Java - Store a list of person objects in SortedSet

Introduction

Person class does not implement the Comparable interface.

The following code creates a SortedSet of persons using a Comparator that sorts the persons using their names:

SortedSet<Person> personsSortedByName = 
     new TreeSet<>(Comparator.comparing(Person::getName));

It uses a method reference to create a lambda expression for creating the Comparator object.

If you add two Person objects with the same name, the second one will be ignored.

personsSortedByName.add(new Person(1, "XML"));
personsSortedByName.add(new Person(2, "Json"));
// A duplicate Person. Will be ignored
personsSortedByName.add(new Person(3, "Json"));

The following code uses a Comparator object to apply custom sorting in a SortedSet.

It uses two custom sortings for Person objects, one by id and one by name.

Demo

import java.util.Comparator;
import java.util.SortedSet;
import java.util.TreeSet;

public class Main {
  public static void main(String[] args) {
    // Create a sorted set sorted by id
    SortedSet<Person> personsById = new TreeSet<>(
        Comparator.comparing(Person::getId));

    // Add some persons to the set
    personsById.add(new Person(1, "XML"));
    personsById.add(new Person(2, "Oracle"));
    personsById.add(new Person(3, "Eve"));
    personsById.add(new Person(4, "Json"));
    personsById.add(new Person(4, "Json")); // A duplicate Person

    // Print the set
    System.out.println("Persons by Id:");
    personsById.forEach(System.out::println);

    // Create a sorted set sorted by name
    SortedSet<Person> personsByName = new TreeSet<>(
        Comparator.comparing(Person::getName));
    personsByName.add(new Person(1, "XML"));
    personsByName.add(new Person(2, "Oracle"));
    personsByName.add(new Person(3, "Eve"));
    personsByName.add(new Person(4, "Json"));
    personsByName.add(new Person(4, "Kip")); // Not a duplicate person

    System.out.println("Persons by Name: ");
    personsByName.forEach(System.out::println);
  }/*from w  ww. j a  v  a2s. c o m*/
}

class Person {
  private int id;
  private String name;

  public Person(int id, String name) {
    this.id = id;
    this.name = name;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  @Override
  public boolean equals(Object o) {
    if (!(o instanceof Person)) {
      return false;
    }

    // id must be the same for two Persons to be equal
    Person p = (Person) o;
    if (this.id == p.getId()) {
      return true;
    }

    return false;
  }

  @Override
  public int hashCode() {
    // A trivial implementation
    return this.id;
  }

  @Override
  public String toString() {
    return "(" + id + ", " + name + ")";
  }
}

Java - Store a list of person objects in SortedSet

Introduction

Demo

Result

Related Topic