how to get values from selected item spinner (Android Studio to Firebase)

Question

First of all, thankyou for clicking my question, hope you can help me to resolve my problem here :(

So, my problem is, i already success to get data from spinner "productname" and after that i want to get "productprice" references from selected item listener.. how to get "product price" ? because when i run the apps, its force closed. hope you can understand what i mean :(

Click here to see my firebase db

package com.example.chocolatte;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.Spinner;
import android.widget.TextView;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import org.w3c.dom.Text;

import java.util.ArrayList;
import java.util.List;

public class transaction_page extends AppCompatActivity {

    private DatabaseReference mDatabase;

    Button bt_min, bt_plus, btOvo, btGopay, btCash, btCheckout;
    TextView txtSumProduct, txtProductSelected;
    String selectedproductprice, selectedproduct;
    Spinner productSpinner;


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

        txtProductSelected = findViewById(R.id.txtProductSelected);

        mDatabase = FirebaseDatabase.getInstance().getReference();

        mDatabase.getRoot().child("PRODUCTS").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull final DataSnapshot dataSnapshot) {
                final List<String> products = new ArrayList<String>();

                for(DataSnapshot productsSnapshot: dataSnapshot.getChildren()) {
                    String productname = productsSnapshot.child("productname").getValue(String.class);
                    products.add(productname);
                }


                Spinner productSpinner = (Spinner) findViewById(R.id.spinner);
                final ArrayAdapter<String> productadapter = new ArrayAdapter<String>(transaction_page.this, android.R.layout.simple_spinner_dropdown_item, products);
                productadapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
                productSpinner.setAdapter(productadapter);

                productSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                    @Override
                    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

                        selectedproduct = productadapter.getItem(position);

                        mDatabase.equalTo(selectedproduct).addValueEventListener(new ValueEventListener() {
                            @Override
                            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                               selectedproductprice = dataSnapshot.child("productprice").getValue().toString();
                            }

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

                            }
                        });

                        txtProductSelected.setText(selectedproduct + " (@" + selectedproductprice + ")");

                    }

                    @Override
                    public void onNothingSelected(AdapterView<?> parent) {

                    }


                });
            }


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

            }


        });


    }
}

and this is for the report

2019-11-25 19:55:44.720 26470-26470/com.example.chocolatte E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.chocolatte, PID: 26470 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference at com.example.chocolatte.transaction_page$1$1$1.onDataChange(transaction_page.java:69) at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.4:75) at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.4:63) at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.4:55) at android.os.Handler.handleCallback(Handler.java:789) at android.os.Handler.dispatchMessage(Handler.java:98) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

the output i expected is

example :

CREAMY CHOCOLATE (@10000)

Answer 1

Your reference is at the root node:

mDatabase = FirebaseDatabase.getInstance().getReference();

If you want to retrieve productprice, you need to do the following:

 mDatabase.child("PRODUCTS").child("PROD1").addValueEventListener(new ValueEventListener() {
    @Override
 public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
       selectedproductprice = dataSnapshot.child("productprice").getValue().toString();
       }

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

Here you add a reference to child PROD1 then you will be able to retrieve the child productprice.