How to add element into ArrayList in HashMap

Question

How to add element into ArrayList in HashMap       HashMap lt String  ArrayList lt Item gt  gt  Items   new HashMap lt String  ArrayList lt Item gt  gt

User · Answer

I know  this is an old question  But just for the sake of completeness  the lambda version   Map lt String  List lt Item gt  gt  items   new HashMap lt  gt     items computeIfAbsent key  k - gt  new ArrayList lt  gt     add item

User · Answer

Typical code is to create an explicit method to add to the list  and create the ArrayList on the fly when adding   Note the synchronization so the list only gets created once    Override public synchronized boolean addToList String key  Item item       Collection lt Item gt  list   theMap get key      if  list    null           list   new ArrayList lt Item gt         or  if you prefer  some other List  a Set  etc          theMap put key  list             return list add item

User · Answer

i m also maintaining insertion order here Map lt Integer ArrayList gt  d new LinkedHashMap lt  gt     for  int i 0  i lt 2  i      int id s nextInt    ArrayList al new ArrayList lt  gt     al add s next        name al add s next        category al add s nextInt       fee d put id  al

User · Answer

HashMap lt String  ArrayList lt Item gt  gt  items   new HashMap lt String  ArrayList lt Item gt  gt      public synchronized void addToList String mapKey  Item myItem        List lt Item gt  itemsList   items get mapKey           if list does not exist create it     if itemsList    null             itemsList   new ArrayList lt Item gt              itemsList add myItem            items put mapKey  itemsList         else              add if item is not already in list         if  itemsList contains myItem   itemsList add myItem

User · Answer

First you have to add an ArrayList to the Map  ArrayList lt Item gt  al   new ArrayList lt Item gt      Items add  theKey   al      then you can add an item to the ArrayLIst that is inside the Map like this   Items get  theKey   add item       item is an object of type Item

[java] How to add element into ArrayList in HashMap

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