java - 从 Firebase 获取最后(5 或 10)个值

Question

我是 Android 的新手，我想从 获取最后的 5 或 10 messages FirebaseDatabase。我试过使用它。

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ChatInteractor.databaseReference.orderByKey().startAt(10).endAt(20).addListenerForSingleValueEvent((ValueEventListener) ActivityChatView.this);

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• 但结果是 NullPointerException。
• Logcat

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java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.Query com.google.firebase.database.DatabaseReference.orderByKey()' on a null object reference

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• 我的DatabaseStructure是这样的

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• 更新之后

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   DatabaseReference databaseReference= FirebaseDatabase.getInstance().getReferenceFromUrl("https://afdbjkaffirebaseio.com/");
        Query query=databaseReference.child(Constants.ARG_CHAT_ROOMS).orderByChild("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3").limitToLast(2);
        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                for(DataSnapshot snapshot:dataSnapshot.getChildren()){
                    Chat chat=snapshot.getValue(Chat.class);
                    Log.e("163","ACV"+chat.fromEmail);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

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• Log的结果

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 E/163: ACVnull

Answer 1

如下所示更新您的代码:

 DatabaseReference databaseReference= FirebaseDatabase.getInstance().getReference();
    Query query=databaseReference.child(Constants.ARG_CHAT_ROOMS).child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3").orderByChild().limitToLast("Number of last messages you want to show");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for(DataSnapshot snapshot:dataSnapshot.getChildren()){
                Chat chat=snapshot.getValue(Chat.class);
                Log.e("163","ACV"+chat.fromEmail);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });