Introduction to Gang of Four (GoF) Design Patterns

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Design patterns are standardized solutions to common software design problems.

The term “Gang of Four (GoF)” refers to 23 classic object‑oriented patterns cataloged by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides in their seminal book.

These patterns provide reusable blueprints for solving recurring design challenges.

The GoF patterns are organized into three groups: Creational, Structural, and Behavioral, each addressing a different aspect of class and object design.

Creational Patterns

Creational patterns abstract and encapsulate object creation to make systems more flexible.

They hide the details of class instantiation and help decouple clients from concrete classes. There are five GoF creational patterns, including:

• Factory Method – Defines an interface for creating an object, letting subclasses decide which class to instantiate. This avoids hard‑coding specific classes.
• Abstract Factory – Provides an interface for creating families of related objects without specifying their concrete classes. It’s essentially a “factory of factories.”
• Singleton – Ensures that a class has only one instance and provides a global access point to it. This is useful for shared resources like configuration or logging.
• Builder – Separates the construction of a complex object from its representation, allowing the same construction process to create different representations. It is useful when an object has many optional parts.
• Prototype – Creates new objects by copying a prototypical instance instead of building from scratch. This can be efficient for expensive object creation.

For example, the Singleton pattern can be implemented in Java as follows:

public class Singleton {
    private static Singleton instance = null;
    private Singleton() {}  // Private constructor

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}
// Usage:
Singleton obj = Singleton.getInstance();

This code ensures only one Singleton object is ever created, matching the pattern’s intent.

Structural Patterns

Structural patterns deal with the composition of classes and objects to form larger structures.

They simplify the design by identifying relationships and responsibilities.

The GoF catalog defines seven structural patterns, such as Adapter, Composite, Proxy, Flyweight, Facade, Bridge, and Decorator. A few examples:

• Adapter – Lets two incompatible interfaces work together by wrapping one with an adapter class. For instance, an adapter can allow old code to interface with a new system.
• Decorator – Attaches additional responsibilities to an object dynamically by wrapping it. This allows behavior to be added without subclassing.
• Facade – Provides a simplified, high‑level interface to a complex subsystem, hiding its inner workings from the client. A facade class directs calls to the subsystem’s components in a unified way.

Each structural pattern helps organize code and promote flexibility.

For example, a Facade can reduce code dependencies by exposing only what a client needs, and an Adapter makes mismatched interfaces compatible.

These patterns ensure that changes in one part of a system have minimal impact on others.

Behavioral Patterns

Behavioral patterns describe how objects interact and assign responsibilities, emphasizing communication and loose coupling.

They focus on algorithms and flow of control between objects.

There are 11 GoF behavioral patterns, including Observer, Strategy, Command, Iterator, Mediator, and others. For example:

• Observer – Establishes a one‑to‑many dependency so that when one object (the subject) changes state, all its dependents (observers) are automatically notified and updated. This is fundamental in event-driven systems.
• Strategy – Defines a family of interchangeable algorithms or behaviors, encapsulating each one and making them interchangeable. This lets an object change its algorithm or behavior at runtime.

Other behavioral patterns include Command (encapsulating requests as objects) and Template Method (defining a skeleton of an algorithm).

In practice, these patterns improve code maintainability by delegating work and communication in a disciplined way.

Summary

The GoF design patterns provide a shared vocabulary and proven solutions for common design problems.

By learning these patterns, beginners gain insight into writing flexible, reusable, and maintainable code.

Each pattern targets a specific problem and offers a concrete solution.

In summary, understanding Creational, Structural, and Behavioral patterns is a foundational step toward better software design in object-oriented programming.

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