java.lang.NullPointerException：尝试调用虚拟方法'com.google.firebase.database

Question

我正在尝试从Firebase数据库中检索数据到RecyclerView但遇到此错误java.lang.NullPointerException：尝试调用虚方法。 'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)' on a null object reference

以下是我的主要活动课程。

    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();
}

}

以下是我的Firebase数据库控制台

Answer 1

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

mRef尚未创建，因此它为null并将抛出NullPointerException

<强>解决方案：

将您的声明更改为：

private DatabaseReference mMovieRef;

并在创建mRef

后为其赋值

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

Answer 2

可能没有初始化DatabaseReference：尝试使用：

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

或者它可以是：

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

而不是

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

Answer 3

您不需要在主体外部初始化或声明<script src="https://unpkg.com/javascript-lp-solver/prod/solver.js"></script> <script> var model = { "optimize": "cost", "opType": "min", "constraints": { "Zr02": {"min": 0.32, "max": 0.34}, "Al203": {"min": 0.49, "max": 0.52}, "Si02": {"min": 0.15, "max": 0.17}, "Na20": {"min": 0.013, "max": 0.02} }, "variables": { "AZS": { "Zr02": 0.30, "Al203": 0.50, "Si02": 0.13, "Na20": 0.018, "cost": 328.95 }, "Fine Salt": { "Zr02": 0.25, "Al203": 0.30, "Si02": 0.43, "Na20": 0.002, "cost": 203.95 }, "ZDCF": { "Zr02": 0.63, "Al203": 0.01, "Si02": 0.33, "Na20": 0.004, "cost": 789.47 }, "AB Fines": { "Zr02": 0.00, "Al203": 0.95, "Si02": 0.01, "Na20": 0.04, "cost": 263.16 }, "HJO": { "Zr02": 0.90, "Al203": 0.01, "Si02": 0.01, "Na20": 0.002, "cost": 1315.79 } }, }; var results = solver.Solve( model ); console.log( results ); let constraintResults = {}; for ( let v in results ) { if ( !( v === 'bounded' || v === 'feasible' || v === 'result' ) ) { for ( let c in model.constraints ) { constraintResults[ c ] = ( constraintResults[ c ] || 0.0 ) + model.variables[ v ][ c ] * results[ v ]; } } } console.log( constraintResults ); </script>。我的意思是不要在外部将其声明为私有变量！

DatabaseReference

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