Why is my function not retrieving data from firebase correctly and how do I fix this?

Question

The purpose of this function is to go through the first 200 hundred days of each user's schedule. Go through a each user's schedule one by one to check if they're available on a certain day. If they are available on a certain day a counter will increment and if the counter matches the amount of users the specified event will be added to the user's schedule. This isn't working however as my code is not reading the data from firebase correctly. Here is the code for the function:

private void findTime(String eventName, int  EventTime)
{

    Boolean dayFound = false;
    countNeeded = TheUserList.size();

    DaySearched = LocalDate.now();
    while (!dayFound) {
        for (int i = 0; i <200; i++)
        {
            // the current count variable tracks the amount of users in the schedule have free tim slots for a specific day
            currentCount = 0;
            Toast.makeText(this, ("The first for loop has begun " + String.valueOf(currentCount)), Toast.LENGTH_SHORT).show();

            for (int e =0; e < TheUserList.size(); e++)
            {
                String currentUser = TheUserList.get(e);
                    Log.i("Firebase", Login.reference.child(currentUser).child(String.valueOf(DaySearched)).child("Time for day").toString());
    

                Login.reference.child(currentUser).child(String.valueOf(DaySearched)).child("Time for day").addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {
                        int TimeForDay = snapshot.getValue(int.class);
                        if (TimeForDay + EventTime < 1441)
                        {
                            currentCount = currentCount + 1;
                        }
                        else
                        {
                            currentCount = 0;
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError error) {
                        Toast.makeText(PlanEvent.this, "Failed to get Data", Toast.LENGTH_SHORT).show();

                    }
                });
            }
            if (currentCount == countNeeded)
            {
                Log.d("FindTime Function", "Found and available day at" + String.valueOf(DaySearched));

                Tasks newEvent = new Tasks(eventName, DaySearched, LocalTime.now(), EventTime);
                Day.enoughTimeChecker(DaySearched, EventTime, newEvent);
                dayFound = true;

            }
            DaySearched = DaySearched.plusDays(1);
        }
    }
}

I have added Toast messages in the while loop to see if it being called (this works) and also one to see if the function has found an appropriate date (this isn't appearing so I assume it cannot find a day for some reason).

Here is an image to understand the structure of the real-time-database

Here is the output of the log.i() line:

Any help would be greatly appreciated.

Answer 1

All data is loaded from Firebase (and all modern cloud APIs) asynchronously, since it may take some time to get it. Instead of waiting (and blocking the user from using your app) your main code continues to run, and a callback to your code (onDataChange here) is then made when the data is available.

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The problem is that you're using currentCount before all of your nested data has loaded. It's easiest to see this if you add some logging:

Log.i("Firebase", "Before loop starts");
while (!dayFound) {
    for (int i = 0; i <200; i++)
    {
        Log.i("Firebase", "Before nested loop starts");

        for (int e =0; e < TheUserList.size(); e++)
        {
            String currentUser = TheUserList.get(e);
            Login.reference.child(currentUser).child(String.valueOf(DaySearched)).child("Time for day").addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot snapshot) {
                    Log.i("Firebase", "Got data, incrementing count");
                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                    Toast.makeText(PlanEvent.this, "Failed to get Data", Toast.LENGTH_SHORT).show();
                }
            });
        }
        Log.i("Firebase", "After loop is done, checking count");
    }
}

The output of this will be something like:

Before loop starts
Before nested loop starts
After loop is done, checking count
Before nested loop starts
After loop is done, checking count
Before nested loop starts
After loop is done, checking count
... Got data, incrementing count
Got data, incrementing count
Got data, incrementing count
Got data, incrementing count

This is probably not the order you expected, but it is working as designed and explains perfectly why your check doesn't work: by the time you check the count, the data hasn't loaded yet!

---

The solution for this is always the same. Any code that needs data from the database, has to be inside onDataChanged, be called from there, or be otherwise synchronized.

The simplest way is usually to move the code into the onDataChange:

private void findTime(String eventName, int  EventTime)
{

    Boolean dayFound = false;
    countNeeded = TheUserList.size();

    DaySearched = LocalDate.now();
    while (!dayFound) {
        for (int i = 0; i <200; i++)
        {
            // the current count variable tracks the amount of users in the schedule have free tim slots for a specific day
            currentCount = 0;
            Toast.makeText(this, ("The first for loop has begun " + String.valueOf(currentCount)), Toast.LENGTH_SHORT).show();

            for (int e =0; e < TheUserList.size(); e++)
            {
                String currentUser = TheUserList.get(e);
                    Log.i("Firebase", Login.reference.child(currentUser).child(String.valueOf(DaySearched)).child("Time for day").toString());
    

                Login.reference.child(currentUser).child(String.valueOf(DaySearched)).child("Time for day").addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {
                        int TimeForDay = snapshot.getValue(int.class);
                        if (TimeForDay + EventTime < 1441)
                        {
                            currentCount = currentCount + 1;
                            // 
                            if (currentCount == countNeeded)
                            {
                                Log.d("FindTime Function", "Found and available day at" + String.valueOf(DaySearched));

                                Tasks newEvent = new Tasks(eventName, DaySearched, LocalTime.now(), EventTime);
                                Day.enoughTimeChecker(DaySearched, EventTime, newEvent);
                                dayFound = true;

                            }
                            DaySearched = DaySearched.plusDays(1);
                            // 
                        }
                        else
                        {
                            currentCount = 0;
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError error) {
                        Toast.makeText(PlanEvent.this, "Failed to get Data", Toast.LENGTH_SHORT).show();

                    }
                });
            }
        }
    }
}

You may need to perform some additional checks inside onDataChange, for example if you want to ensure that DaySearched.plusDays(1); runs at most once.

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For more on this, see:

• getContactsFromFirebase() method return an empty list
• Setting Singleton property value in Firebase Listener