"println needs a message" error in android-CodePudding

I want to read Music_ID of the group with Playlist_ID of 2 in firebase.

The following error occurs.

java.lang.NullPointerException: println needs a message

This is my firebase realtime database.

And this is my code.

database = FirebaseDatabase.getInstance();
storage = FirebaseStorage.getInstance();
dref = FirebaseDatabase.getInstance().getReference();

private void Startplaylist(String mood) {
        DatabaseReference plist = dref.child("Playlist");
        plist.orderByChild("Playlist_ID").equalTo(2).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                // Log.i("Value", dataSnapshot.getValue().toString());
                String music_id = dataSnapshot.child("Music_ID").getValue(String.class);
                Log.i("Value_id", music_id);
                str_musictitle.setText(music_id);

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }

An alarm pops up that an error occurs in this part. Log.i("Value_id", music_id);

I think "music_id" is not being read.

I tried to change part String music_id = dataSnapshot.child("Music_ID").getValue(String.class); to String music_ids = dataSnapshot.child("Music_ID").getValue().toString(); and run it, but I couldn't get the desired result.

CodePudding user response：

When you execute a query against the Firebase Database, there will potentially be multiple results. So the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.

The code in your onDataChange will need to handle this list by looping over dataSnapshot.getChildren(). Something like this:

DatabaseReference plist = dref.child("Playlist");
plist.orderByChild("Playlist_ID").equalTo(2).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        for (DataSnapshot snapshot: dataSnapshot.getChildren()) { //