使用Firebase数据库中的Map检索数据

Question

我试图以Map格式从firebase数据库中检索数据,但我得到一个空指针异常.我已成功使用map将数据插入到我的firebase数据库中.

这是我的代码:

public class Chat extends AppCompatActivity {

  DatabaseReference databaseReference1;

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

    Firebase.setAndroidContext(this);
    databaseReference1 = FirebaseDatabase.getInstance().getReferenceFromUrl("URL");

    databaseReference1.addChildEventListener(new com.google.firebase.database.ChildEventListener() {
        @Override
        public void onChildAdded(com.google.firebase.database.DataSnapshot dataSnapshot, String s) {
            Map map = (Map) dataSnapshot.getValue();
            String message = map.get("text").toString();   //ERROR GOES HERE!!
            String userName = map.get("Username").toString();

        }

        @Override
        public void onChildChanged(com.google.firebase.database.DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(com.google.firebase.database.DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(com.google.firebase.database.DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}


}

Answer 1

在检索时更改您的代码

Map<String, String> map  = (Map) dataSnapshot.getValue();

代替

Map map = (Map) dataSnapshot.getValue();

编辑: 您可以使用以下代码检索录制列表

databaseReference1.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
            Map<String, String> map = (Map) postSnapshot.getValue();
            if (map != null) {
                String message = map.get("text");
                String userName = map.get("Username");
            }
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
});