java - 我需要从 FirebaseDatabse 读取数据

Question

我需要从 firebase 数据库检索数据，但在执行时遇到一些错误，我在下面发布了这些错误以及我正在使用的代码。请仔细检查并通知我哪里做错了。

提前致谢。

这是我的MainActivity.java

public class MainActivity extends AppCompatActivity 
{
private FirebaseDatabase mFirebaseDatabase;
private DatabaseReference mDatabaseReference;
private ChildEventListener mChildEventListener;
private ListView mListView;
private CustomAdapter mAdapterClass;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mListView=(ListView)findViewById(R.id.mainListView);
    List<AdapterClass> list=new ArrayList<>();
    mAdapterClass=new CustomAdapter(this,R.layout.item_list,list);
    mListView.setAdapter(mAdapterClass);


    mFirebaseDatabase=FirebaseDatabase.getInstance();
    mDatabaseReference=mFirebaseDatabase.getReference().child("messages");


    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent=new Intent(MainActivity.this,CreatePost.class);
            startActivity(intent);
        }
    });


    mChildEventListener=new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            AdapterClass data = dataSnapshot.getValue(AdapterClass.class);
            mAdapterClass.add(data);

        }

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

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

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

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    };
    mDatabaseReference.addChildEventListener(mChildEventListener);


}

}

这是我的CustomAdapter.java

public class CustomAdapter extends ArrayAdapter<AdapterClass> {
public CustomAdapter(@NonNull Context context, int resource, @NonNull List<AdapterClass> objects) {
    super(context, resource, objects);
}

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
    View currentView=convertView;
    if(currentView==null)
    {
        currentView= LayoutInflater.from(getContext()).inflate(R.layout.item_list,parent,false);
    }
    AdapterClass adapter=getItem(position);
    TextView txt=(TextView)currentView.findViewById(R.id.listItemTextView);
    txt.setText(adapter.getMessage());
    return currentView;
}
}

这是我的AdapterClass.java

public class AdapterClass {

private String mMessage;

public AdapterClass()
{
}
public AdapterClass(String message)
{
    this.mMessage=message;
}
public String getMessage()
{
    return mMessage;
}
public void setMessage(String message)
{
    mMessage=message;
}

}

我得到的错误是

 FATAL EXCEPTION: main
              Process: com.example.android.vvietproject, PID: 22972
              com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.android.vvietproject.AdapterClasses.AdapterClass
                  at com.google.android.gms.internal.zzepg.zzb(Unknown Source)
                  at com.google.android.gms.internal.zzepg.zza(Unknown Source)
                  at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                  at com.example.android.vvietproject.MainActivity$2.onChildAdded(MainActivity.java:61)
                  at com.google.android.gms.internal.zzegg.zza(Unknown Source)
                  at com.google.android.gms.internal.zzelk.zzcal(Unknown Source)
                  at com.google.android.gms.internal.zzelq.run(Unknown Source)
                  at android.os.Handler.handleCallback(Handler.java:751)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6816)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1565)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1453)

应用程序已终止。

Answer 1

问题出在这一行:

 AdapterClass data = dataSnapshot.getValue(AdapterClass.class);

您无法将 firebase 数据转换为 pojo。

您可以做的一件事是:

public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        String message = (String)dataSnapshot.child("message").getValue();
        AdapterClass data = new Adapter(message);
        mAdapterClass.add(data);

    }