尝试将listView设置为可单击时出现NullPointerException

Question

我的Android应用程序中出现了NullPointerException，据LogCat所知，这是在尝试将列表视图设置为可点击时发生的。这是我的密码。

public class MyActivity extends Activity {
private ListView lstView = (ListView) findViewById(R.id.listView1);
private SQLiteDatabase database;
private static final String fields[] = { "field1", "field2", "field3" };

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    CursorAdapter dataSource;
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    //lstView.setClickable(true);   
private static final String fields[] = { "field1", "field2", "field3" };

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    CursorAdapter dataSource;
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    lstView.setClickable(true);
    DatabaseHelper helper = new DatabaseHelper(this);
    database = helper.getWritableDatabase();
    Cursor data = database.query("mydb", fields, null, null, null, null, null);
    dataSource = new SimpleCursorAdapter(this, R.layout.row, data, fields, new int[] {R.id.field1, R.id.field2});
    database.close();
    lstView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
          @Override
          public void onItemClick(AdapterView<?> arg0, View arg1, int position, long arg3) {
            Intent newActivity = new Intent(getApplicationContext(), NewIntent.class);
            newActivity.putExtra("itemId",lstView.getItemIdAtPosition(position));
            // start activity
            startActivity(newActivity);
          }
        });
}

}

这是LogCat输出

01-10 14:03:53.740: E/AndroidRuntime(30361): FATAL EXCEPTION: main
01-10 14:03:53.740: E/AndroidRuntime(30361): java.lang.RuntimeException: Unable to start activity ComponentInfo{app.jonward.myapp/app.jonward.myapp.MyActivity}: java.lang.NullPointerException
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1821)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1842)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread.access$1500(ActivityThread.java:132)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1038)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.os.Handler.dispatchMessage(Handler.java:99)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.os.Looper.loop(Looper.java:150)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread.main(ActivityThread.java:4263)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at java.lang.reflect.Method.invokeNative(Native Method)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at java.lang.reflect.Method.invoke(Method.java:507)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at dalvik.system.NativeStart.main(Native Method)
01-10 14:03:53.740: E/AndroidRuntime(30361): Caused by: java.lang.NullPointerException
01-10 14:03:53.740: E/AndroidRuntime(30361):    at  app.jonward.castr.CastrActivity.onCreate(MyActivity.java:31)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072)
01-10 14:03:53.740: E/AndroidRuntime(30361):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1785)
01-10 14:03:53.740: E/AndroidRuntime(30361):    ... 11 more

LogCat所说的导致异常的MyActivity第31行的代码是

lstView.setClickable(true);

固定这是工作代码。有一些变化。公共类MyActivity扩展ListActivity{专用SQLiteDatabase数据库；private static final String fields[]=｛"field1"，"field2"，"_id"｝；

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    CursorAdapter dataSource;
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    DatabaseHelper helper = new DatabaseHelper(this);
    database = helper.getWritableDatabase();
    Cursor data = database.query("castr", fields, null, null, null, null, null);
    dataSource = new SimpleCursorAdapter(this, R.layout.row, data, fields, new int[] {R.id.field1, R.id.field2});
    final ListView view = getListView();
    view.setHeaderDividersEnabled(true);
    view.addHeaderView(getLayoutInflater().inflate(R.layout.row, null));
    database.close();
    view.setOnItemClickListener(new AdapterView.OnItemClickListener() {
          @Override
          public void onItemClick(AdapterView<?> arg0, View arg1, int position, long arg3) {
            // Prepare intent
            Intent newActivity = new Intent(getApplicationContext(), NewActivity.class);
            newActivity.putExtra("itemId",view.getItemIdAtPosition(position));
            // start activity
            startActivity(newActivity);
          }
        });
}
}

Answer 1

您应该从布局中获得listView引用。可能会用这样的东西。。

listView = (ListView)findViewById(R.id.nameofyourlistView);

然后将listView设置为可单击。

另一方面，为什么要将列表视图设置为可单击？，您无需执行此操作，默认情况下，列表项是可单击的。

Answer 2

在使用ListView之前，您需要初始化它。

lstView= (ListView) findViewById(R.id.your_listview_id);