Sorting an ArrayList of Objects by Last Name and First Name in Java

In Java, sorting an ArrayList of objects by one or more fields requires implementing the Comparable interface and overriding the compareTo method. When sorting a list of objects with multiple fields, such as last name and first name, we need to define a custom comparison logic that considers both fields.

Here's an example showing how to sort an ArrayList of objects by last name and first name in Java:

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

public class Person implements Comparable<Person> {
    private String firstName;
    private String lastName;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    @Override
    public int compareTo(Person other) {
        int lastNameComparison = lastName.compareTo(other.getLastName());
        if (lastNameComparison != 0) {
            return lastNameComparison;
        }
        return firstName.compareTo(other.getFirstName());
    }

    @Override
    public String toString() {
        return firstName + " " + lastName;
    }

    public static void main(String[] args) {
        ArrayList<Person> people = new ArrayList<>();
        people.add(new Person("John", "Doe"));
        people.add(new Person("Alice", "Smith"));
        people.add(new Person("Mary", "Johnson"));
        people.add(new Person("David", "Brown"));

        // Sort by last name and first name
        Collections.sort(people);

        // Print sorted list
        for (Person person : people) {
            System.out.println(person);
        }
    }
}

In this example, we have a Person class with firstName and lastName fields. The compareTo method is implemented to compare the last names first and then the first names if the last names are the same. This ensures that the list of Person objects is sorted first by last name and then by first name.

To sort the ArrayList, we use the Collections.sort method, which internally uses the overridden compareTo method to compare the objects.

The output of the above code will be:

David Brown
John Doe
Mary Johnson
Alice Smith

The list is sorted in ascending order by last name and then by first name.

Remember, when comparing strings, use the compareTo method instead of the == operator as it performs a lexicographic comparison.

Using this approach, you can easily sort an ArrayList of objects by multiple fields in Java.