获得两个firebase结果的差异

Question

我有以下代码，我打算用它来查找销售和支出之间的利润：

DatabaseReference dref = FirebaseDatabase.getInstance().getReference();
    DatabaseReference dreff = FirebaseDatabase.getInstance().getReference();
    dref = dref.child("KatakaExpenditure");
    dreff = dreff.child("KatakaSales");


    dref.addValueEventListener(new ValueEventListener() {

        @Override
        public void onDataChange(DataSnapshot snapshot) {
            int totalAmount = 0;
            for (DataSnapshot postSnapshot : snapshot.getChildren()) {

                DogExpenditure dogExpenditure = postSnapshot.getValue(DogExpenditure.class);
                totalAmount += dogExpenditure.getAmount();


            }






            textView4.setText("Your total Expenditure for Kataka Unit is:" +Integer.toString(totalAmount));
             amounta = totalAmount;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }


        public void onCancelled(FirebaseError firebaseError) {
            System.out.println("The read failed: " + firebaseError.getMessage());
        }
    });

    dreff.addValueEventListener(new ValueEventListener() {
        int totalAmountt = 0;

        @Override
        public void onDataChange(DataSnapshot snapshot) {

            for (DataSnapshot postSnapshot : snapshot.getChildren()) {

                DogExpenditure dogExpenditure = postSnapshot.getValue(DogExpenditure.class);
                totalAmountt += dogExpenditure.getAmount();


            }


            textView5.setText("Your total Sales for Kataka Unit is:" +Integer.toString(totalAmountt));
            amountb = totalAmountt;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }


        public void onCancelled(FirebaseError firebaseError) {
            System.out.println("The read failed: " + firebaseError.getMessage());
        }
    });

amountc = amountb - amounta;

    textView3.setText("Your total Profit for Kataka Unit is:" +Integer.toString(amountc));

}

然而代码为我的 textView3.setText（＆＃34; Kataka单位的总利润为：＆＃34; + Integer.toString（amountc））; 行给出0值代码..关于如何计算这个数学的任何想法 amountc = amountb - amounta; ???提前谢谢你