class ComplexMathUtils {

  public static ComplexNumber multiply(final ComplexNumber z1, final ComplexNumber z2) {
    ArgumentChecker.notNull(z1, "z1");
    ArgumentChecker.notNull(z2, "z2");
    final double a = z1.getReal();
    final double b = z1.getImaginary();
    final double c = z2.getReal();
    final double d = z2.getImaginary();
    return new ComplexNumber(a * c - b * d, a * d + b * c);
  }

}