How to sort parts of an ArrayList of objects of a class?

Question

I have looked at many questions that have been asked that seem similar but none of them quite address my issue.

I have a class called Student, a class called RollManager, and a driver called RollDriver. The Student class allows a user to enter data for a student, such as their name, major, GPA, classification, etc.

The RollManager class has an ArrayList called classRoll that holds objects of type Student (looks like this: ArrayList<Student> classRoll = new ArrayList<>();

In the RollDriver is a menu that allows a user to do several things. Among them I need to be able to sort the Student objects by their names, and a separate option that allows me to sort them by their GPAs.

The issue is that when I try to use Collections.sort(classRoll), it doesn't know how to sort them. So I made a method in the RollManager class called sortName, but how do I specify that I want to sort specifically by the "name" value of the Student objects? Here's some of my code:

RollManager:

public static void sortName(ArrayList<Student> classRoll)
 {
 for(int i = 0; i < classRoll.size(); i++)
 {
 int pos = i;
 for(int n = i; n < classRoll.size(); n++)
 {
 if(classRoll.get(n).equals(classRoll.get(pos)))
 {
 pos = n;
 }
 }
 }
 }

Option in RollDriver to sort:

else if(choice == 8)
 {
 RollManager.sortName(classRoll);
 System.out.println (classRoll);
 }

I don't get any errors when I run this, but nothing happens either. The objects aren't sorted any differently.

These are some of the pre-added objects I've added to the classRoll to test my code(the classification is an enum):

Student student2 = new Student("John", "Doe", "COMM", 120, 3.65, Classification.FRESHMAN);
Student student3 = new Student("Bob", "Ross", "ARTS", 200, 3.99, Classification.OTHER);
Student student4 = new Student("Liev", "Schreiber", "FILM", 100, 2.53, Classification.GRADUATE);
Student student5 = new Student("Maury", "Povich", "PSCI", 75, 2.24, Classification.JUNIOR);
Student student6 = new Student("Bill", "Leidermann", "CSCI", 90, 2.95, Classification.SENIOR);
classRoll.add (student2);
classRoll.add (student3);
classRoll.add (student4);
classRoll.add (student5);
classRoll.add (student6);

I hope that this is enough info. I can post more of my code if necessary. Thank you for any assistance!

• Andreas: "looks like this: ArrayList classRoll = new ArrayList()" I sure hope not, because that would be using raw generics, and you shouldn't do that. It should be: ArrayList<Student> classRoll = new ArrayList<>()
• faris: docs.oracle.com/javase/7/docs/api/java/util/…
• QuinineDjinn: @Andreas absolutely, that's how it is in my code but I'm not sure how the formatting works on this site so it got rid of <Student> after the first ArrayList somehow

Answer 1

You use the other Collections.sort method: sort(List<T> list, Comparator<? super T> c)

You use it like this:

Collections.sort(classRoll, new Comparator<Student>() {
 @Override
 public int compare(Student s1, Student s2) {
 return s1.getName().compareTo(s2.getName());
 }
});

In Java 8+ it is even easier:

// Sort by name
classRoll.sort(Comparator.comparing(Student::getName));
// Sort by GPA
classRoll.sort(Comparator.comparingDouble(Student::getGpa));

If name is 2 fields (firstName and lastName), you can use thenComparing:

// Sort by last name, then first name
classRoll.sort(Comparator.comparing(Student::getLastName)
 .thenComparing(Student::getFirstName));

Answer 2

The way to doing this is using the Collections.sort with custom Comparators:

Comparator<Student> nameComparator = new Comparator<Student>() {
 public int compare(Student student1, Student student2) {
 return student1.getName().compareTo(student2.getName());
 }
 };
Comparator<Student> gpaComparator = new Comparator<Student>() {
 public int compare(Student student1, Student student2) {
 return student1.getGPA().compareTo(student2.getGPA());
 }
 };

Then you can use different comparator according to your needs:

Collections.sort(classRoll, nameComparator);

This way you have an optimized sorter when compared to your solution with 2 loops above.