Question

findViewById(R.id.myOneButton)为什么会返回null ？

新手学android，求教高手指点一下，十分感觉。

java代码如下：
public class MainActivity extends ActionBarActivity {

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

Button myButton = (Button)findViewById(R.id.myOneButton); //为什么这里是空值异常？
myButton.setText("我的第一个按钮");

if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}

}
}

R.java如下：
public static final class id {
public static final int myOneButton=0x7f05003e;
public static final int myOneTextView=0x7f05003d;
}

mian.xml如下：（注意这里是RelativeLayout ）
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.myandroidgame.MainActivity$PlaceholderFragment" >

<Button
android:id="@+id/myOneButton"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/myOneTextView"/>

</RelativeLayout>

若以下回答无法解决问题，邀请你更新回答

Answer 1

clean下 .............

追问

不行