使用setDisplayShowHomeEnabled(true)程序闪退怎么回事

就是一个点击一个activity里的按钮进入下一个activity，然后第二个activity里的操作栏有返回图标，
书上说使用NavUtils配合元数据实现响应向上按钮，可我一添加meta-data后点击按钮后就闪退怎么把啊

以下是错误
FATAL EXCEPTION: main
Process: com.example.administrator.a3333, PID: 14672
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.administrator.a3333/com.example.administrator.a3333.DetailActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2641)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2710)
at android.app.ActivityThread.access$800(ActivityThread.java:179)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1561)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5803)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1010)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at com.example.administrator.a3333.DetailActivity.onCreate(DetailActivity.java:19)
at android.app.Activity.performCreate(Activity.java:6117)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2594)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2710) 
at android.app.ActivityThread.access$800(ActivityThread.java:179) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1561) 
at android.os.Handler.dispatchMessage(Handler.java:111) 
at android.os.Looper.loop(Looper.java:194) 
at android.app.ActivityThread.main(ActivityThread.java:5803) 
at java.lang.reflect.Method.invoke(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:372) 

要使用getSupportActionBar()过去actionbar