我该如何建模? [英] How do I model something like this?

问题描述

我遇到了机器问题，这是一些棘手的部分之一，这使我有些困惑.

I have this machine problem, and here is one of those tricky parts which gets me a little confused.

这是用例:
学生报名参加一节.
系统不会验证任何计划冲突.
系统验证学生已达到要求的先决条件.

Here is the use case:
Student enrolls in a section.
System verifies no schedule conflicts.
System verifies that the student has taken the required prerequisite(s).

因此，我创建了4个对象，分别是Student，Section，Schedule和Subject. 我只向您展示我的主题"课程，因为这让我感到困惑.

So I created 4 objects, called Student, Section, Schedule, and Subject. I'll only show you my "subject" class because this is where I got confused.

让我感到困惑的部分是:
系统验证学生已达到要求的先决条件.

The part where I get confused is this:
System verifies that the student has taken the required prerequisite(s).

如何正确建模? 我有一个前提条件，它是一个主题对象，但是如果主题没有前提条件，该怎么办?我有些犹豫，因为我不能只做类似的事情:

How do I properly model that? I got a prerequisite which is a subject object, but what if the subject has no prerequisite? I hesitate because I cannot just make something like:

Subject math01 = new Subject(1,"math01",null);

我认为在对象上传递null是不合适的.我想避免nullpointer异常.我该如何解决? :( 我想我做错了.

I think passing null on objects is inappropriate. I'd like to avoid nullpointer exceptions. How do I resolve this? :( I think I'm doing it wrong.

public class Subject {

    //Each subject is worth three (3) units.
    private int units = 3;

    private int subjectID;
    private String subjectName;
    private Subject prerequisite; //<----How do I express that a subject has a prerequisite? Am I doing this right?

    public Subject(int subjectID, String subjectName, Subject prerequisite) {
        this.subjectID = subjectID;
        this.subjectName = subjectName;
        this.prerequisite = prerequisite;
    }

    public int getUnits() {
        return units;
    }

    public int getSubjectID() {
        return subjectID;
    }

    public String getSubjectName() {
        return subjectName;
    }

    public Subject getPrerequisite(){
        return prerequisite;
    }

    public boolean conflictWith(Subject newSubject){
        return (this.subjectID == newSubject.getSubjectID());
    }

}

推荐答案

考虑添加第二个不需要先决条件的构造函数.然后，您不必从API客户端传递null.

Consider adding a second constructor that doesn't take a prerequisite. Then you don't have to pass in null from the client of your API.

我还将添加一个方便的方法hasPrerequisite()，以检查Subject是否确实具有先决条件.这样，您的API客户端也不必自己进行null检查.

I'd also add a convenience method hasPrerequisite() for checking to see if the Subject does have a prerequisite. That way clients of your API also don't have to do a null check themselves.

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