老师，为什么userpwd和role_id允许空值啊？

照着代码敲的，但是程序报错了，就是标红的那部分，我应该怎么改？

老师，这种错误是什么情况，为什么会报创建目录错误？ 我也创建了zoo.cfg了

为什么输入游记后，前面的乐章和文字就不出来了呢？

不使用N+1次查询也可以实现一对一查询，只需引入一个resultMap即可，效率相对于要高

老师，这个为啥不对呀

老师为什么要写copy.copy()呢？不应该是copy.()吗？

       为什么写print("a:",a)呢？不应该是print（“a”）吗？

 不懂就问

这里配置了一个lru，那另外两种也是这般配置吗？

再就是jcache这段文字看不懂哇

表格里面使用<thead><tbody><tfoot>是不是为了增强标签的语义性，跟不加这些 显示的效果页面是一样的吗

老师您好，我看到课堂中的txt文本笔记，是之前学过的吗？这个章节第一节课时老师就说重做系统后开始学习的，但是没有找到这个课程呀？

老师，数据库上的密码没有显示加密的结果。把pwd=pwd改为password=pwd后，反而出现”2000，异常错误“。

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com.bjsxtTestSuper {
    (String[] args) {
        ChildClass().f()}
    FatherClass {
        (){
            = System..println(+)}
    }
    ChildClass FatherClass {
        (){
            = System..println(+)System..println(+.)}

    }
}

为啥我提示

F:\JAVAproject\jlchen003\src\com\bjsxt\TestSuper.java:5:9

java: 无法从静态上下文中引用非静态 变量 this

为什么我预更新会失败？下面是我后台报的错误，会是什么原因呢？

class Benz:pass

class BMW:pass

class BYD:pass

老师这三行后面为什么写pass啊？什么意思？