1 　　@Override
 2     public boolean onKeyDown(int keyCode, KeyEvent event) 
 3     {
 4                  if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN)
 5                  {
 6                          
 7                          if((System.currentTimeMillis()-exitTime) > 2000)  //System.currentTimeMillis()是获取当前时间，返回的是毫秒
 8                          {
 9                           Toast.makeText(getApplicationContext(), "再按一次退出程序",Toast.LENGTH_SHORT).show();                                
10                           exitTime = System.currentTimeMillis();
11                          }
12                          else
13                          {
14                              finish();
15                              System.exit(0);
16                          }
17                                  
18                          return true;
19                  }
20                  return super.onKeyDown(keyCode, event);
21     }