java.lang.NullPointerException:尝试调用虚拟方法';com.google.firebase.database
我试图从Firebase数据库检索数据到RecyclerView中,但遇到以下错误:java.lang.NullPointerException:尝试调用虚拟方法<代码>“com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)”在空对象引用上 下面是我的主要活动课java.lang.NullPointerException:尝试调用虚拟方法';com.google.firebase.database,java,android,firebase,firebase-realtime-database,Java,Android,Firebase,Firebase Realtime Database,我试图从Firebase数据库检索数据到RecyclerView中,但遇到以下错误:java.lang.NullPointerException:尝试调用虚拟方法“com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)”在空对象引用上 下面是我的主要活动课 package oerrride.we.huzykam
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();
}
包oerrride.we.huzykamz.testingappfirebase;
导入android.app.Dialog;
导入android.content.Context;
导入android.support.v7.app.AppActivity;
导入android.os.Bundle;
导入android.support.v7.widget.LinearLayoutManager;
导入android.support.v7.widget.RecyclerView;
导入android.view.view;
导入android.widget.Button;
导入com.google.android.gms.common.GoogleAppAvailability;
导入com.google.android.gms.tasks.Task;
导入com.google.firebase.database.ChildEventListener;
导入com.google.firebase.database.DataSnapshot;
导入com.google.firebase.database.DatabaseError;
导入com.google.firebase.database.DatabaseReference;
导入com.google.firebase.database.FirebaseDatabase;
导入com.google.firebase.database.ValueEventListener;
导入java.util.ArrayList;
导入java.util.List;
公共类MainActivity扩展了AppCompatActivity{
私人按钮,但;
专用数据库参考mRef;
专用MyAdapter适配器;
公共回收视图回收视图;
公开电影名单;
公共环境c;
私人直线酒店经理直线酒店经理;
私有数据库引用mMovieRef=mRef.child(“名称”);
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
电影=新阵列列表();
recyclerView=(recyclerView)findViewById(R.id.recyclerView);
linearLayoutManager=新的linearLayoutManager(此);
recyclerView.setLayoutManager(linearLayoutManager);
recyclerView.setHasFixedSize(true);
mRef=FirebaseDatabase.getInstance().getReferenceFromUrl(“xxxxxxxxxxxxxxxxxxxxxx”);
mRef.addChildEventListener(新的ChildEventListener(){
@凌驾
公共void onChildAdded(DataSnapshot DataSnapshot,字符串s){
LoadData(dataSnapshot);
}
@凌驾
公共void onChildChanged(DataSnapshot DataSnapshot,字符串s){
LoadData(dataSnapshot);
}
@凌驾
ChildRemoved上的公共void(DataSnapshot DataSnapshot){
}
@凌驾
已移动ChildMoved上的公共void(DataSnapshot DataSnapshot,字符串s){
}
@凌驾
已取消的公共void(DatabaseError DatabaseError){
}
});
}
公共void加载数据(DataSnapshot DataSnapshot){
对于(DataSnapshot singlesnapshot:DataSnapshot.getChildren()){
String taskTitle=singlesnapshot.getValue(String.class);
添加(新模型类(taskTitle));
adapter=新的MyAdapter(MainActivity.this,movies);
recyclerView.setAdapter(适配器);
}
}
}
这是我的适配器类。
公共类MyAdapter扩展了RecyclerView.Adapter{
保护上下文c;
列出电影;
公共MyAdapter(上下文c,电影列表){
这个.c=c;
这部电影=电影;
}
@凌驾
公共MyViewHolder onCreateViewHolder(视图组父级,int-viewType){
视图v=LayoutInflater.from(parent.getContext()).flate(R.layout.item_活动,parent,false);
MyViewHolder=新的MyViewHolder(v);
报税表持有人;
}
@凌驾
公共无效onBindViewHolder(MyViewHolder,int位置){
holder.name.setText(movies.get(position.getName());
//PicassoClient.downloadImage(c,movies.get(position.getUrl(),holder.img);
}
@凌驾
public int getItemCount(){
返回电影。大小();
}
}
下面是我的Firebase数据库控制台
mRef尚未创建,因此它为null,将引发NullPointerException
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");
数据库引用可能未初始化:请尝试使用:
private DatabaseReference mMovieRef =new DatabaseReference()
mRef.child("Name");
也可以是:
private DatabaseReference mMovieRef =FirebaseDatabase.getInstance().getReferenceFromUrl("xxxxxxxxxxxxxxxxxxxxxx");
mRef.child("Name");
而不是
private DatabaseReference mMovieRef =mRef.child("Name");
您不需要在main外部初始化或声明您的
数据库引用
。我的意思是不要将它声明为私有变量
DatabaseReference mMovieRef = FirebaseDatabase.getInstance().getReferenceFromUrl("xxxxxxxxxxxxxxxxxxxxxx");
mRef.child("Name");
从这里你可以添加你的监听器please@Ironman这不是一个重复的监听器,不管两者有多少都是空指针,试着通读我的帖子。:)是的,这个错误已经改变了,谢谢,但我正在体验另一个@PoprostyRonin,它是
com.google.firebase.database.DatabaseException:无法将java.util.HashMap类型的值转换为字符串
ooh是的,我知道了这个错误的原因,我的数据格式很差,我已经调整过了,现在它工作得很好。:@@爆米花素
DatabaseReference mMovieRef = FirebaseDatabase.getInstance().getReferenceFromUrl("xxxxxxxxxxxxxxxxxxxxxx");
mRef.child("Name");