Random questions from different child from Firebase to Java andoird studio

Question

I am trying to make a quiz app using Firebase but I need to get 5 questions in total from different topics randomly and add them to ArrayList<object>. I already have generated 1 random question from only 1 topic (child) and I can not make this for the other 4 topics to have 5 questions from different topics My Firebase database is here:

My code is:

final Query questionFromB = FirebaseDatabase.getInstance().getReference().child("B").orderByChild("questionID").equalTo(new Random().nextInt(15));
questionFromB.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot datasnapshot) {
        for (DataSnapshot question : datasnapshot.getChildren()){
            if (questionCount < 1){
                questionView.setText(question.getValue(QuizQuestions.class).getQuestionText());
                answerText.setText(question.getValue(QuizQuestions.class).getAnswerText());

                arrayList.add(new QuizQuestions(question.getValue(QuizQuestions.class).getQuestionID(),question.getValue(QuizQuestions.class).getQuestionText(),question.getValue(QuizQuestions.class).getAnswerText()));
                datasnapshot.getChildrenCount();
                questionCount ++;
            }else{
                nextButton.setVisibility(View.GONE);
                nextActivityButton.setVisibility(View.VISIBLE);
                for (int i=0;i<arrayList.size();i++){
                    textView.append(arrayList.get(i).getQuestionText());
                    textView.append(" \n ");
                    textView.append(" \n ");
                }

            }
        }
    }


    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        Toast.makeText(MainActivity.this, "Error, something was wrong", Toast.LENGTH_SHORT).show();
    }
});

(in the second activity i am just showing ArrayList contents in TextView)

Answer 1

If you want to get results from multiple queries, then you should consider using Query#get(), which returns an object of type Task. That being said, to merge 5 different Firebase Realtime Database requests locally, I recommend you to use Tasks.whenAllSuccess() method. You can achieve this, using the following lines of code:

Query questionFromA = FirebaseDatabase.getInstance().getReference()
        .child("A")
        .orderByChild("questionID")
        .equalTo(new Random()
        .nextInt(15));

Query questionFromB = FirebaseDatabase.getInstance().getReference()
        .child("B")
        .orderByChild("questionID")
        .equalTo(new Random()
        .nextInt(15));

//Create the other three queries.

Task aTask = questionFromA.get();
Task bTask = questionFromb.get();

//Create the other three Task objects.

Task combinedTask = Tasks.whenAllSuccess(aTask, bTask).addOnSuccessListener(new OnSuccessListener<List<Object>>() {
    @Override
    public void onSuccess(List<Object> list) {
         //Do what you need to do with your list
    }
});

And pass all five Task objects to the whenAllSuccess() method. As you can see, when overriding the onSuccess() method the result is a list of objects. So you only have to map each object from the list into an object of type QuizQuestions and do what you need to do with them.