Sorting objects within a Set by a String value that all objects contain

Question

Ok this is a tricky one. I have a list of Sets. I would like to sort the objects in the Sets in an order.

Imagine each set as repressenting a class in a school. Each

Answer 1

You must implement Comparable for your sortable objects (Person etc).

Then:

1. Convert Set to List (some info here) since you can't sort a Set
2. Use Collections.sort

or

1. Convert to a SortedSet (like a TreeSet)
2. Use a Comparator for custom ordering

Examples:

import java.util.*;

class Person implements Comparable {
    private String firstName, lastName;

    public Person(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName;}
    public String getFirstName() {return firstName;}
    public String getLastName() {return lastName;}
    public String getName() {return firstName + " " + lastName;}

    public int compareTo(Person p) {
        return lastName.compareTo(p.lastName);
    }
}

class FirstNameComparator implements Comparator {
    public int compare(Person p1, Person p2){
            return p1.getFirstName().compareTo(p2.getFirstName());
    }
}

class Test {
  public static void log(String s) {
        System.out.println(s);
    }

  public static void main(String[] args) {
        Set people = new HashSet();
        people.add(new Person("Bob", "Jones"));
        people.add(new Person("Alice", "Yetti"));

        log("Sorted list:");
        List peopleList = new LinkedList();
        peopleList.addAll(people);
        Collections.sort(peopleList);
        for (Person p : peopleList) {
            log(p.getName());
        }

        log("TreeSet:");
        TreeSet treeSet = new TreeSet();
        treeSet.addAll(people);
        for (Person p : treeSet) {
            log(p.getName());
        }

        log("TreeSet (custom sort):");
        TreeSet treeSet2 = new TreeSet(new FirstNameComparator());
        treeSet2.addAll(people);
        for (Person p : treeSet2) {
            log(p.getName());
        }
      }
};