java - Android应用程式在启动时当机

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NullPointerException accessing views in onCreate()
                                
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                                5年前关闭。
            
                    
我刚刚开始开发android应用程序，并在打开以下应用程序时显示"Unfortunately your app has crashed"。任何人都可以指出此代码中的错误。

fragmentmain.xml

<LinearLayout 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:background="#ffffbb"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="85dp"
    android:text="Info of Departments and Student Bodies"
    android:textAppearance="?android:attr/textAppearanceLarge" />

<ListView
    android:id="@+id/listView"
    android:layout_width="match_parent"
    android:layout_height="190dp" >
</ListView>

</LinearLayout>

MainActivity.java

public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //adding items to list view
    final ListView listview = (ListView) findViewById(R.id.listView);
    String values[] = new String[]{"Departments" , "Student_bodies"};
    final ArrayList<String> list = new ArrayList<String>();
    for (int i=0 ; i < values.length; ++i){
        list.add(values[i]);
    }

    ArrayAdapter<String> ap = new ArrayAdapter<String>      (this,android.R.layout.simple_list_item_1,values);
    listview.setAdapter(ap);

    listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
                long arg3) {


            if (((TextView)arg1).getText().toString() == "Departments"){
                startActivity(new Intent(MainActivity.this,Departments.class));
            }
            if (((TextView)arg1).getText().toString() == "Student_bodies"){
                startActivity(new Intent(MainActivity.this,Student_bodies.class));
            }
        }
    });
}



@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}

}

这只是列表视图的简单实现，并且在其元素上实现onClick()可以转到不同的Activity。如果有人指出错误，请多谢。

最佳答案

您的布局文件名为FragmentMain，但是您使用activity_main作为setContentView()方法的参数。尝试更改为setContentView(R.layout.FragmentMain);