Question

帮助，SomeBody帮助我..

这是数据库结构

这是我的MainActivity onCreate方法

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    activity_main = (RelativeLayout) findViewById(R.id.activity_main);
    input = (EditText)findViewById(R.id.inputmessage);
    fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(this);

    mAuth = FirebaseAuth.getInstance();
    if(mAuth.getCurrentUser() != null)
    {
        Toast.makeText(MainActivity.this, "Welcome "+mAuth.getCurrentUser().getEmail(), Toast.LENGTH_SHORT).show();
    }
    else{
        finish();
        Intent intent = new Intent(MainActivity.this, Sign_in_form.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        startActivity(intent);
    }
    chatlist = new ArrayList<>();
    listofMsg = (ListView) findViewById(R.id.list_of_messange);
    databaseChat = FirebaseDatabase.getInstance().getReference("chatyoutubemajta");
    databaseChat.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
           // Toast.makeText(MainActivity.this, "onChildAdded:" + dataSnapshot.getKey(), Toast.LENGTH_SHORT).show();
            String id = dataSnapshot.getKey();
            ChatMsg chatmsg = dataSnapshot.child(id).getValue(ChatMsg.class);

            chatlist.add(chatmsg);

            DaftarChat adapter = new DaftarChat(MainActivity.this,chatlist);
            listofMsg.setAdapter(adapter);
        }

        @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) {

        }
    });

}

这是Daftarchat课程：

public class DaftarChat extends ArrayAdapter<ChatMsg> {
private Activity context;
private List<ChatMsg> daftarchat;

public DaftarChat(Activity context,List<ChatMsg> daftarchat){
    super(context,R.layout.list_item,daftarchat);
    this.context = context;
    this.daftarchat = daftarchat;
}

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
    LayoutInflater inflater = context.getLayoutInflater();
    View listViewItem = inflater.inflate(R.layout.list_item,null,true);


    TextView txttext = (TextView)listViewItem.findViewById(R.id.message_text);
    TextView txtuser = (TextView)listViewItem.findViewById(R.id.message_user);
    TextView txttime = (TextView)listViewItem.findViewById(R.id.message_time);

    ChatMsg sampel = daftarchat.get(position);
    txttext.setText(sampel.getMsgText());
    txtuser.setText(sampel.getMsgUser());
    txttime.setText(sampel.getMsgTime());

    return listViewItem;

}

}

问题是它一旦开始就会一直崩溃：

当我删除此语句时（在DaftarChat类中）：

txttext.setText(sampel.getMsgText());
    txtuser.setText(sampel.getMsgUser());
    txttime.setText(sampel.getMsgTime());

它什么都没有显示，但程序可以运行，但我仍然无法检索数据。

下面的logcat：

FATAL EXCEPTION: main
                                                                           Process: com.example.chatapplication, PID: 4384
                                                                           java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.chatapplication.ChatMsg.getMsgText()' on a null object reference
                                                                               at com.example.chatapplication.DaftarChat.getView

请帮助，我无法弄清楚..

由于

Helwa

Answer 1

空对象引用上的ChatMsg.getMsgText（）'

sampel.getMsgText（）为null。添加一些空检查以防止应用程序崩溃，如：

txttext.setText(sampel.getMsgText()!=null?sampel.getMsgText():INSERT_DEFAULT_VALUE_HERE);

Answer 2

好吧因此可能存在一些问题：

在onCreate中仅设置适配器一次：

DaftarChat adapter = new DaftarChat(MainActivity.this, new ArrayList<>());
listofMsg.setAdapter(adapter);

在onChildAdded方法中：

@Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
           // Toast.makeText(MainActivity.this, "onChildAdded:" + dataSnapshot.getKey(), Toast.LENGTH_SHORT).show();
            String id = dataSnapshot.getKey();
            for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
                ChatMsg msg = dataSnapshot.child(id).getValue(ChatMsg.class);
                if(msg != null){
                  listOfMsg.getAdapter().addMsg(chatmsg);
                }
             }
        }

然后在Adapter类中添加此方法：

public void addMsg(ChatMsg chatmsg){
this.daftarchat.add(chatmsg);
notifyDataSetChanged();
}

并修改你的getView方法：

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
    LayoutInflater inflater = context.getLayoutInflater();
    View listViewItem = inflater.inflate(R.layout.list_item,null,true);


    TextView txttext = (TextView)listViewItem.findViewById(R.id.message_text);
    TextView txtuser = (TextView)listViewItem.findViewById(R.id.message_user);
    TextView txttime = (TextView)listViewItem.findViewById(R.id.message_time);


    ChatMsg sampel = daftarchat.get(position);
    if(sampel != null){
      txttext.setText(sampel.getMsgText());
      txtuser.setText(sampel.getMsgUser());
      txttime.setText(sampel.getMsgTime());
    }
    return listViewItem;
}

Answer 3

假设所有这些消息都是Firebase数据库根目录的直接子节点，为了使其正常工作，请使用以下更简单的代码：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<ChatMsg> chatlist = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            ChatMsg chatmsg = dataSnapshot.getValue(ChatMsg.class);
            chatlist.add(chatmsg);
        }
        ListView listofMsg = (ListView) findViewById(R.id.list_of_messange);
        DaftarChat adapter = new DaftarChat(MainActivity.this, chatlist);
        listofMsg.setAdapter(adapter);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
rootRef.addListenerForSingleValueEvent(eventListener);

Answer 4

我刚刚找到答案，我们应该在这种情况下使用迭代器方法。

以下是我从互联网上找到的精彩文章：

onChildAdded Method with iterator

我的应用程序在启动时会一直崩溃

4 个答案: