Android实现长按back键退出应用程序的方法

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) 
{
  if (keyCode == KeyEvent.KEYCODE_BACK) 
  {
   if ((System.currentTimeMillis() - mExitTime) > 2000) 
   {
    mHelperUtils.showToast(mContext, R.string.exit_content);
    mExitTime = System.currentTimeMillis();
   }
   else 
   {
    finish();
   }
   return true;
  }
  return super.onKeyDown(keyCode, event);
}

public boolean onKeyDown(int keyCode, KeyEvent event) 
{ 
 if (keyCode == KeyEvent.KEYCODE_BACK&&event.getRepeatCount()==0) 
 { 
  start=System.currentTimeMillis(); 
  Log.e("start",String.valueOf(start));
  return false; 
 } 
 return super.onKeyDown(keyCode, event);
} 
public boolean onKeyUp(int keyCode,KeyEvent event)
{
 if (keyCode == KeyEvent.KEYCODE_BACK &&event.getRepeatCount()==0) 
 { 
  end=System.currentTimeMillis(); 
  Log.e("end",String.valueOf(end));
  if(start!=-1&&(end-start)>2000)
  {
  AlertDialog.Builder builder = new Builder(MainActivity.this);
  builder.setMessage("确认退出吗？");
  builder.setTitle("提示");
  builder.setPositiveButton("确认",new DialogInterface.OnClickListener()
  {
   public void onClick(DialogInterface dialog,int which)
   {
   dialog.dismiss();
   MainActivity.this.finish();
   System.exit(0);
   }
  });
  builder.setNegativeButton("取消",new DialogInterface.OnClickListener()
  {
   public void onClick(DialogInterface dialog,int which) 
   {
   dialog.dismiss();
   }
  });
  builder.create().show();
  return true;
  }
  else
  {
  return super.onKeyUp(keyCode, event);
  }
 }
 return super.onKeyUp(keyCode, event);
}

public boolean dispatchKeyEvent(KeyEvent event)
{
 int keyCode=event.getKeyCode();
 //Log.e("start",String.valueOf(start));
 switch(keyCode)
 {
  case KeyEvent.KEYCODE_BACK:
  if(event.isLongPress())
  //这句很重要，判断事件是否是长按事件
  {
  AlertDialog.Builder builder = new Builder(MainActivity.this);
  builder.setMessage("确认退出吗？");
  builder.setTitle("提示");
  builder.setPositiveButton("确认",new DialogInterface.OnClickListener()
  {
   public void onClick(DialogInterface dialog,int which)
   {
   dialog.dismiss();
   MainActivity.this.finish();
   System.exit(0);
   }
  });
  builder.setNegativeButton("取消",new DialogInterface.OnClickListener() 
  {
   public void onClick(DialogInterface dialog,int which) 
   {
   dialog.dismiss();
   }
  });
  builder.create().show();
  return true;
  }
  return super.dispatchKeyEvent(event);
  //如果不是长按，则调用原有方法，执行按下back键应有的处理
  default:
  break;
 }
 return super.dispatchKeyEvent(event);
}