Design Patterns in Software Engineering: Concepts, Categories, and Five Essential Patterns

High-level programming languages have existed since the 1950s. Since then, software has been written to solve an enormous range of problems in business, science, entertainment, infrastructure, and more. Although each problem domain is different, experienced programmers gradually noticed that the structures of their solutions often looked familiar.

Sometimes the same kind of classes appeared again and again. Sometimes objects were arranged in similar ways, or responsibilities were split using similar rules. These similarities were not exact enough to turn into a single reusable library or a single algorithm, but they were recognizable as recurring design ideas.

Those recurring ideas became known as design patterns.

1. What Is a Design Pattern?

A design pattern is a general, reusable solution to a commonly occurring problem in software design. It is not a finished piece of code. Instead, it describes:

• The structure of participating classes and objects

• The roles and responsibilities of those objects

• Typical ways they collaborate to solve a particular kind of problem

A design pattern is a conceptual template. Each concrete project adapts the pattern to its own needs, data models, and constraints.

1.1 Design Pattern vs Algorithm

Design patterns are often confused with algorithms, but they serve different purposes.

• An algorithm is a precise, step-by-step procedure that solves a well-defined computational problem.

  • Example: quicksort, Dijkstra’s shortest path, binary search.

  • The steps are fixed and can be followed the same way every time on any suitable input.

• A design pattern describes a structure and a collaboration style, not a precise sequence of operations.

  • Different implementations of the same pattern may have different classes, methods, or control flows.

  • The essence lies in the relationships between parts, not in exact steps.

An analogy makes this clearer.

• A recipe for baking a cake is like an algorithm: do step 1, then step 2, then step 3. Following it leads to the same result, possibly with different ingredients.

• A birthday party is more like a design pattern: there is a recognizable idea (guests, celebration, cake, decorations), but the exact details differ widely. Each host organizes a unique party, yet it still fits the pattern of “birthday party”.

Design patterns operate at this second level: a recognizable template that guides structure, but does not mandate every detail.

2. The “Gang of Four” and the Three Categories of Patterns

In 1994, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides published the book “Design Patterns: Elements of Reusable Object-Oriented Software”. These four authors are often called the Gang of Four (GoF).

That book documented 23 classic design patterns for object-oriented systems and grouped them into three categories:

1. Creational patterns – Focus on how objects are created.

   • Examples: Singleton, Factory Method, Abstract Factory, Builder, Prototype.

2. Structural patterns – Focus on how classes and objects are composed into larger structures.

   • Examples: Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy.

3. Behavioral patterns – Focus on how objects interact, share responsibilities, and communicate.

   • Examples: Strategy, Observer, Command, State, Visitor, Mediator, Iterator, Template Method, Memento, Chain of Responsibility, Interpreter.

Not every developer memorizes all 23 patterns. In practice, familiarity often grows over time. Many programmers discover that they have already applied some of these patterns intuitively, without knowing the formal names.

In day-to-day work, a smaller subset of patterns tends to reappear frequently. The rest form a background toolbox: patterns that can be recognized and applied when a matching problem appears.

The following sections focus on five patterns that are widely useful in real systems:

• Strategy

• Decorator

• Observer

• Singleton

• Facade

Each of these is explained in terms of its intent, structure, and common use cases.

3. Strategy Pattern: Swappable Behaviors

3.1 Intent

The Strategy pattern defines a family of algorithms, encapsulates each one as a separate object, and makes them interchangeable. The client code works with an abstract strategy interface and does not need to know which concrete strategy is used.

Key goals:

• Avoid large if or switch statements that select behavior in many places.

• Allow the algorithm to change at runtime.

• Keep the client code independent from specific implementations.

3.2 Structure

Typical elements in the Strategy pattern:

• Strategy interface

  • Declares the operations that all strategies must support.

• ConcreteStrategy classes

  • Implement the strategy interface in different ways. Each one provides a different behavior.

• Context class

  • Holds a reference to a Strategy object.

  • Delegates behavior to the strategy via the interface.

In UML-style terms:

• Context → has-a → Strategy

• ConcreteStrategyA, ConcreteStrategyB, etc. → implement → Strategy

3.3 Example Scenario

Consider an application that provides navigation. Different travel modes are possible:

• Walking

• Driving

• Cycling

• Public transport

All of these calculate routes, but the details differ: speed, road types, restrictions, and optimization rules.

A Strategy design:

• Define RouteStrategy interface with a method like calculateRoute(origin, destination).

• Implement WalkingRouteStrategy, DrivingRouteStrategy, CyclingRouteStrategy, etc.

• The navigation system (Navigator class) holds a RouteStrategy. When a route is needed, it calls strategy.calculateRoute(...).

Switching behavior is as simple as changing the strategy object; the rest of the system remains unchanged.

3.4 Benefits

• Eliminates long conditional logic that selects between variants.

• Makes it easy to add new behaviors without modifying existing client code.

• Promotes separation of concerns: the context does not need to know algorithm details.

4. Decorator Pattern: Extending Behavior Without Modifying Code

4.1 Intent

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Key goals:

• Extend behavior without changing existing code.

• Wrap objects to add features before or after delegating to the original.

• Keep the original component untouched, following the Open/Closed Principle (open for extension, closed for modification).

4.2 Structure

Core roles in the Decorator pattern:

• Component interface

  • Defines operations that both concrete components and decorators support.

• ConcreteComponent

  • Original implementation of the component’s main behavior.

• Decorator (abstract or base class)

  • Implements the same interface as the component.

  • Holds a reference to a Component (the wrapped object).

  • Delegates operations to the wrapped component by default.

• ConcreteDecorator

  • Extends the base decorator.

  • Overrides operations to add extra behavior before or after delegation.

The important feature is that decorators and components share the same interface. A decorator can be used anywhere a component is expected.

4.3 Example Scenario

A logging system provides a simple Logger interface:

• log(message)

There might be a basic implementation that writes to a file. To extend it:

• A TimestampLoggerDecorator adds timestamps before calling log on the wrapped logger.

• A SeverityLoggerDecorator adds severity tags.

• A ConsoleLoggerDecorator could forward the message to the console and then delegate to the wrapped logger.

These decorators wrap a base logger or even other decorators, layering behavior without altering the original FileLogger class.

4.4 Benefits

• Allows behavior to be layered in flexible combinations at runtime.

• Avoids a deep inheritance tree of subclasses for every behavioral variation.

• Keeps the core component simple and focused.

5. Observer Pattern: Publish–Subscribe Communication

5.1 Intent

The Observer pattern defines a one-to-many dependency between objects so that when one object (the subject) changes state, all its dependents (observers) are notified and updated automatically.

Key goals:

• Decouple the object that generates events from the objects that react to them.

• Allow multiple observers to subscribe or unsubscribe dynamically.

• Support event-driven designs.

5.2 Structure

The pattern typically includes:

• Subject (Publisher)

  • Maintains a collection of observers.

  • Provides methods to attach (subscribe) and detach (unsubscribe) observers.

  • Notifies all observers when a significant event occurs.

• Observer interface (Subscriber)

  • Declares an update method (or similar), called by the subject.

• ConcreteObserver classes

  • Implement the update method to react to changes in the subject.

The subject does not need to know concrete observer types; it interacts only through the observer interface.

5.3 Example Scenario

A generic newsletter system illustrates the idea:

• A NewsletterPublisher acts as the subject. It handles subscribe, unsubscribe, and notify.

• Each Subscriber implements an interface such as update(content) to process a new issue.

• When a new newsletter issue is created, the publisher loops over its internal list of subscribers and calls update for each one.

Only those that subscribed are notified. Others are not involved. The decision to receive updates belongs to each observer.

Similar structures appear throughout software:

• GUI frameworks (buttons notifying listeners on click)

• Event buses and message brokers

• Reactive systems and streams

5.4 Benefits

• Promotes loose coupling between event sources and listeners.

• Supports dynamic subscription and unsubscription.

• Encourages clear separation between “what happened” (in the subject) and “what to do about it” (in the observers).

6. Singleton Pattern: Enforcing a Single Instance

6.1 Intent

The Singleton pattern ensures that a class has only one instance and provides a global point of access to it.

Typical use cases:

• Shared configuration objects

• Centralized logging instances

• Connection pools

• Resource managers

The core idea is to avoid multiple copies of an object that must be unique, such as a database connection manager that should not be duplicated uncontrolled.

6.2 Classic Implementation

A common object-oriented implementation includes:

• A private constructor, so external code cannot call new directly.

• A static field to hold the single instance.

• A static method or property (for example, getInstance()) that:

  • Checks whether the instance already exists

  • Creates it if necessary

  • Returns the same instance thereafter

This pattern can be extended to handle lazy initialization (creating the instance only when first requested) and thread safety (avoiding multiple threads creating separate instances concurrently).

6.3 Thread Safety Considerations

In multithreaded environments, naive implementations are not safe. Two threads could both see a null instance and create separate objects.

Common techniques to address this:

• Synchronization or locking around the instance creation code

• Double-checked locking (check then lock then check again)

• Built-in language features such as lazy initialization primitives in some runtimes

6.4 Benefits and Drawbacks

Benefits:

• Ensures a single, shared instance where necessary.

• Provides a common access point for global resources.

Drawbacks:

• Can introduce hidden global state, making code harder to test and reason about.

• Tight coupling to the singleton can make future refactoring difficult.

• Overuse of singletons can lead to designs that resemble procedural global variables rather than well-structured object-oriented systems.

Because of these drawbacks, Singleton is sometimes considered a pattern that must be used with caution and discipline.

7. Facade Pattern: Simplifying Complex Systems

7.1 Intent

The Facade pattern provides a simple, unified interface to a complex subsystem. It defines a higher-level interface that makes the subsystem easier to use.

Key goals:

• Hide internal complexity from client code.

• Provide a clear entry point for common operations.

• Reduce coupling between client code and many individual subsystem classes.

7.2 Structure

Main elements:

• Subsystem classes

  • Perform various low-level operations, often numerous and detailed.

  • May be difficult to use directly without extensive knowledge.

• Facade class

  • Offers a simpler set of methods for common tasks.

  • Coordinates calls to one or more subsystem classes.

  • Hides many implementation details and configuration choices.

Clients call the Facade’s methods rather than interacting directly with the subsystem layers.

7.3 Example Scenario

Consider a logging library with a poorly designed API that might:

• Require explicit parameters specifying output destinations (file, console, messaging service) for each log call

• Require repetitive configuration options across multiple calls

• Support more features than typical application code needs

A Facade can wrap this library:

• Define a Logger interface with convenient methods such as logInfo, logError, logDebug.

• Implement a SimpleLogger facade that internally calls the complex library, setting default options and hiding extra parameters.

• Application code interacts only with Logger, not with the low-level logging library.

If the underlying logging library changes, only the Facade implementation needs to be updated. The rest of the codebase remains unaffected.

7.4 Benefits

• Makes external libraries and frameworks easier to use.

• Concentrates integration complexity in a single location.

• Reduces coupling, improving maintainability and flexibility.

8. Using Design Patterns Effectively

Design patterns are most useful when treated as tools, not as strict rules. A pattern should emerge naturally from the forces in the problem:

• If there are many interchangeable behaviors: Strategy.

• If behavior must be extendable without modifying existing classes: Decorator.

• If state changes must be broadcast to many listeners: Observer.

• If a single shared instance is required: Singleton (carefully).

• If a complex subsystem needs a simpler API: Facade.

Several guidelines can help in practical use.

8.1 Recognize Before Forcing

Patterns are descriptive, not prescriptive. A design may match a pattern even if that pattern was not consciously chosen at the start. The key is recognizing when a pattern maps well onto a problem:

• Repeated conditional logic that selects implementation suggests Strategy.

• Multiple layers of pre- and post-processing around a core operation suggest Decorator.

• Repeated event notifications to multiple consumers suggest Observer.

• Centralized resource control suggests Singleton or an alternative scoped solution.

• Complex third-party APIs suggest Facade.

Forcing a pattern where it does not fit can lead to over-engineering and unnecessary complexity.

8.2 Communicate Through Pattern Names

One of the greatest advantages of design patterns is shared vocabulary. Saying “use a Strategy here” or “wrap that in a Facade” immediately conveys a structural idea to other engineers.

Names help:

• Compress design discussions

• Transfer intent quickly

• Make architecture documents more concise and expressive

This is most effective when the team has at least a basic familiarity with the classic patterns.

8.3 Combine Patterns When Appropriate

Patterns are not mutually exclusive. They often appear together:

• A Facade might internally use multiple strategies.

• A Singleton might serve as a central event hub using Observer.

• A Decorator might be used to add behavior to strategies or observers.

The aim is not to maximize the number of patterns used, but to support clarity, flexibility, and maintainability.

8.4 Balance Pattern Use With Simplicity

Patterns introduce more classes and interfaces. This is beneficial when they simplify the conceptual model, but can be harmful if they are applied in situations that do not need them.

A simple solution without a named pattern can sometimes be better than a forced pattern. The presence of a pattern is justified when it:

• Removes duplication or uncontrolled complexity

• Clarifies responsibilities

• Makes future extension or replacement easier

9. Conclusion

Design patterns capture and name widely used approaches to recurring design problems in software systems. They sit between concrete code and abstract principles, offering a shared language and set of templates for structuring solutions.

The Gang of Four cataloged 23 such patterns, grouped into creational, structural, and behavioral categories. Among these, several appear especially frequently in everyday development:

• Strategy enables interchangeable algorithms and cleaner behavior selection.

• Decorator extends objects dynamically without modifying their original code.

• Observer models publish–subscribe relationships between subjects and many observers.

• Singleton enforces a single instance where shared resources must not be duplicated.

• Facade hides subsystem complexity behind a simple, unified interface.

Understanding these patterns makes it easier to recognize when designs can be improved by changing relationships between objects rather than by adding more conditionals or duplicating logic. Patterns do not replace algorithms, libraries, or frameworks. Instead, they organize how those building blocks fit together in a clean, maintainable structure.

As systems grow in size and complexity, design patterns remain valuable tools for clarifying structure, guiding refactoring, and enabling effective communication among engineers.