Design patterns represent proven solutions to recurring problems in software development. These standardized approaches have been validated through extensive practical application and provide architects with reliable templates for building maintainable, flexible, and scalable systems.

Categories of Design Patterns

Design patterns are broadly categorized into three main types based on their purpose and scope with in a software system.

Creational Patterns

Creational patterns focus on the object creation mechanism, abstracting the instantiation process from the client code. By delegating the creation responsibility to subclasses, these patterns promote loose coupling and improve code reusability.

The most commonly used creational patterns include:

• Singleton Pattern: Guarantees a class has only one instance and provides a global access point
• Factory Method Pattern: Defines an interface for creating objects, allowing subclasses to determine which class to instantiate
• Abstract Factory Pattern: Provides an interface for creating related object families without specifying concrete classes
• Builder Pattern: Separates the construction of complex objects from their representation
• Prototype Pattern: Creates new objects by copying existing instances

Structural Patterns

Structural patterns address how classes and objects are composed to form larger structures. They utilize composition relationships beyond simple inheritance to achieve greater flexibility in system design.

Key structural patterns include:

• Adapter Pattern: Converts one interface into another that clients expect
• Bridge Pattern: Decouples an abstraction from its implementation
• Composite Pattern: Composes objects into tree structures for part-whole hierarchies
• Decorator Pattern: Attaches additional responsibilities to objects dynamically
• Facade Pattern: Provides a unified interface to a set of interfaces
• Flyweight Pattern: Uses sharing to support large numbers of fine-grained objects
• Proxy Pattern: Provides a surrogate or placeholder for another object

Behavioral Patterns

Behavioral patterns focus on communication between objects, describing how objects interact and how responsibilities are distributed among them.

Notable behavioral patterns include:

• Strategy Pattern: Defines a family of algorithms and makes them interchangeable
• Template Method Pattern: Defines the skeleton of an algorithm, deferring some steps to subclasses
• Observer Pattern: Defines a one-to-many dependency between objects
• Iterator Pattern: Provides a way to access aggregate objects sequentially
• State Pattern: Allows an object to alter its behavior when its internal state changes
• Chain of Responsibility Pattern: Passes requests along a chain of handlers
• Interpreter Pattern: Defines a representation for a grammar
• Memento Pattern: Captures and externalizes an object's internal state
• Visitor Pattern: Represents an operation to be performed on elements of a object structure
• Mediator Pattern: Defines how objects interact through a mediator object

Code Examples

Singleton Pattern Implementation

The following example demonstrates a singleton configuration manager:

public class ConfigManager {
    private static ConfigManager configInstance;
    private Map<String, String> settings;

    private ConfigManager() {
        settings = new HashMap<>();
    }

    public static ConfigManager getInstance() {
        if (configInstance == null) {
            configInstance = new ConfigManager();
        }
        return configInstance;
    }

    public void setParameter(String key, String value) {
        settings.put(key, value);
    }

    public String getParameter(String key) {
        return settings.get(key);
    }
}

This implementation ensures thread safety through lazy initialization. The private constructor prevents external instantiation, while the static getInstance() method provides global access to the single instance.

Adapter Pattern Implementation

The adapter pattern enables compatibility between incompatible interfaces. The following example shows a legacy payment processor adapted to work with a modern interface:

public class LegacyPaymentGateway {
    public void processPaymentWithLegacyApi(double amount) {
        System.out.println("Processing payment: " + amount);
    }
}

public interface PaymentProcessor {
    void processPayment(double amount);
}

public class PaymentGatewayAdapter implements PaymentProcessor {
    private LegacyPaymentGateway legacyGateway;

    public PaymentGatewayAdapter(LegacyPaymentGateway legacyGateway) {
        this.legacyGateway = legacyGateway;
    }

    @Override
    public void processPayment(double amount) {
        legacyGateway.processPaymentWithLegacyApi(amount);
    }
}

Comparing Creational and Structural Patterns

Understanding the distinction between creational and structural patterns helps developers choose the right approach for specific problems.

Creational patterns primarily address the "how" of object creation. They abstract the instantiation process, separating object creation from usage. This separation reduces coupling and increases flexibility. By encapsulating creation logic, these patterns allow systems to remain independent of how their objects are created, composed, and represented.

Structural patterns focus on the "how" of object composition. They solve problems related to class and object assembly, addressing concerns such as extensibility, encapsulation, and interaction design. These patterns provide solutions for building flexible hierarchies and managing relationships between objects.

The key difference lies in their primary concern: creational patterns manage object lifecycle, while structural patterns manage object relationships and interactions.

When applying design patterns, consider the specific requirements of your system. Avoid over-engineering by implementing patterns only when they provide clear benefits to code organization, maintainability, or performance.