if里的逻辑判断顺序不一样，效果不一样？

Question

当没选择checkbox的时候老师的代码成功进入循环体，我改了个顺序就报错了-----------------------------------------------String[] isUseCookies = request.getParameterValues("isUseCookie");       if(isUseCookies!=null&&isUseCookies.length>0)老师写的成功了----------------------------------------------- String[] isUseCookies = request.getParameterValues("isUseCookie");       if(isUseCookies.length>0&&isUseCookies!=null)这样写就报空指针错误了，why???-----------------------------------------------org.apache.jasper.JasperException: An exception occurred processing JSP page /dologin.jsp at line 3532:        request.setCharacterEncoding("utf-8");33:        //棣栧厛鍒ゆ柇鐢ㄦ埛鏄惁閫夋嫨浜嗚浣忕櫥褰曠姸鎬�34:        String[] isUseCookies = request.getParameterValues("isUseCookie");35:        if(isUseCookies.length>0&&isUseCookies!=null)36:        {37:           //鎶婄敤鎴峰悕鍜屽瘑鐮佷繚瀛樺湪Cookie瀵硅薄閲岄潰38:           String username = URLEncoder.encode(request.getParameter("username"),"utf-8");

Answer 1

isUseCookies 应该先判断是字符串否为空，不为空再取length判断字符串长短。因为如果字符串为空，就是null，取它的length,就类似于数组越界，就是对一个不存在的对象取值，会报空指针异常。