how to add the sum off children from different root nodes in firebase database

Question

I have tried many ways to get the sum of all the amounts from all the child's but not working.

Here is my whole main activity of that particular task:

public class hostelincome extends AppCompatActivity {

   EditText name,amount,status,mod,seriesnumber;;
   Button storedata;
   TextView prevnumber;
   FirebaseDatabase firebaseDatabase;
   DatabaseReference reference,reference1;
String name1,amount1,status1,mod1,date,count1,count2,incometotal1;
double amount2,incometotal;
int total;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_hostelincome);

    name = (EditText)findViewById(R.id.name);
    amount = (EditText)findViewById(R.id.amount);
    status = (EditText)findViewById(R.id.status);
    final String currentDateTimeString = DateFormat.getDateTimeInstance().format(new Date());
    mod = (EditText)findViewById(R.id.mod);
    storedata = (Button)findViewById(R.id.storedata);
    firebaseDatabase = FirebaseDatabase.getInstance();
    date = getIntent().getStringExtra("date");

    reference = 
    firebaseDatabase.getReference("Hostel").child(date).child("Income").child(currentDateTimeString 
    );



    storedata.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            name1 = name.getText().toString();
            amount1 = amount.getText().toString();
            status1 = status.getText().toString();
            mod1 = mod.getText().toString();



            reference.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                   total = 0;

                    for(DataSnapshot ds : dataSnapshot.getChildren()) {
                        String amount = ds.child("amount").getValue(String.class);
                        int value = Integer.valueOf(amount);
                        total =+ value;
                    }
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });

            count1 = Integer.toString(total);

            Toast.makeText(getApplicationContext(),count1,Toast.LENGTH_LONG).show();


            if (amount1.isEmpty()){}
            else{
            amount2 = Double.parseDouble(amount1);}

            incomeinfo incomeinfo;
            incomeinfo = new incomeinfo(name1,amount1,status1,mod1);
            reference.setValue(incomeinfo);
        }
      });

     }
   }

Date is taken as a string adding integer values of day/month/year together.

When I run this program I am getting an error to add the values that the:

  Process: org.electromob.budget, PID: 21949
  java.lang.NumberFormatException: s == null
    at java.lang.Integer.parseInt(Integer.java:570)
    at java.lang.Integer.valueOf(Integer.java:794)
    at org.electromob.budget.hostelincome$1$1.onDataChange(hostelincome.java:71)
    at 
  com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase- 
  database@@19.2.0:75)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase- 
  database@@19.2.0:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase- 
  database@@19.2.0:55)
    at android.os.Handler.handleCallback(Handler.java:790)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:7000)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:441)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)'''

my firebase info taking class

    public class incomeinfo {

    public String payeename;
    public String amount;
    public String status;
    public String mod;

    public incomeinfo() {
    }


    public incomeinfo(String payeename, String amount, String status, String mod) {
    this.payeename = payeename;
    this.amount = amount;
    this.status = status;
    this.mod = mod;
    }

    public String getPayeename() {
    return payeename;
    }

    public void setPayeename(String payeename) {
    this.payeename = payeename;
    }

    public String getAmount() {
    return amount;
    }

    public void setAmount(String amount) {
    this.amount = amount;
    }

    public String getStatus() {
    return status;
    }

    public void setStatus(String status) {
    this.status = status;
    }

    public String getMod() {
    return mod;
    }

    public void setMod(String mod) {
    this.mod = mod;
    }

    }

Firebase structure is added below of my project:

Can someone help me with a solution to my error?

Answer 1

It looks like you're reading one level too low in the JSON. Your onDataChange iterates over the individual amount, mod, payeename and status properties, which then don't have an amount child anymore.

If this is indeed the case, you can fix it by attaching your listener one level higher:

reference = firebaseDatabase.getReference("Hostel").child(date).child("Income");