Question

所以我有一个应用程序，为此，我在checkData()中运行方法onCreate以确保所需的子节点存在，如果不存在，则创建它们（因为我想确保一切都适用于第一个用户）。我在updateUI()中致电onStart。这将从相应节点检索数据并分别更新几个文本字段。在添加了一些不使用这些方法之一的代码后，我突然开始收到错误（即使删除了所述代码后错误仍然存在，所以不是问题）。

如果我删除updateUI()电话，节点会正确创建，并且所有内容都可以在后续的程序运行中运行，但在updateUI()之后立即调用checkData()告诉我我的数据不存在，也不创建节点。我找不到这里出了什么问题，我不知所措。

public void checkData()
    {
        //Checks if the "users" table exists in the database, and populates it with the current user if it doesn't
       // mDatabase.addListenerForSingleValueEvent(this);
        mDatabase.addListenerForSingleValueEvent(new ValueEventListener()
        {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot)
            {
                if (!dataSnapshot.hasChild("users"))
                {
                    Calendar cal = Calendar.getInstance();
                    User user = new User(mUser.getUid(), mUser.getDisplayName(), mUser.getEmail(), "", "Chennai", cal);
                    mDatabase.child("users").child(mUser.getUid()).setValue(user);
                    mUsers = mDatabase.child("users");
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError)
            {

            }
        });

        //Checks if the current user exists in the "users" table, and adds it if it doesn't
        mUsers.addListenerForSingleValueEvent(new ValueEventListener()
        {
            @Override
            public void onDataChange(DataSnapshot snapshot)
            {
                //Log.e("Data Change", snapshot.getValue().toString());
                if (!snapshot.hasChild(mUser.getUid()))
                {
                    Calendar cal = Calendar.getInstance();
                    User user = new User(mUser.getUid(), mUser.getDisplayName(), mUser.getEmail(), "", "Chennai", cal);
                    mUsers.child(mUser.getUid()).setValue(user);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError)
            {

            }
        });
    }


public void updateUI()
{
    Log.e("Update UI", "Called");

   eText_passName = findViewById(R.id.eText_passName);
   eText_passName.setText(mUser.getDisplayName());
   eText_date = findViewById(R.id.eText_Date);
   eText_time = findViewById(R.id.eText_Time);

   //Gets DateTime String and sets it in the fields
   mCurrentUser = mUsers.child(mUser.getUid());
   mCurrentUser.addListenerForSingleValueEvent(new ValueEventListener()
   {
       @Override
       public void onDataChange(DataSnapshot dataSnapshot)
       {
           User u = dataSnapshot.getValue(User.class);


           String dt = u.getDt();
           String dtArr[] = dt.split(" ");

           eText_date.setText(dtArr[0]);
           eText_time.setText(dtArr[1]);

       }

       @Override
       public void onCancelled(DatabaseError databaseError)
       {

       }
   });

}

错误：

12-29 00:54:30.720 26411-26411/com.namsnath.cabplanner E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         Process: com.namsnath.cabplanner, PID: 26411
                                                                         java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.namsnath.cabplanner.User.getDt()' on a null object reference
                                                                             at com.namsnath.cabplanner.MainActivity$3.onDataChange(MainActivity.java:152)
                                                                             at com.google.firebase.database.zzp.onDataChange(Unknown Source)
                                                                             at com.google.android.gms.internal.zzegf.zza(Unknown Source)
                                                                             at com.google.android.gms.internal.zzeia.zzbyc(Unknown Source)
                                                                             at com.google.android.gms.internal.zzeig.run(Unknown Source)
                                                                             at android.os.Handler.handleCallback(Handler.java:751)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                             at android.os.Looper.loop(Looper.java:154)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:6123)
                                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)

错误是在updateUI（）中调用u.getDt（）似乎数据写入在某处被中断，但我找不到哪里。日志也没有显示出任何结果。

编辑：这可能是因为互联网连接缓慢，在调用数据之前不允许数据库更新？如果是这样，Firebase是否应该存储本地副本？这是不是被访问了？

编辑2：

    public class User
{
    String uid, username, email, group, destination;
    //CustomCal c;
    String dt;

    User()
    {}

    User(String uid, String username, String email, String group, String destination, Calendar cal)
    {
        this.uid = uid;
        this.username = username;
        this.email = email;
        this.group = group;
        this.destination = destination;

        SimpleDateFormat format = new SimpleDateFormat("dd-MM-yyyy KK:mm");
        dt = null;
        try
        {
            dt = format.format(cal.getTime());
        }
        catch (Exception e)
        {

        }

        /*int year = cal.get(Calendar.YEAR);
        int day = cal.get(Calendar.DATE);
        int month = cal.get(Calendar.MONTH);*/
    }
}

我写的数据是这种形式：

cabplanner
  users
    userUID
      Data from user class

Answer 1

datasnapshot.getValue可能会返回多个结果。试试这个：

for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
   User u = snapshot.getValue(User.class);
   ...
}

Firebase数据被推送但在Null指针结果后立即检索

1 个答案: