Object-Oriented Design and Unit Testing

You are tasked with implementing a small, in-memory model for a company and its employees. The interviewer will provide only high-level input/output expectations, requiring you to clarify detailed requirements and design clean, intuitive APIs.

Part 1: Employee Object Design

Design an Employee object that must include at least the following attributes:

• id: A unique identifier for the employee.
• name: The employee's full name.
• email: The employee's email address.

Part 2: Company Object Design

Design a Company object responsible for managing a collection of employees. This object must support the following core operations:

• Add an employee to the company.
• Read or look up an employee by their unique id.
• Update an existing employee's email address, identified by their id.
• Delete or remove an employee from the company by their id.

Part 3: Behavior and Validation

Define and implement robust behavior for various edge cases and clarify any assumptions made during design. Consider scenarios such as:

• Attempting to add an employee with an id that already exists.
• Attempting to look up, update, or delete an employee using an id that does not exist within the company.
• Handling invalid email formats, if you choose to implement email validation.

Part 4: Testing Requirement

Write comprehensive unit tests for each part and operation designed. These tests must be executable and successfully pass. Ensure test coverage for:

• All "happy path" scenarios where operations proceed as expected.
• Key edge cases and the corresponding error handling mechanisms.

Part 5: Code Quality Expectations

Your solution should demonstrate high code quality, aiming for:

• Clear, descriptive method names and well-defined return values.
• Appropriate use of exceptions or other error reporting mechanisms.
• A maintainable structure, exhibiting good separation of concerns and minimal coupling between components.