java - 初始化变量的 NullPointerException？

Question

我有一个 TextView [?] 的问题，我初始化了 textview 因为我想让它滚动但是我有一个错误。

public void onCreate(Bundle savedInstanceState)
{
     super.onCreate(savedInstanceState);
     dc = new DigitalClock(this);
     textViewMesssage = new TextView(this);
     dc = (DigitalClock) findViewById(R.id.digitalClock1);
     textViewMesssage = (TextView) findViewById(R.id.textV);
     textViewMesssage.setMovementMethod(new ScrollingMovementMethod());
     setContentView(R.layout.main);
     addListenerOnButton();

}
public void addListenerOnButton() {
    buttonTest = (Button) findViewById(R.id.button_id);
    buttonTest.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View arg0) {
                                  Calendar c = Calendar.getInstance();
                                  SimpleDateFormat sdf = new SimpleDateFormat("dd:MMMM:yyyy HH:mm:ss a");
                                  String strDate = sdf.format(c.getTime());
        }

    });
}

主.xml

<TextView  
    android:id="@+id/textV"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:text="MainActivity"
    android:layout_gravity="left"
    android:scrollbars = "vertical"
    />
<Button
    android:id="@+id/button_id"  
    android:layout_width="100dp" 
    android:layout_height="wrap_content" 
    android:text="Hello"
    android:layout_gravity="right"

    />

和错误

20:36:04.407 18740 XXX.X ERROR AndroidRuntime Caused by: java.lang.NullPointerException 20:36:04.407 18740 XXX.X ERROR AndroidRuntime at XXX.X.MainActivity.onCreate(MainActivity.java:40) 40 line is textViewMesssage.setMovementMethod(new ScrollingMovementMethod());

我正在使用 API 级别 8

Answer 1

setContentView(R.layout.main); 必须在尝试从布局 XML 获取任何 View 之前调用。