为什么 postSnapshot 为空?

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英文:

why postSnapshot is looking null

问题

public  void setClock( int hour,  int minute){
    Alarm alm = findAlarm();
    if(alm == null){
        String pushkey = databaseReference.push().getKey();
        Log.e("push", pushkey);
        Alarm alarm = new Alarm(hour, minute, firebaseUser.getUid(), pushkey);
        databaseReference.child(pushkey).setValue(alarm);
    }
    else{
        alm.setHour(hour);
        alm.setMin(minute);
        databaseReference.child(alm.getPushKey()).setValue(alm);
    }
}

public Alarm findAlarm(){
    final Alarm alarm2 = new Alarm(); // Create a new Alarm instance instead of initializing as null

    databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Log.e("hour", String.valueOf(postSnapshot.child("hour").getValue(int.class)));
                Log.e("userid", postSnapshot.child("user").getValue(String.class));
                Log.e("userid2", firebaseUser.getUid());

                if(postSnapshot.child("user").getValue(String.class).equals(firebaseUser.getUid())) {
                    alarm2.setHour(postSnapshot.child("hour").getValue(int.class));
                    alarm2.setMin(postSnapshot.child("min").getValue(int.class));
                    alarm2.setPushKey(postSnapshot.child("pushKey").getValue(String.class));
                    alarm2.setUser(postSnapshot.child("user").getValue(String.class));
                }
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

    return alarm2;
}

Logcat

E/hour: 23
E/userid: E3CFesWWcrM7WAJCPwDENbQCk2i2
E/userid2: E3CFesWWcrM7WAJCPwDENbQCk2i2
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.sozluk, PID: 7185
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.sozluk.Alarm.setHour(int)' on a null object reference
at com.example.sozluk.Scroll$2.onDataChange(Scroll.java:111)
英文:

I try to create/update alarm data in firebase realtime database.

If there is an exinting alarm data for current user I want to update it.

I have a setClock method and try to control if there is an existing data so I write a new method(findAlarm) but I got a null object reference error.

I used Log.e to control that it is null or not but it is looking full.

I don't understand what is my fault.
please help

public  void setClock( int hour,  int minute){
Alarm alm=findAlarm();
if(alm==null){
String pushkey= databaseReference.push().getKey();
Log.e("push",pushkey);
Alarm alarm = new Alarm(hour,minute,firebaseUser.getUid(),pushkey);
databaseReference.child(pushkey).setValue(alarm);
}
else{
alm.setHour(hour);
alm.setMin(minute);
databaseReference.child(alm.getPushKey()).setValue(alm);
}
}
public Alarm findAlarm(){
final Alarm alarm2 = null;
databaseReference.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
Log.e("hour", String.valueOf(postSnapshot.child("hour").getValue(int.class)));
Log.e("userid",postSnapshot.child("user").getValue(String.class));
Log.e("userid2",firebaseUser.getUid());
if(postSnapshot.child("user").getValue(String.class).equals(firebaseUser.getUid())) {
alarm2.setHour(postSnapshot.child("hour").getValue(int.class));
alarm2.setMin(postSnapshot.child("min").getValue(int.class));
alarm2.setPushKey(postSnapshot.child("pushKey").getValue(String.class));
alarm2.setUser(postSnapshot.child("user").getValue(String.class));
}
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
return alarm2;
}

Logcat

E/hour: 23
E/userid: E3CFesWWcrM7WAJCPwDENbQCk2i2
E/userid2: E3CFesWWcrM7WAJCPwDENbQCk2i2
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.sozluk, PID: 7185
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.sozluk.Alarm.setHour(int)' on a null object reference
at com.example.sozluk.Scroll$2.onDataChange(Scroll.java:111)

答案1

得分: 0

alarm2 最初是 null,因为你设置了它的值为:

final Alarm alarm2 = null;

而且你从未给它赋新的值,因此在执行以下代码时它仍然保持为 null,从而导致你的应用崩溃:

alarm2.setHour(postSnapshot.child("hour").getValue(int.class));

在调用方法之前,你需要给它赋值。不幸的是,你将它声明为 final,以便在数据库回调内部使用。

为了正确使用 Firebase,你需要学习如何按设计异步地使用它的 API。查询不会立即返回结果。监听器会在某个时间点后被调用,即在查询完成后。你只能在监听器内部使用快照中的数据。

英文:

alarm2 is initially null, because you set it that way:

final Alarm alarm2 = null;

And you never gave it a new value, so it stays null as this line executes, crashing your app:

alarm2.setHour(postSnapshot.child("hour").getValue(int.class));

You will need to give it a value before calling methods on it. Unforutunately, you can't because you made it final so that you can use it inside the database callback.

In order to work with Firebase correctly, you will need to learn how to work with its APIs asynchronously, as they are designed. Queries don't return immediately. Your listener is going to get called some time later, after the query completes. You can only use the data from the snapshot in the listener.

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  • 本文由 发表于 2020年4月11日 07:27:13
  • 转载请务必保留本文链接:https://go.coder-hub.com/61150147.html
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