关于使用getActionBar()报空指针异常的错误

当你在onCreate()方法中添加如下代码的时候

<span style="font-size:14px;">ActionBar actionBar = getActionBar();</span>

有可能在控制台报出如下错误：

<span style="font-size:14px;">01-26 09:20:02.958: D/AndroidRuntime(18779): Shutting down VM
01-26 09:20:02.959: E/AndroidRuntime(18779): FATAL EXCEPTION: main
01-26 09:20:02.959: E/AndroidRuntime(18779): Process: com.example.tabwithslidingdrawer, PID: 18779
01-26 09:20:02.959: E/AndroidRuntime(18779): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tabwithslidingdrawer/com.example.tabwithslidingdrawer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2314)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.access$800(ActivityThread.java:148)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.os.Handler.dispatchMessage(Handler.java:102)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.os.Looper.loop(Looper.java:135)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.main(ActivityThread.java:5312)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at java.lang.reflect.Method.invoke(Native Method)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at java.lang.reflect.Method.invoke(Method.java:372)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
01-26 09:20:02.959: E/AndroidRuntime(18779): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.example.tabwithslidingdrawer.MainActivity.onCreate(MainActivity.java:95)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.Activity.performCreate(Activity.java:5953)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1128)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
01-26 09:20:02.959: E/AndroidRuntime(18779):    ... 10 more</span>

这时候当你点击getActionBar()跳到它的方法申明处，

<span style="font-size:14px;">package android.app;

public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback {

/**
* Retrieve a reference to this activity's ActionBar.
*
* @return The Activity's ActionBar, or null if it does not have one.
*/
@Nullable
public ActionBar getActionBar() {
initWindowDecorActionBar();
return mActionBar;
}
}</span>

发现这个包是android.app,而发现你当前自己创建的activity却不是直接继承的android.app.Activity,你的activity很有可能继承的

android.support.v7.app中的activity，所以这里就不能用getActionBar(),应该使用getSupportActionBar()方法，点击它方法申明处:

package android.support.v7.app;public class AppCompatActivity extends FragmentActivity implements AppCompatCallback,TaskStackBuilder.SupportParentable, ActionBarDrawerToggle.DelegateProvider {/*** Support library version of {@link android.app.Activity#getActionBar}.** <p>Retrieve a reference to this activity's ActionBar.** @return The Activity's ActionBar, or null if it does not have one.*/@Nullablepublic ActionBar getSupportActionBar() {return getDelegate().getSupportActionBar();}}

查看注释发现，这个方法是support 版本独有的，只能用这个方法在支持包中获得actionbar的引用。[/code]