How do I model something like this?

问题

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

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).

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.

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

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);

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());
    }

}

回答1:

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.

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.

回答2:

Use Null Object pattern. My recommendation is for you to use Guava library so that you can use the Optional class. Here is the tutorial explaining its usage.

来源：https://stackoverflow.com/questions/8730313/how-do-i-model-something-like-this

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modeling