Android Studio -（java.lang.NullPointerException: 尝试调用虚拟方法 'com.google.firebase.database.ChildEventListener'）

Question

我正在尝试从 Firebase 实时数据库检索数据以显示在 Android Studio 的 RecyclerView 中，但是在运行模拟器上的应用程序之后，它只能显示 1 行数据，所以我检查了logcat，这是错误

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

我现在只是尝试检索小贩的姓名和描述

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);
}
}

Hawker.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

Output in emulator These are the data in firebase

Answer 1

我认为你需要检查你的 Hawker class 是否与 firebase 中的数据匹配，你的 java class 中的属性名称和属性数量应该匹配到 firebase

Android Studio -（java.lang.NullPointerException: 尝试调用虚拟方法 'com.google.firebase.database.ChildEventListener'）

Android Studio - (java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener')

java

android

android-recyclerview

firebase-realtime-database

firebaseui