Sorting objects within a Set by a String value tha

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 set contains person objects. A person object holds a String value for name. I'd like to arrange the Persons in the Set by name before I loop through and write them out.

Is there anywahy to use Collections.sort(); or something similar to achieve this?

for (Set<Person> s : listOfAllChildren) {       
      for (Person p : s) {
        if(p.getClass().equalsIgnoreCase("Jones")){
          System.out.println(p.getName());
          }
         else if...//carry on through other classes 
        }                              
      }        

I do know that 2+ children in a class may share the same name but please ignore this

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<Person> {
    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<Person> {
    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<Person> people = new HashSet<Person>();
        people.add(new Person("Bob", "Jones"));
        people.add(new Person("Alice", "Yetti"));

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

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

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

Answer 2

You may want to look at using a SortedSet for example a TreeSet. This allows you to provide a Comparator which in your case can compare the name of the Person.

Answer 3

You can consider using TreeSet to store objects. And when sorting create new TreeSet with custom comparator for your Person objects. I do not suggest using Collection.sort because AFAIR it can sort only lists.

Answer 4

You could make your Person class implement the Comparable interface as shown here and then sort them accordingly.

Answer 5

With Java 8 you can sort the Set of persons and generate List of persons which are sorted as follows.

List<Person> personList = personSet.stream().sorted((e1, e2) -> 
e1.getName().compareTo(e2.getName())).collect(Collectors.toList());

Answer 6

A Set has no notion of ordering because, well, it's a set.

There is a SortedSet interface implemented by TreeSet class that you can use. Simply provide an appropriate Comparator to the constructor, or let your Person class implements Comparable.