android退出activity提示再按一次返回键

@Override  
public boolean onKeyDown(int keyCode, KeyEvent event)//主要是对这个函数的复写  
{  
    // TODO Auto-generated method stub   

    if((keyCode == KeyEvent.KEYCODE_BACK)&&(event.getAction() == KeyEvent.ACTION_DOWN))  
    {  
        if(System.currentTimeMillis() - temptime >2000) // 2s内再次选择back键有效  
        {  
            System.out.println(Toast.LENGTH_LONG);  
            Toast.makeText(this, "请在按一次返回退出", Toast.LENGTH_LONG).show();  
            temptime = System.currentTimeMillis();  
        }  
        else {  
               finish();   
               System.exit(0); //凡是非零都表示异常退出!0表示正常退出!  
        }  

        return true;   

    }  
    return super.onKeyDown(keyCode, event);  
}