Understanding Java's Comparator Interface with Example Code

What is a Comparator in Java? Use it with code

In Java, a Comparator is an interface that defines a method for comparing two objects of a specific type. It is used to provide custom ordering for collections of objects, such as sorting a list or an array. The Comparator interface is part of the java.util package and has a single method, compare(T o1, T o2), which compares its two arguments for order.

Key Points:

• A Comparator can be used to define multiple sorting orders for the same class.
• It is often used in conjunction with collections like List, Set, or arrays.
• You can implement the Comparator interface either by creating a separate class or using a lambda expression (Java 8 and later).

Example Code

Here’s an example of how to use a Comparator to sort a list of Person objects by their age:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

// Define a Person class
class Person {
    String name;
    int age;

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

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

public class ComparatorExample {
    public static void main(String[] args) {
        // Create a list of Person objects
        List<Person> people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 35));

        // Sort the list using a Comparator
        Collections.sort(people, new Comparator<Person>() {
            @Override
            public int compare(Person p1, Person p2) {
                return Integer.compare(p1.age, p2.age); // Compare by age
            }
        });

        // Print the sorted list
        System.out.println("Sorted by age:");
        for (Person person : people) {
            System.out.println(person);
        }

        // Alternatively, using a lambda expression (Java 8 and later)
        Collections.sort(people, (p1, p2) -> p1.name.compareTo(p2.name)); // Sort by name

        // Print the sorted list by name
        System.out.println("\nSorted by name:");
        for (Person person : people) {
            System.out.println(person);
        }
    }
}

Explanation:

1. Person Class: A simple class with name and age attributes.
2. List of People: We create a list of Person objects.
3. Sorting by Age: We use Collections.sort() with a custom Comparator to sort the list by age.
4. Sorting by Name: We demonstrate sorting by name using a lambda expression, which is more concise.

Output:

Sorted by age:
Bob (25)
Alice (30)
Charlie (35)

Sorted by name:
Alice (30)
Bob (25)
Charlie (35)

This example illustrates how to use a Comparator to sort objects in different ways based on different attributes.