Summary
Software design patterns are standardised solutions to frequently occurring problems in software design. They provide a shared vocabulary for design decisions, independent of programming language or specific framework.
This article sets out the fundamentals of software design patterns, including their historical roots, the classic categorisation into creational, structural and behavioural patterns, and the role patterns play in modern development practice. We also look at popular examples and at their relationship with principles such as SOLID and object-oriented design.
Key take-aways from this article:
What software design patterns are and why they originated
How patterns are described and categorised
Overview of the three main groups: creational, structural and behavioural
Examples of well-known patterns and when to use them
The role of design patterns in modern development and their relationship with frameworks
Introduction and definition of software design patterns
Software design patterns are descriptions of proven solutions to common problems in software design. A pattern is not ready-made code, but a conceptual template that indicates how particular responsibilities can be distributed across classes, objects or components. The aim is to promote the reuse of good solutions and to avoid reinventing the wheel.
The term design pattern became widely known in the software world through the book "Design Patterns: Elements of Reusable Object-Oriented Software", published in the 1990s. The book defined a set of patterns, outlined their structure and explained why they are useful. Since then, patterns have been regarded as a way to capture the experience of designers and make it transferable.
A design pattern is usually described using fixed sections. Typical elements include the name of the pattern, the problem it solves, the context in which it occurs, the structure of the participating classes and objects, a description of their collaboration and the consequences, such as for maintainability and flexibility. Thanks to this standard form, developers can recognise patterns in existing code and apply them consciously in new designs.
A short code illustration can help make the abstract nature clearer. The pseudo-code below shows the essence of a simple factory pattern, without being tied to any specific programming language:
interface Shape {
draw()
}
class Circle implements Shape {
draw() { /* draw circle */ }
}
class Square implements Shape {
draw() { /* draw square */ }
}
class ShapeFactory {
create(type) {
if (type == "circle") return new Circle()
if (type == "square") return new Square()
throw Error("Unknown type")
}
}
// Usage
factory = new ShapeFactory()
shape = factory.create("circle")
shape.draw()
In this snippet the pattern is not the syntax but the way object creation is centralised in a factory. This makes it easier to add new shapes later without changing client code.
Classification and basic structure of design patterns
Design patterns are often grouped into three main categories based on their goal and focus. This subdivision appears frequently in documentation, literature and online references. The categories are creational patterns, structural patterns and behavioural patterns. Each group addresses a different aspect of software design.
Creational patterns describe how objects are created. The goal is to isolate creation logic and prevent code from becoming tightly coupled to specific implementations. Examples include Factory Method, Abstract Factory, Builder, Prototype and Singleton. These patterns offer alternatives to direct object construction and allow you to vary concrete implementations at runtime.
Structural patterns focus on how classes and objects are composed into larger structures. They describe how objects can be combined to create new functionality without changing the internal implementation of the individual objects. Frequently cited patterns are Adapter, Facade, Composite, Decorator, Proxy and Bridge. These patterns help reduce dependencies, simplify complex subsystem interfaces and extend behaviour through composition rather than inheritance.
Behavioural patterns, finally, describe interaction and responsibility allocation between objects. They focus on communication, collaboration and message exchange. Well-known examples are Strategy, Observer, Command, State, Template Method, Iterator and Mediator. These patterns make behaviour flexible and interchangeable, so code can more easily adapt to future changes in functionality without rewriting the core structure.
A popular online introduction to these patterns is available in video form. A representative example that explains the main groups, basic principles and some core patterns is the following YouTube video:
Creational patterns
Creational patterns solve problems around object creation, such as tight coupling to concrete classes or complex construction logic scattered throughout the codebase. By hiding creation behind an abstraction, code can work with interfaces or base types, which increases extensibility and testability.
The Factory Method pattern defines an interface for creating objects but lets subclasses decide which concrete class is instantiated. This reduces dependencies on specific implementations. Abstract Factory goes a step further and provides a family of related objects, for example different UI components that share the same theme, without client code needing to know about the differences between concrete families.
The Builder pattern splits the construction of a complex object into small steps. This is useful when objects have many optional parameters or an intricate internal structure. By decoupling construction from representation, different representations can be built with the same set of steps.
Singleton is a pattern that guarantees exactly one instance of a class and provides a global access point to that instance. Although widely used, modern guidelines often highlight its drawbacks, such as hidden dependencies and testing difficulties. As a result, Singleton is used sparingly in contemporary codebases or replaced by explicit dependency injection.
Structural patterns
Structural patterns describe how objects are combined to form larger structures. They focus on composition, interface compatibility and simplifying subsystems. Rather than deeply modifying existing classes, they provide a layer around them that enables integration or extension.
The Adapter pattern allows an existing class to be used through a different interface than the one it originally offers. This is helpful when an existing component needs to work with newly written code while the original code cannot or should not be modified. Adapter acts as an intermediary that translates incoming requests into the format the existing component expects.
Facade offers a simplified interface on top of a complex subsystem. Instead of client code interacting directly with many classes, a single class is introduced that bundles common actions. This reduces cognitive load for developers and prevents internal details of the subsystem from leaking out.
Composite defines tree structures of objects in which individual objects and compositions of objects are treated the same way. This is useful for hierarchical data such as menu structures or bill-of-materials. Decorator provides an alternative to inheritance by adding behaviour to objects dynamically via wrapper objects. This enables combinations of additional functionality without an explosion of subclasses.
Proxy, finally, introduces a stand-in object that controls access to another object. It can be used for lazy loading, access control, caching or remote communication. Client code continues to use the same interface, while the proxy can execute extra logic before forwarding the request to the underlying object.
Behavioural patterns
Behavioural patterns focus on interaction between objects and the distribution of responsibilities. They help make behaviour configurable, decouple communication and manage complex dependencies. Many modern frameworks and libraries implicitly use these patterns in their API design.
The Strategy pattern defines a family of algorithms, encapsulates them and makes them interchangeable. The client knows only the abstract strategy interface and can switch concrete strategies at runtime. This is valuable when calculation logic varies, for example different sorting algorithms or price calculations, without needing to modify client code.
Observer describes a one-to-many relationship between objects, where a change in one object is automatically propagated to all dependent objects. This pattern is common in event systems and user-interface libraries. The subject maintains a list of observers and notifies them whenever its state changes.
Command encapsulates a request as an object, allowing requests to be queued, logged or undone. Each command object implements, for example, an execute method and optionally an undo. This makes it possible to model operations as objects that can be stored or replayed. A simple pseudo-code example illustrates the structure:
interface Command {
execute()
}
class AddItemCommand implements Command {
constructor(cart, item) { ... }
execute() { cart.add(item) }
}
class Invoker {
commands = []
addCommand(cmd) { commands.append(cmd) }
run() {
for cmd in commands:
cmd.execute()
}
}
State, lastly, lets an object alter its behaviour when its internal state changes. Instead of large if or switch constructs, each state is represented by a separate class that implements the specific behaviour. A context object refers to the current state and delegates to that class. This simplifies logic and makes adding new states more manageable.
Patterns in modern software development
In modern software development the role of design patterns has shifted from literally applying catalogues to recognising the underlying principles in frameworks and architectures. Many contemporary libraries and frameworks, from web frameworks to UI toolkits, are internally built around one or more patterns such as MVC, Observer, Strategy or Dependency Injection.
Design patterns are often used alongside design principles such as SOLID and clean architecture. Patterns give these principles concrete form. For instance, using Strategy and Factory helps apply dependency inversion because code depends on abstractions rather than implementations, improving testability and replaceability.
At the same time, recent literature and discussions highlight potential pitfalls. Excessive use of patterns can lead to unnecessary complexity, where simple solutions are replaced by elaborate structures that are elegant only in theory. The advice is to apply patterns pragmatically, only when they genuinely solve a concrete problem.
With the rise of new programming languages, functional paradigms and microservice architectures, some classic patterns have evolved or become integrated into language constructs. Patterns such as Iterator and Observer are often provided directly by languages or standard libraries. Nevertheless, software design patterns remain relevant as a vocabulary for communicating design ideas and formulating solutions at a higher level of abstraction, independent of specific technology.
What exactly are software design patterns
Software design patterns are general, reusable solutions to frequently occurring design problems in software. They are conceptual descriptions, not concrete code snippets. They explain how classes and objects can collaborate so that the software becomes more flexible, easier to maintain and simpler to extend.
Why are design patterns still relevant in modern development
Design patterns remain relevant because they offer a common language for design decisions. Even with modern frameworks and languages, the same kinds of problems recur, such as object creation, dependency management and communication between components. Patterns help address these problems systematically and make code more understandable to developers who use the same terminology.
What is the difference between creational, structural and behavioural patterns
Creational patterns focus on how objects are created and how creation logic is organised. Structural patterns concentrate on how classes and objects are composed into larger structures. Behavioural patterns describe interaction and responsibility distribution between objects, emphasising communication and algorithms. This categorisation helps you choose the right type of pattern for a particular design problem.
Are design patterns tied to a specific programming language
Design patterns are language-agnostic; they are not tied to a single language. The concepts arose in the context of object-oriented languages, but the underlying ideas are applicable in many environments. Some patterns may be easier or less relevant in a specific language, depending on available language constructs and standard libraries.
Can design patterns also have drawbacks
Yes, design patterns can have drawbacks if they are applied thoughtlessly or excessively. Overuse can lead to too much abstraction, extra layers and complicated class diagrams, whereas a simpler solution would have sufficed. This is sometimes referred to as over-engineering. It is therefore important to employ patterns only when they solve a clearly identifiable problem, not merely for theoretical reasons.
How do design patterns relate to SOLID principles
Design patterns and SOLID principles complement each other. SOLID offers broad guidelines for good object-oriented design, such as single responsibility and dependency inversion. Design patterns provide concrete structures that help put these principles into practice. For example, Strategy and Factory Method support the idea that code should depend on abstractions rather than implementations, which aligns with the dependency inversion principle.
Are all well-known patterns equally important in microservices and cloud environments
Not all patterns have the same relevance in every architecture. Some classic object-oriented patterns are applied less explicitly when frameworks already solve the underlying problems. At the same time, new patterns emerge at a higher level, focusing on distribution, fault tolerance and scalability. Many fundamental patterns, however, remain useful, especially within individual services, because object-oriented structures and interactions are still designed there.
