Invoice.cs

public class Invoice {
  private bool isForXDepartment;
  
  public Invoice(bool isForXDepartment) {
    // บริษัทนี้ถ้าเป็นใบเสร็จของแผนก X จะต้องคำนวณภาษีและส่วนลดแบบพิเศษ
    this.isForXDepartment = isForXDepartment;
  }

  public double calculateSumPrice () {
    if (this.isForXDepartment) {
      return this.calculateSumPriceForDepartmentX();
    }
    else {
      return this.calculateSumPrice();
    }
  }

  private double calculateSumPriceForDepartmentX () {
  
  }
  
  private double calculateSumPrice() {
  
  }

}