Listing 1: Sorting String Instances Using Arrays.sort

import java.util.Arrays;

.
.
.

String animals[] = new String[4];
animals[0] = "snake";
animals[1] = "kangaroo";
animals[2] = "wombat";
animals[3] = "bird";

for (int i=0; i<4; i++) {
  System.out.println("animal " + i + " : " + animals[i]);
}

Arrays.sort(animals);

for (int i=0; i<4; i++) {
  System.out.println("animal " + i + " : " + animals[i]);
}

If you run the program, the first for loop gives you the name of animals as follows:

animal 0 : snake
animal 1 : kangaroo
animal 2 : wombat
animal 3 : bird

And, the second for loop prints the animals sorted alphabetically.

animal 0 : bird
animal 1 : kangaroo
animal 2 : snake
animal 3 : wombat

Listing 2: Sorting String Instances Using Collections.sort

import java.util.ArrayList;
import java.util.Collections;

.
.
.

ArrayList insects = new ArrayList();
insects.add("mosquito");
insects.add("butterfly");
insects.add("dragonfly");
insects.add("fly");

int size = insects.size();
for (int i=0; i<size; i++) {
  System.out.println("insect " + i + " : " + (String) insects.get(i));
}

Collections.sort(insects);

for (int i=0; i<size; i++) {
  System.out.println("insect " + i + " : " + (String) insects.get(i));
}

The first for loop in Listing 2 produces the following output:

insect 0 : mosquito
insect 1 : butterfly
insect 2 : dragonfly
insect 3 : fly

The second for loop prints the following:

insect 0 : butterfly
insect 1 : dragonfly
insect 2 : fly
insect 3 : mosquito

Listing 3: The Person Class

class Person {
  private String firstName;
  private String lastName;
  private int age;

  public String getFirstName() {
    return firstName;
  }

  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }

  public String getLastName() {
    return lastName;
  }

  public void setLastName(String lastName) {
    this.lastName = lastName;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    this.age = age;
  }
}

In another part of the program, we construct four instances of the Person class and populate them with names and ages:

  Person[] persons = new Person[4];
  persons[0] = new Person();
  persons[0].setFirstName("Elvis");
  persons[0].setLastName("Goodyear");
  persons[0].setAge(56);

  persons[1] = new Person();
  persons[1].setFirstName("Stanley");
  persons[1].setLastName("Clark");
  persons[1].setAge(8);

  persons[2] = new Person();
  persons[2].setFirstName("Jane");
  persons[2].setLastName("Graff");
  persons[2].setAge(16);

  persons[3] = new Person();
  persons[3].setFirstName("Nancy");
  persons[3].setLastName("Goodyear");
  persons[3].setAge(69);

How do we sort these Person instances by age or by name? Using the java.util.Arrays class' sort method, as in:

Arrays.sort(persons);

will throw a ClassCastException.

You can, of course, write your own code to sort them using an algorithm such as quick sort, bubble sort, or others, but that's impractical. The easy solution is to implement the java.lang.Comparable interface.

Using the java.lang.Comparable Interface

Implement the Comparable interface to make class instances comparable. This interface has one method, compareTo, which determines how to compare two instances of the class. The signature of this method is:

public int compareTo(Object o)

The compareTo method accepts Object, so you can pass it an instance of any type. However, chances are that you want to make sure to compare two instances of the same type. It does not make sense to compare an elephant with an ant, for example. Therefore, you can throw a java.lang.ClassCastException if the argument of this method is not the same type as your class.

The compareTo method returns zero if the object passed is equal to this instance. It returns a positive integer or a negative integer if this object is greater or smaller than the passed object, respectively.

Listing 4: The Person Class That Implements the Comparable Interface

package comparable.ex01;

class Person implements Comparable {
  private String firstName;
  private String lastName;
  private int age;

  public String getFirstName() {
    return firstName;
  }

  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }

  public String getLastName() {
    return lastName;
  }

  public void setLastName(String lastName) {
    this.lastName = lastName;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    this.age = age;
  }

  public int compareTo(Object anotherPerson) throws ClassCastException {
    if (!(anotherPerson instanceof Person))
      throw new ClassCastException("A Person object expected.");
    int anotherPersonAge = ((Person) anotherPerson).getAge();  
    return this.age - anotherPersonAge;    
  }
}

Listing 5: The comparable.ex01.Testing Class

package comparable.ex01;

import java.util.Arrays;
import java.util.ArrayList;

public class Testing {

  public static void main(String[] args) {
    Person[] persons = new Person[4];
    persons[0] = new Person();
    persons[0].setFirstName("Elvis");
    persons[0].setLastName("Goodyear");
    persons[0].setAge(56);

    persons[1] = new Person();
    persons[1].setFirstName("Stanley");
    persons[1].setLastName("Clark");
    persons[1].setAge(8);

    persons[2] = new Person();
    persons[2].setFirstName("Jane");
    persons[2].setLastName("Graff");
    persons[2].setAge(16);

    persons[3] = new Person();
    persons[3].setFirstName("Nancy");
    persons[3].setLastName("Goodyear");
    persons[3].setAge(69);

    System.out.println("Natural Order");

    for (int i=0; i<4; i++) {
      Person person = persons[i];
      String lastName = person.getLastName();
      String firstName = person.getFirstName();
      int age = person.getAge();
      System.out.println(lastName + ", " + firstName + ". Age:" + age);
    }

    Arrays.sort(persons);

    System.out.println();
    System.out.println("Sorted by age");

    for (int i=0; i<4; i++) {
      Person person = persons[i];
      String lastName = person.getLastName();
      String firstName = person.getFirstName();
      int age = person.getAge();
      System.out.println(lastName + ", " + firstName + ". Age:" + age);
    }
  }
}

The result of the code in Listing 5 is as follows:

Natural Order
Goodyear, Elvis. Age:56
Clark, Stanley. Age:8
Graff, Jane. Age:16
Goodyear, Nancy. Age:69

Sorted by age
Clark, Stanley. Age:8
Graff, Jane. Age:16
Goodyear, Elvis. Age:56
Goodyear, Nancy. Age:69