Android ActionBar不可见

Question

I've updated the API 21, tools and extras up to date, the ActionBar in some projects has gone invisible. I did this following code, but app crashes showing NullPointerException.

ActionBar actionBar = getActionBar();
actionBar.show();

Logcat Output

02-23 18:03:36.269: E/AndroidRuntime(3327): FATAL EXCEPTION: main 02-23 18:03:36.269: E/AndroidRuntime(3327): Process: com.example.twaapp, PID: 3327 02-23 18:03:36.269: E/AndroidRuntime(3327): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.twaapp/com.example.twaapp.MainActivity}: java.lang.NullPointerException 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread.access$800(ActivityThread.java:139) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.os.Handler.dispatchMessage(Handler.java:102) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.os.Looper.loop(Looper.java:136) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread.main(ActivityThread.java:5086) 02-23 18:03:36.269: E/AndroidRuntime(3327): at java.lang.reflect.Method.invokeNative(Native Method) 02-23 18:03:36.269: E/AndroidRuntime(3327): at java.lang.reflect.Method.invoke(Method.java:515) 02-23 18:03:36.269: E/AndroidRuntime(3327): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785) 02-23 18:03:36.269: E/AndroidRuntime(3327): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) 02-23 18:03:36.269: E/AndroidRuntime(3327): at dalvik.system.NativeStart.main(Native Method) 02-23 18:03:36.269: E/AndroidRuntime(3327): Caused by: java.lang.NullPointerException 02-23 18:03:36.269: E/AndroidRuntime(3327): at com.example.twaapp.MainActivity.onCreate(MainActivity.java:82) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.Activity.performCreate(Activity.java:5248) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.Instrumentatio n.callActivityOnCreate(Instrumentation.java:1110) 02-23 18:03:36.269: E/AndroidRuntime(3327): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162) 02-23 18:03:36.269: E/AndroidRuntime(3327): ... 11 more

How to make AB visible?

Answer 1

Your Activity class should extend like this:

public class MainActivity extends ActionBarActivity{

    ....

}

instead of

public class MainActivity extends FragmentActivity{

    ....

}

Also, you need to call

ActionBar = getSupportActionBar();

instead of

ActionBar = getActionBar();

Try this. This will work.

Answer 2

Make sure that your activity extends the ActionBarActivity . Like this: MainActivity extends ActionBarActivity . I found that when I remove this and extend something else that the action bar would disappear. Also check the theme of the xml and change it.

Check this out:

can I use a compatible action bar AND a fragment in the same activity?

Answer 3

如果您正在使用Activity类，那么您的最小SDK应该更大，以显示操作栏。否则在Android支持库中使用ActionbarActivity使您的应用向后兼容并使用getSupportActionBar（）来获取操作栏实例

Answer 4

Since ActionBarActivity is deprecated, AppCompatActivity should be used. As-

public class LoginActivity extends AppCompatActivity{

}