefore  depending on your List s items  it may or may not be possible to convert it to a Map  Does your List s items have no duplicates  Does each item have a unique key  If so then it s possible to put them in a Map

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Short and sweet  Using Java 8 you can do following   Map lt Key  Value gt  result  results                         stream                           collect Collectors toMap Value  getName Function identity       Value can be any object you use

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Using java-8 streams  results stream   collect Collectors toMap e- gt   Integer e 0    e- gt  String e 1

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A Java 8 example to convert a List lt   gt  of objects into a Map lt k  v gt    List lt Hosting gt  list   new ArrayList lt  gt     list add new Hosting 1   liquidweb com   new Date      list add new Hosting 2   linode com   new Date      list add new Hosting 3   digitalocean com   new Date         example 1 Map lt Integer  String gt  result1   list stream   collect      Collectors toMap Hosting  getId  Hosting  getName     System out println  Result 1       result1      example 2 Map lt Integer  String gt  result2   list stream   collect      Collectors toMap x - gt  x getId    x - gt  x getName        Code copied from  https   www mkyong com java8 java-8-convert-list-to-map

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Apache Commons MapUtils populateMap If you don t use Java 8 and you don t want to use a explicit loop for some reason  try MapUtils populateMap from Apache Commons  MapUtils populateMap Say you have a list of Pairs  List lt ImmutablePair lt String  String gt  gt  pairs   ImmutableList of      new ImmutablePair lt  gt   quot A quot    quot aaa quot        new ImmutablePair lt  gt   quot B quot    quot bbb quot       And you now want a Map of the Pair s key to the Pair object  Map lt String  Pair lt String  String gt  gt  map   new HashMap lt  gt     MapUtils populateMap map  pairs  new Transformer lt Pair lt String  String gt   String gt          Override   public String transform Pair lt String  String gt  input        return input getKey             System out println map    gives output   A  A aaa   B  B bbb    That being said  a for loop is maybe easier to understand   This below gives the same output   Map lt String  Pair lt String  String gt  gt  map   new HashMap lt  gt     for  Pair lt String  String gt  pair   pairs      map put pair getKey    pair     System out println map

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Universal method  public static  lt K  V gt  Map lt K  V gt  listAsMap Collection lt V gt  sourceList  ListToMapConverter lt K  V gt  converter        Map lt K  V gt  newMap   new HashMap lt K  V gt         for  V item   sourceList            newMap put  converter getKey item   item              return newMap     public static interface ListToMapConverter lt K  V gt        public K getKey V item

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Many solutions come to mind  depending on what you want to achive   Every List item is key and value  for  Object o   list         map put o o       List elements have something to look them up  maybe a name   for  MyObject o   list         map put o name o       List elements have something to look them up  and there is no guarantee that they are unique  Use Googles MultiMaps  for  MyObject o   list         multimap put o name o       Giving all the elements the position as a key   for  int i 0  i lt list size  i           map put i list get i             It really depends on what you want to achive   As you can see from the examples  a Map is a mapping from a key to a value  while a list is just a series of elements having a position each  So they are simply not automatically convertible

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There is also a simple way of doing this using Maps uniqueIndex      from Google guava libraries

[java] Java: How to convert List to Map

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