Comparable Interface

The Comparable interface in Java is used to define a natural ordering for objects of a class. This interface imposes a total ordering on the objects of each class that implements it. The natural ordering is used by various utility methods, such as Collections.sort() and Arrays.sort(), to sort objects.

The purpose is to allow objects of a class to be compared to each other for sorting purposes. Typically implemented by the class whose objects need to be ordered.

Definition

public interface Comparable<T> {
    int compareTo(T o);
}

It contains a single method compareTo(T o) : Compares this object with the specified object (passed in method parameter) for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

Code implementing Comparable in a Custom Class with multiple attributes

class Student implements Comparable<Student> {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public int compareTo(Student other) {
        int ageComparison = Integer.compare(this.age, other.age);
        if (ageComparison != 0) {
            return ageComparison;
        }
        return this.name.compareTo(other.name);
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

public class ComparableMultipleAttributesExample {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student("Alice", 20));
        students.add(new Student("Bob", 22));
        students.add(new Student("Charlie", 18));
        students.add(new Student("David", 25));
        students.add(new Student("Eve", 22));

        // Sort students by age, then by name
        Collections.sort(students);

        // Print sorted students
        for (Student student : students) {
            System.out.println(student);
        }
    }
}