空检查后抛出错误吗？

Question

有人可以请解释为什么scoreTextSwitcher即使我用if语句检查也会得到空错误吗？改变屏幕方向 - - 错误才刚刚开始活动后，再次抛出改变屏幕的方向 - 然后出现错误...

if(scoreTextSwitcher != null) { 
     scoreTextSwitcher.setText(String.valueOf(a)); 
} 

我试图寻找下面的链接的答案，但无法弄清楚它是如何涉及到我的代码。如果是这样，有人可以澄清？非常感谢。

• NullReferenceException thrown after null check?
• Null Pointer Exception after checking with 'if' statement
• NullPointerException thrown after field is check if it is null

---

这里是整个方法（不知道是否在一个线程是要紧）

private void setScoreTextSwitcher(final int a){ 
    Thread setScore = new Thread() { 
     public void run() { 
      if(scoreTextSwitcher == null) { 
       scoreTextSwitcher = (TextSwitcher) findViewById(R.id.score_text_switcher); 

      } 
      GameActivity.this.runOnUiThread(new Runnable() { 
       public void run() { 
        if(scoreTextSwitcher != null) { 
         scoreTextSwitcher.setText(String.valueOf(a)); 
        } 
       } 
      }); 
     } 
    }; 
    setScore.start(); 
} 

---

下面是错误信息：

08-27 11:02:10.226 2780-2780/com.major.color D/AndroidRuntime﹕ Shutting down VM 
08-27 11:02:10.226 2780-2780/com.major.color W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x409961f8) 
08-27 11:02:10.247 2780-2780/com.major.color E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.NullPointerException 
      at android.widget.TextSwitcher.setText(TextSwitcher.java:78) 
      at com.major.color.GameActivity$6$1.run(GameActivity.java:600) 
      at android.os.Handler.handleCallback(Handler.java:605) 
      at android.os.Handler.dispatchMessage(Handler.java:92) 
      at android.os.Looper.loop(Looper.java:137) 
      at android.app.ActivityThread.main(ActivityThread.java:4340) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 

---

这是我的计划，也越来越空指针错误的另一部分。我相信这是同样的问题，因为它只发生在来回切换屏幕方向时。希望这第二个例子可以帮助缩小一点。 onCreate方法的

private void startCountDownTimer(){ 
    isTimerOn = true; 
    timerTextSwitcher = (TextSwitcher) findViewById(R.id.timer_text_switcher); 
    timer = new CountDownTimer(timerCount, 1000) { 
     public void onTick(long millisUntilFinished) { 
      isTimerOn = true; 
      timerCount = millisUntilFinished; 

      if (millisUntilFinished < 10001) { 
       TextView TextSwTextView = (TextView) timerTextSwitcher.getChildAt(0); 
       TextSwTextView.setTextColor(Color.RED); 
       TextSwTextView.setText(String.valueOf(millisUntilFinished/1000)); 
      }else 
       if(timerTextSwitcher != null){ 
        timerTextSwitcher.setText(String.valueOf(millisUntilFinished/1000));}else { 
        timerTextSwitcher = (TextSwitcher) findViewById(R.id.timer_text_switcher); 
        timerTextSwitcher.setFactory(new ViewSwitcher.ViewFactory() { 

         public View makeView() { 
          // Create a new TextView and set properties 
          TextView textView = new TextView(getApplicationContext()); 
          textView.setLayoutParams(new TextSwitcher.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT)); 
          textView.setTextSize(17); 
          textView.setTextColor(Color.BLUE); 

          return textView; 
         } 
        }); 
        timerTextSwitcher.setText(String.valueOf(millisUntilFinished/1000)); 
       } 
     } 
    public void onFinish() { 
      timerTextSwitcher.post(new Runnable() { 
       @Override 
       public void run() { 
        createToast("GAME OVER!"); 
       } 
      }); 
      isTimerOn = false; 

      saveScore(); 
      DialogFragment endDialog = new EndGameFragment(); 
      Dialog = endDialog; 
      endgameVisible = true; 
      endDialog.show(getSupportFragmentManager(), "EndGameDialogFragment"); 
     } 
    }; 
    timer.start(); 
} 

---

部分中显示我怎么设置的TextSwitcher的得分和活动的时间。

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
      WindowManager.LayoutParams.FLAG_FULLSCREEN); 
    setContentView(R.layout.activity_game) 
//more code... 

timerTextSwitcher = (TextSwitcher) findViewById(R.id.timer_text_switcher); 
    timerTextSwitcher.setFactory(new ViewSwitcher.ViewFactory() { 

     public View makeView() { 
      // Create a new TextView and set properties 
      TextView textView = new TextView(getApplicationContext()); 
      textView.setLayoutParams(new TextSwitcher.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT)); 
      textView.setTextSize(17); 
      textView.setTextColor(Color.BLUE); 

      return textView; 
     } 
    }); 
    scoreTextSwitcher = (TextSwitcher) findViewById(R.id.score_text_switcher); 
    scoreTextSwitcher.setFactory(new ViewSwitcher.ViewFactory() { 

     public View makeView() { 
      // Create a new TextView and set properties 
      TextView textView = new TextView(getApplicationContext()); 
      textView.setLayoutParams(new TextSwitcher.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT)); 
      textView.setTextSize(17); 
      textView.setTextColor(Color.BLUE); 
      textView.setText("0"); 
      return textView; 
     } 
    }); 

    // Declare the animations and initialize them 
    Animation in = AnimationUtils.loadAnimation(this, android.R.anim.slide_in_left); 
    Animation out = AnimationUtils.loadAnimation(this, android.R.anim.slide_out_right); 

    // set the animation type to textSwitcher 
    timerTextSwitcher.setInAnimation(in); 
    timerTextSwitcher.setInAnimation(out); 
    scoreTextSwitcher.setInAnimation(in); 
    scoreTextSwitcher.setInAnimation(out); 
} 

Answer 1

以下行导致不希望的行为：

private void startCountDownTimer(){ 
    ... 
    timerTextSwitcher = (TextSwitcher) findViewById(R.id.timer_text_switcher); 

和：

private void setScoreTextSwitcher(final int a){ 
    ... 
    scoreTextSwitcher = (TextSwitcher) findViewById(R.id.score_text_switcher); 

对findViewById()的这些调用正在重新初始化切换器，导致它们丢失onCreate()中的工厂和动画设置，并在setText()方法中导致内部NullPointerException。因为，看起来，切换器是在Activity的范围内声明的，您只需要删除这些行。

Answer 2

试试这个

private void setScoreTextSwitcher(final int a){ 
scoreTextSwitcher = (TextSwitcher) findViewById(R.id.score_text_switcher); 
Thread setScore = new Thread() { 
    public void run() { 
     if(scoreTextSwitcher.isEmpty()) { 
      Log.e("","Empty"); 

     } 
     GameActivity.this.runOnUiThread(new Runnable() { 
      public void run() { 
       if(scoreTextSwitcher != null) { 
        scoreTextSwitcher.setText(String.valueOf(a)); 
       } 
      } 
     }); 
    } 
}; 
setScore.start(); 

}