java - Android Firebase - Why Null Values in Map fields are ignored -

i discovered when saving pojo map field using firebase on android, if map contains nulls in value property of map, whole field ignored.

the workaround easy (non-null values result in map saving successfully), want understand why so?

model

public class game {      private string owner;     private map<string, string> characters;      public game() {         // empty constructor firebase     }      public string getowner() {         return owner;     }      public void setowner(string owner) {         this.owner = owner;     }      public map<string, string> getcharacters() {         return characters;     }      public void setcharacters(map<string, string> characters) {         this.characters = characters;     } } 

calling code

final game game = new game();         game.setowner("owner");         game.setcharacters(new hashmap<string, string>());         game.getcharacters().put("bugs bunny", null);         game.getcharacters().put("batman", null);         final firebase ref = new firebase("<firebaseurl>/test/" + system.currenttimemillis());         ref.setvalue(game); 

resulting object in firebase

{     "1456421340781": {         "owner": "owner"     } } 

they're not ignored. when give firebase null value property/path, indicate want property or path deleted.

from documentation on firebase's javascript set() method:

passing null new value equivalent calling remove(); data @ location or child location deleted.

so if set value with:

ref.child("keith").setvalue(47649); 

then following delete it:

ref.child("keith").setvalue(null); 

this behavior useful when use updatechildren(), works equally when call setvalue().