java 将 ArrayList 添加到类中 - 它有什么作用？
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Adding an ArrayList into a class - what does it do?
提问by
Following on from this Q of mine: What's the best way to make this Java program?
继我的这个问题之后：制作这个 Java 程序的最佳方法是什么？
I was recommended to store a list in Lecturer class and Course class. So I did, and it looks something like this:
我被建议在讲师班和课程班中存储一个列表。所以我做了，它看起来像这样：
public class Lecturer
{
private String id;
private String name;
List<Course> courses = new ArrayList<Course>(); // a list to hold the courses
public Lecturer(String idIn, String nameIn) // arguments of the constructor
{
id = idIn;
name = nameIn;
}
}
Same thing for the Course class except it has Lecturer list. But what I dont get is what does placing a list there exactly do? cos I dont know where to put the methods of the ArrayList such as adding and removing lecturers from it?
Course 类也一样，除了它有讲师列表。但我不明白的是，在那里放置一个列表到底有什么作用？因为我不知道将ArrayList的方法（例如从中添加和删除讲师）放在哪里？
Can someone explain the purpose of this?
有人可以解释一下这样做的目的吗？
I use another method which is basically placing the arraylists and its methods in two seperate classes for the lecturer and course, and then I simply add into the Course and Lecturer class as an attribute eg:
我使用另一种方法，它基本上将数组列表及其方法放在讲师和课程的两个单独的类中，然后我简单地将作为属性添加到课程和讲师类中，例如：
public class Lecturer
{
private String id;
private String name;
private CourseList courses; // COurseList is the class with the arraylist and methods
public Lecturer(String idIn, String nameIn) // arguments of the constructor
{
id = idIn;
name = nameIn;
courses = new CourseList();
}
}
I hope i'm making sense, cos ive been stuck on one thing for the last 2 weeks which no seems to understand.
我希望我说得有道理，因为过去 2 周我一直坚持做一件事，这似乎没有人理解。
Thank you
谢谢
回答by Outlaw Programmer
With the first method, you need to expose methods that allow client code to add stuff to those lists. So, you could have:
使用第一种方法，您需要公开允许客户端代码向这些列表添加内容的方法。所以，你可以有：
public class Lecturer
{
List<Course> courses = new ArrayList<Course>(); // a list to hold the courses
public Lecturer(String idIn, String nameIn)
{
/* do stuff */
}
public void addCourse(Course newCourse)
{
this.courses.add(newCourse);
}
}
You can do something similar for the Course class. Once you have those set up, you can do something like this:
您可以为 Course 类做类似的事情。完成这些设置后，您可以执行以下操作：
public static void main(String[] args)
{
Lecturer bob = new Lecturer(1, "Bob Smith");
Course math = new Course("Math 101");
// wire them together such that Bob teaches Math 101
bob.addCourse(math);
math.addLecturer(bob);
}
I think that solves what you're asking, but having this 2-way, circular relationship is sometimes a sign of a bad design. Only you know what you're real assignment is, though, so I hope this helps!
我认为这可以解决您的问题，但是这种双向循环关系有时是设计不佳的标志。不过，只有你知道你真正的任务是什么，所以我希望这会有所帮助！
回答by Humphrey Bogart
I'd advise using Map<Lecturer, List<Course>>, as someone replied in the previous question, which means, "an association (Map) between different lecturers (Lecturer) to a list of the courses they teach (List<Course>), instead, which you would instantiate as
new HashMap<Lecturer, List<Course>>. Otherwise, storing a list for each, you'll be duplicating functionality.
我建议使用Map<Lecturer, List<Course>>，正如有人在上一个问题中回答的那样，这意味着“Map不同讲师 ( Lecturer)之间的关联 ( ) 与他们教授的课程列表 ( List<Course>) 相反，您可以将其实例化为
new HashMap<Lecturer, List<Course>>。否则，存储一个列出每个，您将复制功能。
Once you declared the Courses c1, c2, ..., cna Lecturerlteaches, you associate them in the Mapmas m.put(l, c)where cis a Listof the courses, declared as new LinkedList<Course>(), and added as c.add(c1); c.add(c2); ... c.add(cn);.
声明Courses c1, c2, ..., cna 后Lecturerl，您将它们关联到课程的Mapmas m.put(l, c)where cis aList中，声明为new LinkedList<Course>()，并添加为c.add(c1); c.add(c2); ... c.add(cn);。
If I explain myself clearly.
如果我解释清楚。
You can read http://java.sun.com/docs/books/tutorial/collections/interfaces/map.htmland http://java.sun.com/docs/books/tutorial/collections/implementations/map.htmlfor more help on using Maps.
您可以阅读http://java.sun.com/docs/books/tutorial/collections/interfaces/map.html和http://java.sun.com/docs/books/tutorial/collections/implementations/map.html有关使用Maps 的更多帮助。
To obtain the reverse association, you can easily use the following code:
要获得反向关联，您可以轻松使用以下代码：
Collection<Course> courses = m.values();
Map<Course, List<Lecturer>> reverseAssociation = new HashMap<Course, List<Lecturer>>;
for (Course course : courses) {
List<Lecturer> lecturersTeachingCourse = new LinkedList<Lecturer>();
for (Lecturer lecturer : m.keySet()) {
if (lecturer.teaches(course)) {
lecturersTeachingCourse.add(lecturer);
}
}
coursesTaught.put(course, lecturersTeachingCourse);
}
Which, as long as Lecturer.teaches(Course)queries whether the lecturer teaches the passed course, sets reverseAssociationas the courses-lecturers association. (Naturally, you should encapsulate that code as a method in Lecturers).
其中，只要Lecturer.teaches(Course)查询讲师是否教授通过的课程，设置reverseAssociation为课程讲师协会。（当然，您应该将该代码封装为 Lecturers 中的方法）。
Good luck JavaNoob! We were all there once!
祝 JavaNoob 好运！我们都去过一次！
