小编mrA*_*ool的帖子

我对Android很新.这是添加到Eclipse ADT中引用的MyFirstApp指南的几行.我只是想在TextView中更改文本.我很尴尬,我已经在这几个小时了,仍然无法找到解决方案.任何帮助表示赞赏:-)

MainActivity.Java

public class MainActivity extends ActionBarActivity {

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

        TextView objTV = (TextView)this.findViewById(R.id.tv_id);
        objTV.setText("test");

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

fragment_main.xml

<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.myfirstapp.MainActivity$PlaceholderFragment" >

<TextView
    android:id="@+id/tv_id"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

</RelativeLayout>

logcat的

03-22 13:44:36.793: E/AndroidRuntime(1710): FATAL EXCEPTION: main
03-22 13:44:36.793: E/AndroidRuntime(1710): Process: com.example.myfirstapp, PID: 1710
03-22 13:44:36.793: E/AndroidRuntime(1710): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}: java.lang.NullPointerException
03-22 13:44:36.793: …