CODEWITHSUNDEEP
java
WHOATEMYNOODLES
WHOATEMYNOODLES

Reputation: 897

Value for map.compute is always null

I'm trying to use .compute() on my map but the else statement is never fired.

public void onInviteUser(int adapterPosition) {
        Log.d(TAG, "onInviteUser: " + adapterPosition); //not null
        mapInvitedFriends.compute(adapterPosition, (k, v) -> {
            Log.d(TAG, "onInviteUser: " + adapterPosition); 
            if (v == null) {
                Log.d(TAG, "onInviteUser: " + v);
                mapInvitedFriends.put(adapterPosition, listFriends.get(adapterPosition));
                Log.d(TAG, "onInviteUser: " + listFriends.get(adapterPosition).toString());
                return v;
            } else {
                //Never gets fired
                Log.d(TAG, "onInviteUser: " + v);
                mapInvitedFriends.remove(adapterPosition);
                return v;
            }
        });
}

I've logged everthing, but I can't figure out why the value v is always null.

Edit:

The types for the map is:

private final Map<Integer, Friend> mapInvitedFriends = new HashMap<>();
private final List<Friend> listFriends = new ArrayList<>();

Upvotes: 1

Views: 180

Answers (1)

Eran
Eran

Reputation: 393986

You are not use compute correctly:

The remapping function should not modify this map during computation.

Therefore you shouldn't call to mapInvitedFriends.put or mapInvitedFriends.remove inside the function.

Instead, you can try:

    mapInvitedFriends.compute(adapterPosition, (k, v) -> {
        Log.d(TAG, "onInviteUser: " + adapterPosition); 
        if (v == null) {
            return listFriends.get(adapterPosition); // this will cause a new mapping to be added
        } else {
            return null; // this will cause the existing mapping to be removed
        }
    });

or simply:

mapInvitedFriends.compute(adapterPosition,
                          (k, v) -> v == null ? listFriends.get(adapterPosition) : null);

Upvotes: 4

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