In Java 8 how do I transform a Map K V to another Map K V using a lambda

Question

I ve just started looking at Java 8 and to try out lambdas I thought I d try to rewrite a very simple thing I wrote recently   I need to turn a Map of String to Column into another Map of String to Column where the Column in the new Map is a defensive copy of the Column in the first Map   Column has a copy constructor   The closest I ve got so far is       Map lt String  Column gt  newColumnMap  new HashMap lt  gt         originalColumnMap entrySet   stream   forEach x - gt  newColumnMap put x getKey    new Column x getValue         but I m sure there must be a nicer way to do it and I d be grateful for some advice

User · Answer

If you don t mind using 3rd party libraries  my cyclops-react lib has extensions for all JDK Collection types  including Map  You can directly use the map or bimap methods to transform your Map  A MapX can be constructed from an existing Map eg      MapX lt String  Column gt  y   MapX fromMap orgColumnMap                                  map c- gt new Column c getValue       If you also wish to change the key you can write     MapX lt String  Column gt  y   MapX fromMap orgColumnMap                                  bimap this  newKey c- gt new Column c getValue       bimap can be used to transform the keys and values at the same time   As MapX extends Map the generated map can also be defined as    Map lt String  Column gt  y

User · Answer

Keep it Simple and use Java 8 -   Map lt String  AccountGroupMappingModel gt  mapAccountGroup CustomerDAO getAccountGroupMapping     Map lt String  AccountGroupMappingModel gt  mapH2ToBydAccountGroups                  mapAccountGroup entrySet   stream                             collect Collectors toMap e- gt e getValue   getH2AccountGroup                                                       e - gt e getValue

User · Answer

If you use Guava  v11 minimum  in your project you can use Maps  transformValues   Map lt String  Column gt  newColumnMap   Maps transformValues    originalColumnMap    Column  new    equivalent to  x - gt  new Column x       Note  The values of this map are evaluated lazily  If the transformation is expensive you can copy the result to a new map like suggested in the Guava docs    To avoid lazy evaluation when the returned map doesn t need to be a view  copy the returned map into a new map of your choosing

User · Answer

You could use a Collector   import java util    import java util stream Collectors   public class Defensive      public static void main String   args        Map lt String  Column gt  original   new HashMap lt  gt         original put  foo   new Column         original put  bar   new Column          Map lt String  Column gt  copy   original entrySet            stream            collect Collectors toMap Map Entry  getKey                                    e - gt  new Column e getValue            System out println original       System out println copy          static class Column       public Column          public Column Column c

User · Answer

The way without re-inserting all entries into the new map should be the fastest it won t because HashMap clone internally performs rehash as well   Map lt String  Column gt  newColumnMap   originalColumnMap clone    newColumnMap replaceAll  s  c  - gt  new Column c

User · Answer

Map lt String  Integer gt  map   new HashMap lt  gt     map put  test1   1   map put  test2   2    Map lt String  Integer gt  map2   new HashMap lt  gt     map forEach map2  put    System out println  map      map   System out println  map2      map2      Output     map    test2 2  test1 1     map2   test2 2  test1 1    You can use the forEach method to do what you want   What you re doing there is   map forEach new BiConsumer lt String  Integer gt           Override     public void accept String s  Integer integer            map2 put s  integer                    Which we can simplify into a lambda   map forEach  s  integer  - gt   map2 put s  integer      And because we re just calling an existing method we can use a method reference  which gives us   map forEach map2  put

User · Answer

Here is another way that gives you access to the key and the value at the same time  in case you have to do some kind of transformation   Map lt String  Integer gt  pointsByName   new HashMap lt  gt     Map lt String  Integer gt  maxPointsByName   new HashMap lt  gt      Map lt String  Double gt  gradesByName   pointsByName entrySet   stream            map entry - gt  new AbstractMap SimpleImmutableEntry lt  gt                   entry getKey      double  entry getValue                             maxPointsByName get entry getKey       100d            collect Collectors toMap Map Entry  getKey  Map Entry  getValue

[java] In Java 8 how do I transform a Map<K,V> to another Map<K,V> using a lambda?

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