Java Android firebase – why ignore null values in map fields

When I use firebase on Android to save POJOs using the map field, I find that if the map contains null in the value attribute of the map, the whole field will be ignored

The solution is simple (non null values will successfully save the map), but I want to know why?

Model

public class Game {

    private String owner;
    private Map<String, String> characters;

    public Game() {
        // empty constructor for 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;
    }
}

Call 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);

Result object in firebase

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

resolvent:

They are not actually ignored. When you provide firebase with a null value for the property / path, it means that you want to delete the property or path

Start with documentation on firebase's JavaScript set() method:

Therefore, if you set the value using the following method:

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

Then delete it as follows:

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

This behavior is most useful when you use updatechildren(), but it is also valid when you call setvalue()