What Are Design Patterns? The 10 Most Commonly Used Patterns

Design patterns are reusable solution approaches to common design problems encountered during software development. They help make code more understandable and maintainable. So, what is a design pattern, what types are there, and how do the 10 most commonly used design patterns work?

What Is a Design Pattern?

A design pattern is a general design approach that provides solutions to recurring problems in software development. It is not tied to a specific programming language or a ready-made piece of code. Instead, it describes how classes, objects, and components can be organized.

Design patterns are frequently used in object-oriented programming. For example, different patterns can help manage various notification types, control the object creation process, or reduce dependencies between components in an application.

The main purpose of using design patterns is to benefit from proven approaches rather than designing a solution from scratch for every problem. However, not every design pattern is suitable for every project. Choosing the right pattern requires evaluating the problem's requirements and the application's complexity.

What Are the Types of Design Patterns?

In the approach known as Gang of Four (GoF), design patterns are divided into three main categories. This classification makes it easier to understand the types of design problems each pattern addresses.

• Creational patterns: Organize how objects are created. Singleton and Factory Method are examples.
• Structural patterns: Focus on how classes and objects are combined. Adapter and Decorator are examples.
• Behavioral patterns: Organize how objects communicate with one another and distribute responsibilities. Observer and Strategy belong to this category.

These three categories provide a good starting point for learning design patterns. Let's explore 10 design patterns commonly encountered in software projects and their use cases.

What Are the 10 Most Commonly Used Design Patterns?

1. What Is the Singleton Pattern?

Singleton is a design pattern that aims to ensure that only one instance of a class is created within an application and provides a shared access point to that instance. It can be used when a particular resource needs to be managed through a single object.

For example, consider an object responsible for managing an application's configuration settings. The Singleton approach may be useful when different components need to access the same configuration object.

However, Singleton should be used carefully because it can introduce global state and make testing more difficult. If a single instance is not genuinely necessary, injecting dependencies from outside the class may offer greater flexibility.

2. What Is the Factory Method Pattern?

Factory Method is a design pattern that manages object creation through a method rather than directly tying the process to a specific class. This separates the decision about which concrete class to instantiate from the code that uses the creation mechanism.

For example, a notification application may support email and SMS notifications. Using a creation method instead of having different parts of the application instantiate specific notification classes directly can make it easier to introduce new notification types.

Factory Method is particularly useful in applications where object creation decisions may change and different product types need to be managed.

3. What Is the Abstract Factory Pattern?

Abstract Factory is a design pattern that allows related groups of objects to be created without depending directly on their concrete classes. Unlike Factory Method, which focuses on creating individual objects, Abstract Factory focuses on creating families of compatible objects.

For example, imagine a desktop application that uses different interface components for Windows and macOS. Abstract Factory can ensure that compatible button and window components are created together for each operating system.

This approach helps maintain consistency between objects when adding support for different platforms or product families.

4. What Is the Builder Pattern?

Builder is a design pattern that allows complex objects with many properties to be constructed step by step. It can help avoid long, difficult-to-read constructors, especially when objects have numerous optional parameters.

For example, a report object may include different options such as a title, date range, filters, and output format. With the Builder approach, these properties can be configured in specific steps, making object creation more readable.

Builder is useful when the object creation process involves multiple steps. For simple objects, however, it may introduce unnecessary complexity.

5. What Is the Adapter Pattern?

Adapter is a design pattern that allows two incompatible interfaces to work together. It can be used when modifying an existing class is not possible or when an external system's interface does not match the application's expectations.

For example, an application may expect a particular payment service to provide a pay() method. If the external service uses a different method name and parameter structure, an Adapter can act as a translator between the two interfaces.

This makes it possible to integrate different systems without rewriting the existing business logic from scratch.

6. What Is the Decorator Pattern?

Decorator is a design pattern that adds responsibilities to an object without changing its existing behavior or rewriting its class. This approach allows new features to be added to an object dynamically when needed.

For example, a text-editing application may allow basic text to be enhanced with bold formatting, italics, or other visual features. The Decorator approach helps manage these features as separate components instead of combining them in one large class.

This pattern provides flexibility, particularly in systems where different features can be combined in various ways.

7. What Is the Facade Pattern?

Facade provides a simpler and more understandable interface for accessing the functionality of a complex subsystem. The client code does not need to manage numerous classes or operations within the subsystem individually.

For example, a video conversion application may involve several steps, including file reading, format decoding, audio processing, and output generation. Facade can allow these operations to be initiated through a single convertVideo() method.

Facade simplifies the use of complex systems. However, it should be used to streamline only the necessary operations rather than combining every subsystem feature into a single point of access.

8. What Is the Observer Pattern?

Observer is a behavioral design pattern that notifies other objects when an object's state changes. This creates a more flexible relationship between the object managing the state and the components receiving updates.

For example, in an e-commerce application, a change in an order's status may require notifying the customer and updating the administration panel. With the Observer approach, these components can listen for changes in the order status and perform their respective actions.

This pattern can be used in notification systems, event management, and user interfaces. However, when many observers are involved, it is important to monitor and manage the notification flow.

9. What Is the Strategy Pattern?

Strategy allows different algorithms that perform the same task to be defined as separate components. This separates the selection of an algorithm from the main code that uses it.

For example, an e-commerce application may calculate shipping costs based on standard delivery, express delivery, or free shipping options. Each calculation method can be defined as a separate strategy, allowing the appropriate method to be selected as needed.

Strategy makes it easier to manage changing business rules. When a new calculation method is introduced, a new strategy can be created instead of modifying the main workflow.

10. What Is the Command Pattern?

Command is a design pattern that encapsulates the information required to perform an operation within a separate object. This separates the component requesting an operation from the component executing it.

For example, in a text editor, copy, paste, and undo operations can be modeled as separate commands. Each command defines its own behavior and is executed by the appropriate component of the application.

Command can be used in task queues, job scheduling, operation history, and undo mechanisms. It is particularly useful in applications where commands need to be executed later or recorded for future reference.

What Should You Consider When Choosing a Design Pattern?

Using a design pattern simply because it is popular can make code unnecessarily complex. First, identify the problem you are trying to solve and evaluate whether the pattern genuinely addresses that problem.

Consider the following factors when choosing a design pattern:

• Type of problem: Identify the specific need, such as object creation, component composition, or communication between objects.
• Existing code structure: Take the project's architecture and technologies into account.
• Need for extensibility: Consider whether new features or alternative behaviors may need to be added in the future.
• Readability: The chosen pattern should make the code easier to understand.
• Testability: Components should be easy to test independently, and their dependencies should be manageable.

Design patterns do not replace software architecture. Choosing the right pattern for the right problem—and recognizing when no pattern is necessary—is also an important part of good software design.