java - 写入 Firebase 时出错。应用程序不断崩溃

Question

所以，我是 android 和 firebase 的新手。我正在尝试创建一个消息传递应用程序。但是只要我点击发送按钮，我的应用程序就会崩溃。我确切地知道应用程序崩溃的位置。在我尝试将数据放入数据库时​​单击发送按钮后，它立即崩溃了。检查了 Logcat，一切仍然无法弄清楚如何修复它。下面是 MainActivity.java 的代码

导入 com.google.firebase.database.DatabaseReference；导入 com.google.firebase.database.FirebaseDatabase;

公共(public)类 MainActivity 扩展 AppCompatActivity {

private Button sendButton;
private TextView editMessage;
private ListView messageList;

private FirebaseDatabase firebaseDatabase;
private DatabaseReference qDatabaseReference;

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


    try {
        firebaseDatabase = FirebaseDatabase.getInstance();
        qDatabaseReference = qDatabaseReference.getRef().child("messages");
    }catch(Exception e){
        Toast.makeText(this,"Database Error!", Toast.LENGTH_SHORT);
        e.printStackTrace();
    }

    sendButton = findViewById(R.id.sendButton);
    editMessage = findViewById(R.id.messageEdit);
    messageList = findViewById(R.id.messageList);

    sendButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Message message = new Message("user","TEST");
            //app crashes here at the line below
            qDatabaseReference.push().setValue(message);
            editMessage.setText("");
        }
    });
}
}

我的消息类代码

public class Message {

private int maxLength = 1000;

private String name;
private String text;

Message(){
    name = null;
    text = null;
}

Message(String name, String message){
    this.name = name;
    this.text = message;
}
}

终于在LogCat中找到了错误

java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.push()' on a null object reference
    at com.codeswingstudios.codewise.MainActivity$1.onClick(MainActivity.java:42)
    at android.view.View.performClick(View.java:5293)
    at android.view.View$PerformClick.run(View.java:21660)
    at android.os.Handler.handleCallback(Handler.java:815)
    at android.os.Handler.dispatchMessage(Handler.java:104)
    at android.os.Looper.loop(Looper.java:227)
    at android.app.ActivityThread.main(ActivityThread.java:6100)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:859)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:749)

Answer 1

尝试像这样初始化您的 DatabaseReference:

DatabaseReference qDatabaseReference = FirebaseDatabase.getInstance().getReference().child("messages");