Math Class (Java)

// Math with Natural Numbers
class NMath {
  final long NOT_NATURAL;
  static final long ZERO_FACTORIAL = 1;
  NMath(long notNatural) {
    this.NOT_NATURAL = notNatural;
  }
  // Triangle Number
  long tri(long n) {
    if (n < 0) {
      return this.NOT_NATURAL;
    }
    else if (n == 0) {
      return 0;
    }
    else {
      return n + this.tri(n - 1);
    }
  }
  // Factorial
  long fac(long n) {
    if (n < 0) {
      return this.NOT_NATURAL;
    }
    else if (n == 0) {
      return NMath.ZERO_FACTORIAL;
    }
    else {
      return n * this.fac(n - 1);
    }
  }
}

// Math with Whole Numbers
class ZMath extends NMath {
  ZMath() {
    super(-666);
  }
  // Absolute Value
  long abs(long n) {
    return (n < 0) ? -n : n;
  }
}

// Math with Rational Numbers
class QMath extends ZMath {
  QMath() {
    super();
  }
  // Triangle Number
  long tri(long n) {
    if (n < 0) {
      return this.NOT_NATURAL;
    }
    else {
      return (n*(n+1))/2;
    }
  }
}
// Quiz: What output does the following code produce?
class MathClass {
  public static void main(String[] args) {
    NMath nMath = new NMath(-1);
    QMath qMath = new QMath();
    System.out.println(qMath.abs(qMath.NOT_NATURAL));
    System.out.println(nMath.tri(5));
    System.out.println(qMath.tri(10));
    System.out.println(qMath.fac(3));
    System.out.println(qMath.fac(-3));
    System.out.println(nMath.tri(-1));
    System.out.println(qMath.tri(-1));
  }
}