JAVA EE: State Design pattern

Saturday, 25 January 2014

State Design pattern - Introduction

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What is the State Design Pattern?

The State Design Pattern is a behavioral design pattern cataloged by the Gang of Four (GoF). It allows an object to alter its behavior when its internal state changes, making it appear as if the object has dynamically changed its class.

In software systems, domain objects often behave differently depending on their status. For example, a document behaves differently in Draft, Moderation, and Published states. When actions are triggered, traditional approaches rely on massive conditional ladders (if-else or switch-case statements), resulting in fragile and tangled code.

Core Concepts & Participants

The State pattern resolves this complexity by turning states into separate polymorphic classes:

Context: The primary interface for clients. It maintains a reference to a concrete state object that represents the current state of the machine or process and delegates state-specific requests to it.
State Interface: An interface declaring the methods that all concrete state implementations must fulfill.
Concrete State Classes: Individual classes that implement the State interface, providing state-specific logic and coordinating state transitions.

Why Use the State Pattern?

Integrating the State design pattern into your applications offers key architectural advantages:

Elimination of Massive Conditionals: Replaces bloated conditional blocks with clean polymorphic dispatch.
Single Responsibility Principle: Organizes behavior corresponding to particular states into dedicated, easily testable classes.
Open/Closed Principle: Lets you introduce new states without modifying existing state classes or the context.