#Design Patterns

Design a Locker Delivery System for an E-commerce platform. Describe the flow where a delivery agent requests a locker, the system assigns an available locker, and the process for customers to use the locker to collect their packages. Consider how to make the system extensible to accommodate different package attributes.

Describe the Observer Design Pattern. Implement a basic example where one subject notifies multiple observers about state changes. Briefly explain the code and its components.

How would you design a thread-safe singleton class in Java? Explain the different methods and their trade-offs.