Solution of the exercise on the class TimeOfDay

public class TimeOfDay {

  // we represent a time of the day by the total number of seconds since midnight
  private int totsec;

  public TimeOfDay(int h, int m, int s) {
    totsec = h*3600 + m*60 + s;
  }

  public void add(TimeOfDay t) {
    this.totsec += t.totsec;
    if (this.totsec > 24*3600)
      this.totsec -= 24*3600;
  }

  public void subtract(TimeOfDay t) {
    this.totsec -= t.totsec;
    if (this.totsec < 0)
      this.totsec += 24*3600;
  }

  public boolean precedes(TimeOfDay t) {
    return this.totsec < t.totsec;
  }

  public boolean equalTo(TimeOfDay t) {
    return this.totsec == t.totsec;
  }

  public String toString() {
    int h = totsec / 3600;
    int m = (totsec - h*3600) / 60;
    int s = (totsec - h*3600 - m*60);
    String hh = (h < 10) ? " " + h : Integer.toString(h);
    String mm = (m < 10) ? " " + m : Integer.toString(m);
    String ss = (s < 10) ? " " + s : Integer.toString(s);
    return hh + ":" + mm + ":" + ss;
  }
}