Cannot retrieve data from firebase using dataSnapshot

Question

My Firebase database looks like above I'm trying to retrieve the value of 'a' for 'alind' and similarly for another child node 'kartik' I'm using Android Studio. but i receive this error-

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.example.minorproject.facultyfreeslots.matchslots(facultyfreeslots.java:119)
        at com.example.minorproject.facultyfreeslots$1.onClick(facultyfreeslots.java:48)
        at android.view.View.performClick(View.java:6614)

I've tried setting different paths to access this using Datasnapshot.

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_facultyfreeslots);
        t1=findViewById(R.id.textView50);
        name=findViewById(R.id.editText4);
        name2=findViewById(R.id.editText5);
        b1=findViewById(R.id.button9);
        day=findViewById(R.id.spinner18);
        time=findViewById(R.id.spinner19);
        FirebaseApp.initializeApp(this);
        d1= FirebaseDatabase.getInstance().getReference("teacher");
        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                matchslots();
            }
        });
    }

void matchslots()
    {  // getdata();
        name11=name.getText().toString();
        name12=name.getText().toString();
        daystring=day.getSelectedItem().toString().trim();
        timestring=time.getSelectedItem().toString().trim();
        d1.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                dvalue1=dataSnapshot.child(name11).child(daystring).child(timestring1).getValue().toString().trim();
                dvalue2=dataSnapshot.child(name12).child(daystring).child(timestring1).getValue().toString().trim();
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Log.d(TAG, "Failed to read value.");
            }
        });
 //line119- 
  if(dvalue1.equals("no class"))
        {
            a=0;
        }

        else {
            a=1;
        }
        if(dvalue2.equals("no class"))
        {
            b=0;
        }
        else
        {
            b=1;

I want to get the value of a or b or c. nam11=alind or any othername, daystring="monday"/"tuesday" etc
timestring="a"/"b"/"c" etc

Not even remotely. It's a race condition, and still a duplicate. — Zoe, Apr 28 '19 at 14:51

score 2 · Accepted Answer · answered Apr 28 '19 at 14:31

Since onDataChange() is asynchronous, then you have to do the following:

d1.addValueEventListener(new ValueEventListener() {
 @Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
   dvalue1=dataSnapshot.child(name11).child(daystring).child(timestring1).getValue().toString().trim();
   dvalue2=dataSnapshot.child(name12).child(daystring).child(timestring1).getValue().toString().trim();
   if(dvalue1.equals("no class"))
    {
        a=0;
    }

    else 
    {
        a=1;
    }
    if(dvalue2.equals("no class"))
    {
        b=0;
    }
    else
    {
        b=1;
     }
   }
 @Override
public void onCancelled(@NonNull DatabaseError databaseError) {
 Log.d(TAG, "Failed to read value.");
  }
});

The code after your listener is getting run before retrieving all the data, therefore you get the error that dvalue1 is null.

What change do i have to make exactly? I'm new to Android hence the doubts. — Kartik Singh, Apr 28 '19 at 14:35
you need to check if dvalu1 is equal to `"no class"` inside `onDataChange` and not outside.. — Peter Haddad, Apr 28 '19 at 14:36

Cannot retrieve data from firebase using dataSnapshot

1 Answers1