Java Language
Comparable and Comparator
Sorting a List using Comparable or a Comparator
Example
Say we are working on a class representing a Person by their first and last names. We have created a basic class to do this and implemented proper equals and hashCode methods.
public class Person {
private final String lastName; //invariant - nonnull
private final String firstName; //invariant - nonnull
public Person(String firstName, String lastName){
this.firstName = firstName != null ? firstName : "";
this.lastName = lastName != null ? lastName : "";
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public String toString() {
return lastName + ", " + firstName;
}
@Override
public boolean equals(Object o) {
if (! (o instanceof Person)) return false;
Person p = (Person)o;
return firstName.equals(p.firstName) && lastName.equals(p.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
}
Now we would like to sort a list of Person objects by their name, such as in the following scenario:
public static void main(String[] args) {
List<Person> people = Arrays.asList(new Person("John", "Doe"),
new Person("Bob", "Dole"),
new Person("Ronald", "McDonald"),
new Person("Alice", "McDonald"),
new Person("Jill", "Doe"));
Collections.sort(people); //This currently won't work.
}
Unfortunately, as marked, the above currently won't compile. Collections.sort(..) only knows how to sort a list if the elements in that list are comparable, or a custom method of comparison is given.
If you were asked to sort the following list : 1,3,5,4,2, you'd have no problem saying the answer is 1,2,3,4,5. This is because Integers (both in Java and mathematically) have a natural ordering, a standard, default comparison base ordering. To give our Person class a natural ordering, we implement Comparable<Person>, which requires implementing the method compareTo(Person p):
public class Person implements Comparable<Person> {
private final String lastName; //invariant - nonnull
private final String firstName; //invariant - nonnull
public Person(String firstName, String lastName) {
this.firstName = firstName != null ? firstName : "";
this.lastName = lastName != null ? lastName : "";
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public String toString() {
return lastName + ", " + firstName;
}
@Override
public boolean equals(Object o) {
if (! (o instanceof Person)) return false;
Person p = (Person)o;
return firstName.equals(p.firstName) && lastName.equals(p.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
@Override
public int compareTo(Person other) {
// If this' lastName and other's lastName are not comparably equivalent,
// Compare this to other by comparing their last names.
// Otherwise, compare this to other by comparing their first names
int lastNameCompare = lastName.compareTo(other.lastName);
if (lastNameCompare != 0) {
return lastNameCompare;
} else {
return firstName.compareTo(other.firstName);
}
}
}
Now, the main method given will function correctly
public static void main(String[] args) {
List<Person> people = Arrays.asList(new Person("John", "Doe"),
new Person("Bob", "Dole"),
new Person("Ronald", "McDonald"),
new Person("Alice", "McDonald"),
new Person("Jill", "Doe"));
Collections.sort(people); //Now functions correctly
//people is now sorted by last name, then first name:
// --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald
}
If, however, you either do not want or are unable to modify class Person, you can provide a custom Comparator<T> that handles the comparison of any two Person objects. If you were asked to sort the following list: circle, square, rectangle, triangle, hexagon you could not, but if you were asked to sort that list based on the number of corners, you could. Just so, providing a comparator instructs Java how to compare two normally not comparable objects.
public class PersonComparator implements Comparator<Person> {
public int compare(Person p1, Person p2) {
// If p1's lastName and p2's lastName are not comparably equivalent,
// Compare p1 to p2 by comparing their last names.
// Otherwise, compare p1 to p2 by comparing their first names
if (p1.getLastName().compareTo(p2.getLastName()) != 0) {
return p1.getLastName().compareTo(p2.getLastName());
} else {
return p1.getFirstName().compareTo(p2.getFirstName());
}
}
}
//Assume the first version of Person (that does not implement Comparable) is used here
public static void main(String[] args) {
List<Person> people = Arrays.asList(new Person("John", "Doe"),
new Person("Bob", "Dole"),
new Person("Ronald", "McDonald"),
new Person("Alice", "McDonald"),
new Person("Jill", "Doe"));
Collections.sort(people); //Illegal, Person doesn't implement Comparable.
Collections.sort(people, new PersonComparator()); //Legal
//people is now sorted by last name, then first name:
// --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald
}
Comparators can also be created/used as an anonymous inner class
//Assume the first version of Person (that does not implement Comparable) is used here
public static void main(String[] args) {
List<Person> people = Arrays.asList(new Person("John", "Doe"),
new Person("Bob", "Dole"),
new Person("Ronald", "McDonald"),
new Person("Alice", "McDonald"),
new Person("Jill", "Doe"));
Collections.sort(people); //Illegal, Person doesn't implement Comparable.
Collections.sort(people, new PersonComparator()); //Legal
//people is now sorted by last name, then first name:
// --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald
//Anonymous Class
Collections.sort(people, new Comparator<Person>() { //Legal
public int compare(Person p1, Person p2) {
//Method code...
}
});
}
Java SE 8
Lambda expression based comparators
As of Java 8, comparators can also be expressed as lambda expressions
//Lambda
Collections.sort(people, (p1, p2) -> { //Legal
//Method code....
});
Comparator default methods
Furthermore, there are interesting default methods on the Comparator interface for building comparators : the following builds a comparator comparing by lastName and then firstName.
Collections.sort(people, Comparator.comparing(Person::getLastName)
.thenComparing(Person::getFirstName));
Inversing the order of a comparator
Any comparator can also easily be reversed using the reversedMethod which will change ascending order to descending.
