Retrieve strings from real time database in a 2D ArrayList Android

I have a Firebase Realtime Database structure like this :

How do I retrieve it into a 2D ArrayList like this using Java.

private String mChoices [][] = {
        {"Mutare", "Harare", "Bulawayo","Mozambique"},
        {"Johhannesburg", "Pretoria", "Duran","Capetown"},
        {"Nairobi", "Lusaka", "Windhoek","Lilongwe"},
        {"Harare", "Lilongwe", "Pretoria","Nairobi"}
};

As @SardorbekKhujaev mentioned in his answer, the arrays are fixed data-types. To solve the problem, we need to use an ArrayList, as the size can be increased dynamically. Once we populate the ArrayList we can simply convert it into an 2D array, like in the following lines of code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference choicesRef = rootRef.child("choices");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<String[]> choices = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            int size = (int) ds.getChildrenCount();
            String[] choice = new String[size];
            int count = 0;
            for(DataSnapshot dSnapshot : ds.getChildren()) {
                choice[count++] = dSnapshot.getValue(String.class);
            }
            choices.add(choice);
        }
        String choicesArray[][] = new String[choices.size()][];
        for (int j = 0; j < choices.size(); j++) {
            choicesArray[j] = choices.get(j);
        }
        //Test the array
        for(String[] array : choicesArray) {
            Log.d(TAG, Arrays.toString(array));
        }
        //Do what you need to do with your 2D array
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d("TAG", databaseError.getMessage()); //Don't ignore potential errors!
    }
};
choicesRef.addListenerForSingleValueEvent(valueEventListener);

The result of this code will be the exact 2D array that you are looking for.

You are asking to work with a 2D array, which makes the code more complex, as arrays are not dynamic. Taking into account that firebase data is dynamic and the general convenience of array lists in Java, I recommend you to use ArrayList.
This solution focuses on the matter of the question - a 2D array

private String[][] mChoices;
private void initializeMChoices(int i, int j) {
mChoices = new String[i][j];
}

Wherever you need to fill the array with the String values, paste this code:

        //Get the reference to the "choices" node:
DatabaseReference ref = FirebaseDatabase.getInstance().getReference("choices");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot snapshot) {
// We first go through the whole node to count it's child nodes
int i = 0, j = 0;
while(snapshot.child(Integer.toString(i)).exists())
{
j = 0;
while(snapshot.child(Integer.toString(i)).child(Integer.toString(i)).exists())
{
j++;
}
i++;
}
// we have the numbers, now we initialize the array
initializeMChoices(i, j);
//we go through the whole node to put child-node values in the 2D array
i = 0;
j = 0;
while(snapshot.child(Integer.toString(i)).exists())
{
while(snapshot.child(Integer.toString(i)).child(Integer.toString(i)).exists())
{
j++;
mChoices[i][j] = snapshot.child(Integer.toString(i)).child(Integer.toString(i)).toString();
}
i++;
}
}
@Override
public void onCancelled(@NonNull DatabaseError error) {
}
});