Structural Design Patterns

Structural patterns are concerned with how classes and objects are composed to form larger structures. Structural class patterns use inheritance to compose interfaces or implementations.

Importance of Structural Design Patterns

• This pattern is particularly useful for making independently developed class libraries work together.
• Structural object patterns describe ways to compose objects to realize new functionality.
• It added flexibility of object composition comesfrom the ability to change the composition at run-time, which is impossible with static class composition.

When to ue Structural Design Patterns

• Adapting to Interfaces: Use structural patterns like the Adapter pattern when you need to make existing classes work with others without modifying their source code. This is particularly useful when integrating with third-party libraries or legacy code.
• Organizing Object Relationships: Structural patterns such as the Decorator pattern are useful when you need to add new functionalities to objects by composing them in a flexible and reusable way, avoiding the need for subclassing.
• Simplifying Complex Systems: When dealing with complex systems, structural patterns like the Facade pattern can be used to provide a simplified and unified interface to a set of interfaces in a subsystem.
• Managing Object Lifecycle: The Proxy pattern is helpful when you need to control access to an object, either for security purposes, to delay object creation, or to manage the object’s lifecycle.
• Hierarchical Class Structures: The Composite pattern is suitable when dealing with hierarchical class structures where clients need to treat individual objects and compositions of objects uniformly.

Advantages of Structural Design Patterns

• Flexibility and Adaptability: Structural patterns enhance flexibility by allowing objects to be composed in various ways. This makes it easier to adapt to changing requirements without modifying existing code.
• Code Reusability: These patterns promote code reuse by providing a standardized way to compose objects. Components can be reused in different contexts, reducing redundancy and improving maintainability.
• Improved Scalability: As systems grow in complexity, structural patterns provide a scalable way to organize and manage the relationships between classes and objects. This supports the growth of the system without causing a significant increase in complexity.
• Simplified Integration: Structural patterns, such as the Adapter pattern, facilitate the integration of existing components or third-party libraries by providing a standardized interface. This makes it easier to incorporate new functionalities into an existing system.
• Easier Maintenance: By promoting modularity and encapsulation, structural patterns contribute to easier maintenance. Changes to one part of the system are less likely to affect other parts, reducing the risk of unintended consequences.
• Solves Recurring Design Problems: These patterns encapsulate solutions to recurring design problems. By applying proven solutions, developers can focus on higher-level design challenges unique to their specific applications.

Disadvantages of Structural Design Patterns

• Complexity: Introducing structural patterns can sometimes lead to increased complexity in the codebase, especially when multiple patterns are used or when dealing with a large number of classes and interfaces.
• Overhead: Some structural patterns, such as the Composite pattern, may introduce overhead due to the additional layers of abstraction and complexity introduced to manage hierarchies of objects.
• Maintenance Challenges: Changes to the structure of classes or relationships between objects may become more challenging when structural patterns are heavily relied upon. Modifying the structure may require updates to multiple components.
• Limited Applicability: Not all structural patterns are universally applicable. The suitability of a pattern depends on the specific requirements of the system, and using a pattern in the wrong context may lead to unnecessary complexity.

Designing object-oriented software is hard, and designing reusable object-oriented software is even harder.

Christopher Alexander says, “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice”.

Even though Alexander was talking about patterns in buildings and towns, what he says is true about object-oriented design patterns.