應用程序在模擬器中崩潰

Question

我搜索了很多並且沒有發現任何內容來防止此錯誤。我正在製作一個簡單的秒錶應用程序，當我嘗試測試它時，它正在模擬器中崩潰。

的事項代碼：

public class MyActivity extends Activity implements ActionBar.TabListener { 

    /** 
    * The {@link android.support.v4.view.PagerAdapter} that will provide 
    * fragments for each of the sections. We use a 
    * {@link FragmentPagerAdapter} derivative, which will keep every 
    * loaded fragment in memory. If this becomes too memory intensive, it 
    * may be best to switch to a 
    * {@link android.support.v13.app.FragmentStatePagerAdapter}. 
    */ 

    SectionsPagerAdapter mSectionsPagerAdapter; 

    /** 
    * The {@link ViewPager} that will host the section contents. 
    */ 
    ViewPager mViewPager; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_my); 
     Button start; 
     final TextView stopwatch_num; 
     final double mcount = 0.0; 
     start = (Button)findViewById(R.id.start_button); 
     stopwatch_num = (TextView)findViewById(R.id.timer_num); 
     start.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       mcount = mcount + .01; 
       stopwatch_num.setText(""+mcount); 
      } 


     }); 

的onclicklistener內部，在第一mcount，它說，它「不能賦值給最後一個變量‘mcount’」。

的logcat的：

06-13 19:29:04.280  851-851/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 60K, 10% free 3452K/3804K, paused 36ms, total 38ms 
06-13 19:29:04.480  851-851/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 3K, 9% free 3670K/4028K, paused 21ms, total 21ms 
06-13 19:29:04.500  851-851/martinproductions.time_me I/dalvikvm-heap﹕ Grow heap (frag case) to 6.283MB for 2536936-byte allocation 
06-13 19:29:04.540  851-861/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed <1K, 6% free 6148K/6508K, paused 37ms, total 37ms 
06-13 19:29:04.720  851-851/martinproductions.time_me D/AndroidRuntime﹕ Shutting down VM 
06-13 19:29:04.720  851-851/martinproductions.time_me W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0xb1ab0ba8) 
06-13 19:29:04.730  851-851/martinproductions.time_me E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: martinproductions.time_me, PID: 851 
    java.lang.RuntimeException: Unable to start activity ComponentInfo{martinproductions.time_me/martinproductions.time_me.MyActivity}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
      at android.app.ActivityThread.access$800(ActivityThread.java:135) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:136) 
      at android.app.ActivityThread.main(ActivityThread.java:5017) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:515) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at martinproductions.time_me.MyActivity.onCreate(MyActivity.java:49) 
      at android.app.Activity.performCreate(Activity.java:5231) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
            at android.app.ActivityThread.access$800(ActivityThread.java:135) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:136) 
            at android.app.ActivityThread.main(ActivityThread.java:5017) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:515) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
            at dalvik.system.NativeStart.main(Native Method) 
06-13 19:29:18.550  851-851/martinproductions.time_me I/Process﹕ Sending signal. PID: 851 SIG: 9 
device not found 

更新的logcat：

06-13 19:56:50.810  987-987/martinproductions.time_me D/dalvikvm﹕ Not late-enabling CheckJNI (already on) 
06-13 19:56:53.550  987-987/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 71K, 10% free 3452K/3812K, paused 36ms, total 38ms 
06-13 19:56:53.690  987-987/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 3K, 10% free 3670K/4036K, paused 19ms, total 20ms 
06-13 19:56:53.710  987-987/martinproductions.time_me I/dalvikvm-heap﹕ Grow heap (frag case) to 6.283MB for 2536936-byte allocation 
06-13 19:56:53.740  987-996/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed <1K, 6% free 6148K/6516K, paused 27ms, total 27ms 
06-13 19:56:53.890  987-987/martinproductions.time_me D/AndroidRuntime﹕ Shutting down VM 
06-13 19:56:53.890  987-987/martinproductions.time_me W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0xb1a8fba8) 
06-13 19:56:53.900  987-987/martinproductions.time_me E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: martinproductions.time_me, PID: 987 
    java.lang.RuntimeException: Unable to start activity ComponentInfo{martinproductions.time_me/martinproductions.time_me.MyActivity}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
      at android.app.ActivityThread.access$800(ActivityThread.java:135) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:136) 
      at android.app.ActivityThread.main(ActivityThread.java:5017) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:515) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at martinproductions.time_me.MyActivity.onCreate(MyActivity.java:38) 
      at android.app.Activity.performCreate(Activity.java:5231) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
            at android.app.ActivityThread.access$800(ActivityThread.java:135) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:136) 
            at android.app.ActivityThread.main(ActivityThread.java:5017) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:515) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
            at dalvik.system.NativeStart.main(Native Method) 
06-13 19:56:57.120  987-987/martinproductions.time_me I/Process﹕ Sending signal. PID: 987 SIG: 9 
06-13 19:57:03.190 1030-1030/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 79K, 10% free 3452K/3820K, paused 102ms, total 104ms 
06-13 19:57:03.520 1030-1030/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed 3K, 10% free 3670K/4044K, paused 20ms, total 21ms 
06-13 19:57:03.540 1030-1030/martinproductions.time_me I/dalvikvm-heap﹕ Grow heap (frag case) to 6.283MB for 2536936-byte allocation 
06-13 19:57:03.580 1030-1039/martinproductions.time_me D/dalvikvm﹕ GC_FOR_ALLOC freed <1K, 6% free 6148K/6524K, paused 33ms, total 33ms 
06-13 19:57:03.730 1030-1030/martinproductions.time_me D/AndroidRuntime﹕ Shutting down VM 
06-13 19:57:03.730 1030-1030/martinproductions.time_me W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0xb1a8fba8) 
06-13 19:57:03.740 1030-1030/martinproductions.time_me E/AndroidRuntime﹕ FATAL EXCEPTION: main 

Answer 1

由於mcount是最終的，它只能被分配一次。您可以讓mcount成爲MyActivity類的成員，因此可以在OnClickListener之內訪問，而不是最終的。

代碼應該是這樣的：

public class MyActivity extends Activity implements ActionBar.TabListener { 

SectionsPagerAdapter mSectionsPagerAdapter; 
ViewPager mViewPager; 
double mcount; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_my); 
    Button start; 
    final TextView stopwatch_num; 
    mcount = 0.0; 
    start = (Button)findViewById(R.id.start_button); 
    stopwatch_num = (TextView)findViewById(R.id.timer_num); 
    start.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View view) { 
      mcount = mcount + .01; 
      stopwatch_num.setText(""+mcount); 
     } 


    });