State Pattern Explained: How Objects Can Change Behavior Based on Internal State

The State Pattern, defined by the Gang of Four, is a behavioral design pattern that allows an object to alter its behavior when its internal state changes, making it appear to change its class. A common real-world example is an e-commerce order system, where operations like pay, ship, cancel, and refund depend on the order's current status. Without the pattern, managing multiple states and operations leads to complex cascading conditionals — seven states and six operations can produce up to 42 conditions to maintain. The pattern solves this by separating each state into its own class implementing a shared interface, so each state handles only its own valid transitions and throws exceptions for invalid ones. Adding a new state, such as an anti-fraud hold, requires only creating a new class without modifying existing states or the core Order class, naturally respecting the Open/Closed Principle.