Structural design patterns focus on how classes and objects are composed. They simplify relationships and ensure parts of a system work together, even with incompatible interfaces. This makes code easier to extend, modify, and reuse without altering existing logic.

The Adapter pattern

It enables incompatible interfaces to work together, acting as a bridge between new and old systems. For example, if a payment gateway expects a different method signature, an adapter converts your method into the required format without changing the original code. Pseudo-code for Adapter:

The Composite pattern

It lets you treat individual objects and groups of objects uniformly. It’s common in tree-like structures, such as a file system where files and folders are handled the same way, even though folders can contain more files or folders.

The Proxy pattern

This one provides a placeholder to control access to another object. It’s useful for expensive operations, access control, or lazy initialization—for example, an image loader proxy that loads the image only when it’s actually needed.

The Flyweight pattern

The Flyweight pattern reduces memory usage by sharing common data among many objects. It’s especially useful with large numbers of similar objects. For example, a text editor where characters share font data instead of duplicating it for every letter.

Each structural pattern solves a different concern: some connect incompatible interfaces, others manage part-whole hierarchies, some control access, and others optimize memory use.

Structural patterns are key to clean architecture as applications grow, reducing complexity with reusable solutions that prevent duplication and ease future extensions.