Lutaaya Huzaifah Idris Lutaaya Huzaifah Idris - 2 months ago 21
Java Question

java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database

I am trying to retrieve data into a RecyclerView from Firebase Database but am experiencing this error java.lang.NullPointerException: Attempt to invoke virtual method.

'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)' on a null object reference


Below is my main activity class .

package oerrride.we.huzykamz.testingappfirebase;

import android.app.Dialog;
import android.content.Context;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.widget.Button;

import com.google.android.gms.common.GoogleApiAvailability;
import com.google.android.gms.tasks.Task;
import com.google.firebase.database.ChildEventListener;
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 java.util.ArrayList;
import java.util.List;


public class MainActivity extends AppCompatActivity {

private Button but;
private DatabaseReference mRef;
private MyAdapter adapter;
public RecyclerView recyclerView;

public List<ModelClass> movies_;
public Context c;
private LinearLayoutManager linearLayoutManager;

private DatabaseReference mMovieRef =mRef.child("Name");

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
movies_ = new ArrayList<ModelClass>();

recyclerView =(RecyclerView) findViewById(R.id.recyclerview);

linearLayoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(linearLayoutManager);
recyclerView.setHasFixedSize(true);
mRef = FirebaseDatabase.getInstance().getReferenceFromUrl("xxxxxxxxxxxxxxxxxxxxxx");


mRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
LoadData(dataSnapshot);
}

@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
LoadData(dataSnapshot);
}

@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {

}

@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {

}

@Override
public void onCancelled(DatabaseError databaseError) {

}
});




}



public void LoadData (DataSnapshot dataSnapshot){
for(DataSnapshot singlesnapshot :dataSnapshot.getChildren()){
String taskTitle =singlesnapshot.getValue(String.class);

movies_.add(new ModelClass(taskTitle));

adapter = new MyAdapter(MainActivity.this, movies_);
recyclerView.setAdapter(adapter);



}

}

}


And this is my Adapter class .







public class MyAdapter extends RecyclerView.Adapter<MyViewHolder> {




protected Context c;
List<ModelClass> movies;

public MyAdapter(Context c, List<ModelClass> movies) {
this.c = c;
this.movies = movies;
}

@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_activity,parent,false);
MyViewHolder holder = new MyViewHolder(v);
return holder;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {

holder.name.setText(movies.get(position).getName());
// PicassoClient.downloadImage(c,movies.get(position).getUrl(),holder.img);

}

@Override
public int getItemCount() {
return movies.size();
}


}

Below is my Console of Firebase database

Firebase Console data am trying to fetch

Answer
private DatabaseReference mMovieRef =mRef.child("Name");

mRef is not created yet so It is null and will throw NullPointerException

Solution:

Change your declaration to:

private DatabaseReference mMovieRef;

and assign value to it after you created mRef

mRef =  FirebaseDatabase.getInstance().getReferenceFromUrl("xxxxxxxxxxxxxxxxxxxxxx");
mMovieRef = mRef.child("Name");