Java 8 List V into Map K V

Question

I want to translate a List of objects into a Map using Java 8 s streams and lambdas   This is how I would write it in Java 7 and below   private Map lt String  Choice gt  nameMap List lt Choice gt  choices            final Map lt String  Choice gt  hashMap   new HashMap lt  gt             for  final Choice choice   choices                hashMap put choice getName    choice                     return hashMap      I can accomplish this easily using Java 8 and Guava but I would like to know how to do this without Guava   In Guava   private Map lt String  Choice gt  nameMap List lt Choice gt  choices        return Maps uniqueIndex choices  new Function lt Choice  String gt                Override         public String apply final Choice input                return input getName                          And Guava with Java 8 lambdas   private Map lt String  Choice gt  nameMap List lt Choice gt  choices        return Maps uniqueIndex choices  Choice  getName

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I was trying to do this and found that  using the answers above  when using Functions identity   for the key to the Map  then I had issues with using a local method like this  localMethodName to actually work because of typing issues   Functions identity   actually does something to the typing in this case so the method would only work by returning Object and accepting a param of Object  To solve this  I ended up ditching Functions identity   and using s- gt s instead   So my code  in my case to list all directories inside a directory  and for each one use the name of the directory as the key to the map and then call a method with the directory name and return a collection of items  looks like   Map lt String  Collection lt ItemType gt  gt  items   Arrays stream itemFilesDir listFiles File  isDirectory    map File  getName   collect Collectors toMap s- gt s  this  retrieveBrandItems

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If you don t mind using 3rd party libraries  AOL s cyclops-react lib  disclosure I am a contributor  has extensions for all JDK Collection types  including List and Map   ListX lt Choices gt  choices  Map lt String  Choice gt  map   choices toMap c- gt  c getName   c- gt c

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It s possible to use streams to do this  To remove the need to explicitly use Collectors  it s possible to import toMap statically  as recommended by Effective Java  third edition    import static java util stream Collectors toMap   private static Map lt String  Choice gt  nameMap List lt Choice gt  choices        return choices stream   collect toMap Choice  getName  it - gt  it

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This can be done in 2 ways  Let person be the class we are going to use to demonstrate it   public class Person        private String name      private int age       public String getAge             return age            Let persons be the list of Persons to be converted to the map  1 Using Simple foreach and a Lambda Expression on the List  Map lt Integer List lt Person gt  gt  mapPersons   new HashMap lt  gt     persons forEach p- gt mapPersons put p getAge   p      2 Using Collectors on Stream defined on the given List    Map lt Integer List lt Person gt  gt  mapPersons               persons stream   collect Collectors groupingBy Person  getAge

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As an alternative to guava one can use kotlin-stdlib  private Map lt String  Choice gt  nameMap List lt Choice gt  choices        return CollectionsKt associateBy choices  Choice  getName

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Map lt String  Set lt String gt  gt  collect   Arrays asList Locale getAvailableLocales    stream   collect Collectors                  toMap l - gt  l getDisplayCountry    l - gt  Collections singleton l getDisplayLanguage

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I use this syntax  Map lt Integer  List lt Choice gt  gt  choiceMap    choices stream   collect Collectors groupingBy choice - gt  choice getName

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Here s another one in case you don t want to use Collectors toMap    Map lt String  Choice gt  result      choices stream   collect HashMap lt String  Choice gt   new                               m  c  - gt  m put c getName    c                               m  u  - gt

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Most of the answers listed  miss a case when the list has duplicate items  In that case there answer will throw IllegalStateException  Refer the below code to handle list duplicates as well   public Map lt String  Choice gt  convertListToMap List lt Choice gt  choices        return choices stream            collect Collectors toMap Choice  getName  choice - gt  choice               oldValue  newValue  - gt  newValue

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For example  if you want convert object fields to map   Example object   class Item          private String code          private String name           public Item String code  String name                this code   code              this name   name                       getters and setters         And operation convert List To Map   List lt Item gt  list   new ArrayList lt  gt     list add new Item  code1    name1     list add new Item  code2    name2      Map lt String String gt  map   list stream         collect Collectors toMap Item  getCode  Item  getName

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Here is solution by StreamEx  StreamEx of choices  toMap Choice  getName  c - gt  c

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One more option in simple way   Map lt String Choice gt  map   new HashMap lt  gt     choices forEach e- gt map put e getName   e

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You can create a Stream of the indices using an IntStream and then convert them to a Map    Map lt Integer Item gt  map    IntStream range 0 items size              boxed             collect Collectors toMap  i - gt  i  i - gt  items get i

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Map lt String Choice gt  map list stream   collect Collectors toMap Choice  getName  s- gt s      Even  serves this purpose for me   Map lt String Choice gt  map   list1 stream   collect   - gt  new HashMap lt String Choice gt                   r s  - gt  r put s getString   s   r s  - gt  r putAll s

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List lt V gt  choices     your list Map lt K V gt  result   choices stream   collect Collectors toMap choice  getKey   choice      assuming class  V  has a method to get the key  this method must handle case of duplicates too and provide a unique key

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Based on Collectors documentation it s as simple as   Map lt String  Choice gt  result       choices stream   collect Collectors toMap Choice  getName                                                Function identity

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If your key is NOT guaranteed to be unique for all elements in the list  you should convert it to a Map lt String  List lt Choice gt  gt  instead of a Map lt String  Choice gt   Map lt String  List lt Choice gt  gt  result    choices stream   collect Collectors groupingBy Choice  getName

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I will write how to convert list to map using generics and inversion of control  Just universal method   Maybe we have list of Integers or list of objects  So the question is the following  what should be key of the map   create interface  public interface KeyFinder lt K  E gt        K getKey E e       now using inversion of control     static  lt K  E gt  Map lt K  E gt  listToMap List lt E gt  list  KeyFinder lt K  E gt  finder            return  list stream   collect Collectors toMap e - gt  finder getKey e    e - gt  e            For example  if we have objects of book   this class is to choose key for the map    public class BookKeyFinder implements KeyFinder lt Long  Book gt         Override     public Long getKey Book e            return e getPrice

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Another possibility only present in comments yet  Map lt String  Choice gt  result   choices stream   collect Collectors toMap c - gt  c getName    c - gt  c      Useful if you want to use a parameter of a sub-object as Key  Map lt String  Choice gt  result   choices stream   collect Collectors toMap c - gt  c getUser   getName    c - gt  c

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Use getName   as the key and Choice itself as the value of the map   Map lt String  Choice gt  result       choices stream   collect Collectors toMap Choice  getName  c - gt  c

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If every new value for the same key name has to be overridden   public Map  lt  String  Choice  gt  convertListToMap List  lt  Choice  gt  choices        return choices stream            collect Collectors toMap Choice  getName              Function identity                 oldValue  newValue  -  gt  newValue        If all choices have to be grouped in a list for a name   public Map  lt  String  Choice  gt  convertListToMap List  lt  Choice  gt  choices        return choices stream   collect Collectors groupingBy Choice  getName

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String array       ASDFASDFASDF   AA    BBB    CCCC    DD    EEDDDAD        List lt String gt  list   Arrays asList array       Map lt Integer  String gt  map   list stream                collect Collectors toMap s - gt  s length    s - gt  s   x  y  - gt                    System out println  Dublicate key    x                   return x                  - gt  new TreeMap lt  gt   s1 s2 - gt s2 compareTo s1          System out println map     Dublicate key AA  12 ASDFASDFASDF  7 EEDDDAD  4 CCCC  3 BBB  2 AA

[java] Java 8 List<V> into Map<K, V>

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