Drill 13: Rewrite it — Classes and objects

public record Course(String slug, String title, int lessons) {
    public Course {
        if (lessons < 1) throw new IllegalArgumentException("lessons");
    }
}

public final class Progress {
    private int completed;

    public void markComplete() {
        completed += 1;
    }

    public int completed() {
        return completed;
    }
}