Can i use a set or List in this example?

public class Student implements java.io.Serializable {

private long studentId;
private String studentName;
private Set<Course> courses = new HashSet<Course>(0);

public Student() {
}

public Student(String studentName) {
    this.studentName = studentName;
}

public Student(String studentName, Set<C开发者_JAVA百科ourse> courses) {
    this.studentName = studentName;
    this.courses = courses;
}

public long getStudentId() {
    return this.studentId;
}

public void setStudentId(long studentId) {
    this.studentId = studentId;
}

public String getStudentName() {
    return this.studentName;
}

public void setStudentName(String studentName) {
    this.studentName = studentName;
}

public Set<Course> getCourses() {
    return this.courses;
}

public void setCourses(Set<Course> courses) {
    this.courses = courses;
}

}

Here they are using Hashset to get the courses. My doubt is can i use a list to get the courses here. I read in internet that list get the vaues in a specified order and allows duplicates inside the list. whereas in set it doesnt have any order and wont allow duplicates. I want to know where i should uses sets and lists? Can anyone suggest?

It feels like you answered your own question already. If you need a Collection of items, and you want the collection of items to have no duplicates, use a Set. You can use a SortedSet to impose ordering.

If your collection is allowed to have duplicates, then you can use a List. I think in your example, a Set works since a student would probably never take the same course twice.

The fundamental difference between List and Set is (as you said) that Set does not allow duplicates, while List does. So in your case a Set is more appropriate, since a student should not be able to enroll in course twice. Actually, he should be able, but on in the same semester. So you may have each student have a set of CourseEnrollment objects, rather than Course objects.

Note that preserving order is not impossible for a Set - there are implementations (like LinkedHashSet) that preserve the order of elements, and other, like TreeSet, which keep to elements sorted.