Android实现两次按下返回键退出

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) 
    {
			     if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN)
			     {
			             
					     if((System.currentTimeMillis()-exitTime) > 2000)  //System.currentTimeMillis()无论何时调用，肯定大于2000
					     {
					      Toast.makeText(getApplicationContext(), "再按一次退出程序",Toast.LENGTH_SHORT).show();                                
					      exitTime = System.currentTimeMillis();
					     }
					     else
					     {
					         finish();
					         System.exit(0);
					     }
					             
					     return true;
			     }
			     return super.onKeyDown(keyCode, event);
    }

第二种：

http://blog.csdn.net/carterjin/article/details/7677069