java - Android Studio - (java.lang.NullPointerException:尝试调用虚拟方法'com.google.firebase.database.ChildEventListener')
问题描述
我正在尝试从 Firebase 实时数据库中检索数据以显示在 Android Studio 的 RecyclerView 中,但是在模拟器上运行应用程序后,它只显示了 1 行数据,所以我检查了 logcat,这是错误
java.lang.RuntimeException:无法启动活动 ComponentInfo{com.example.test/com.example.test.MainActivity}:java.lang.NullPointerException:尝试调用虚拟方法 'com.google.firebase.database.ChildEventListener com. google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' 在空对象引用上
我现在只是试图检索小贩的名称和描述
MainActivity.java
public class MainActivity extends AppCompatActivity {
DatabaseReference ref;
private FirebaseRecyclerOptions<Hawker> options;
private FirebaseRecyclerAdapter<Hawker, myViewHolder> adapter;
private RecyclerView recyclerView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ref = FirebaseDatabase.getInstance().getReference().child("hawkers");
recyclerView=findViewById(R.id.recycler);
recyclerView.setHasFixedSize(false);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
options = new FirebaseRecyclerOptions.Builder<Hawker>().setQuery(ref, Hawker.class).build();
adapter = new FirebaseRecyclerAdapter<Hawker, myViewHolder>(options) {
@Override
protected void onBindViewHolder(@NonNull myViewHolder holder, int position, @NonNull Hawker model) {
holder.textViewID.setText(model.getName());
holder.textViewName.setText(model.getDescription());
}
@NonNull
@Override
public myViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.single_view_layout,parent,false);
return new myViewHolder(v);
}
};
adapter.startListening();
recyclerView.setAdapter(adapter);
}
}
myViewHolder.java
public class myViewHolder extends RecyclerView.ViewHolder{
TextView textViewID, textViewName;
public myViewHolder(@NonNull View itemView) {
super(itemView);
textViewID = itemView.findViewById(R.id.textViewID);
textViewName = itemView.findViewById(R.id.textViewName);
}
}
小贩.java
public class Hawker {
private String name;
private String description;
public Hawker() {
}
public Hawker(String name, String description) {
this.name = name;
this.description = description;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
}
logcat 中的完整错误
2021-03-05 00:24:30.691 6466-6466/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.test, PID: 6466
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
at com.firebase.ui.database.FirebaseArray.onCreate(FirebaseArray.java:54)
at com.firebase.ui.common.BaseObservableSnapshotArray.addChangeEventListener(BaseObservableSnapshotArray.java:97)
at com.firebase.ui.database.FirebaseRecyclerAdapter.startListening(FirebaseRecyclerAdapter.java:52)
at com.example.test.MainActivity.onCreate(MainActivity.java:67)
at android.app.Activity.performCreate(Activity.java:8000)
at android.app.Activity.performCreate(Activity.java:7984)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
2021-03-05 00:24:30.703 6466-6466/? I/Process: Sending signal. PID: 6466 SIG: 9
解决方案
我认为您需要检查您的 Hawker 类是否与 firebase 中的数据匹配,您的 java 类中的属性名称和属性数应与 firebase 匹配