Developed the foundational Invoice Management System for the KShiMin/Hospital_Management_System repository, focusing on establishing core billing functionality. Designed and implemented an Invoice class in Java using object-oriented principles, providing attributes and lifecycle management methods to support invoice creation, management, and printing. Introduced a main demo application to validate and demonstrate the end-to-end invoice workflow, enabling basic billing previews and setting the stage for future enhancements such as persistence and reporting. The modular architecture supports extensibility, allowing for integration of additional features over time. Emphasized robust testing practices throughout development, ensuring a reliable base for future invoicing capabilities within the system.