雙擊返回鍵回到主屏幕

@Override

public boolean onKeyDown(int keyCode, KeyEvent event)
 {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
             if (backTip)
          {
             backTip = false;
             Toast.makeText(this,"再按一次返回鍵回到主屏幕", Toast.LENGTH_SHORT).show();
             new Thread()
             {
                public void run()
                {
                   try
                   {
                      Thread.sleep(3000);
                      backTip = true;
                   } catch (InterruptedException e)
                   {
                      e.printStackTrace();
                   }
                };
             }.start();
          }
          else
          {
                Intent backHome = new Intent(Intent.ACTION_MAIN);
             backHome.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
             backHome.addCategory(Intent.CATEGORY_HOME);
             startActivity(backHome);
             System.exit(0);
             }
        return true;
    }
    return super.onKeyDown(keyCode, event); }