Module 1: Functional Requirements

Understanding Functional Requirements

Functional requirements describe functions the software must perform. They define what the system should do, such as:

• The order number retriever function shall throw an Exception when the provided order number is invalid
• The ATM shall log a user in when they provide a valid card and matching PIN
• The game shall merge two blocks with the same value when the player swipes them together

Difference Between Functional and Non-Functional Requirements

While functional requirements define what a system should do, non-functional requirements describe how the system should perform:

• The program shall require no more than 6GB of memory
• The service shall retrieve the orders for an order ID in less than 500ms
• The online chat shall display messages from 1000 simultaneous users

Example Java Implementation

public class OrderProcessor {
    /**
     * Processes an order based on functional requirements
     * @param orderId The unique identifier for the order
     * @throws IllegalArgumentException if the orderId is invalid
     * @return The processed order details
     */
    public Order processOrder(String orderId) {
        // Check precondition - valid orderId
        if (orderId == null || orderId.isEmpty()) {
            throw new IllegalArgumentException("OrderId cannot be null or empty");
        }
        
        // Process the order
        Order order = retrieveOrder(orderId);
        
        // Check postcondition - order must be valid
        if (order == null) {
            throw new IllegalStateException("Could not retrieve a valid order");
        }
        
        return order;
    }
}