Software Design Patterns Cheat Sheet

1. Introduction to Software Design Patterns
Software design patterns are essential solutions to common problems encountered in software design. They provide a template for solving issues in a way that is reusable and flexible. Understanding these patterns can significantly improve code quality and facilitate better communication among developers. In this cheat sheet, we will explore the most widely used design patterns, their classifications, and examples.

2. Creational Patterns
Creational patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. Here are the key creational patterns:

2.1 Singleton Pattern
The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. This is useful when exactly one object is needed to coordinate actions across the system.

Example: A configuration manager class that reads configuration settings from a file. You only want one instance of this class to ensure consistent configuration across the application.

2.2 Factory Method Pattern
The Factory Method pattern provides an interface for creating objects but allows subclasses to alter the type of objects that will be created. This pattern is particularly useful when a class cannot anticipate the type of objects it needs to create.

Example: A document creation application where the document type (e.g., PDF, Word) is determined at runtime.

2.3 Abstract Factory Pattern
The Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes. This pattern is useful when the system needs to be independent of how its products are created, composed, and represented.

Example: A UI toolkit that supports multiple look-and-feel styles (e.g., Windows, Mac). The toolkit uses an abstract factory to create UI components that match the style.

2.4 Builder Pattern
The Builder pattern separates the construction of a complex object from its representation, allowing the same construction process to create different representations. This pattern is ideal for constructing complex objects step by step.

Example: A meal preparation process where you can create different types of meals (e.g., vegetarian, non-vegetarian) using the same preparation steps.

2.5 Prototype Pattern
The Prototype pattern creates new objects by copying an existing object, known as the prototype. This pattern is useful when the cost of creating a new instance of an object is more expensive than copying an existing one.

Example: An application that needs to clone complex objects with many attributes, such as a graphics editor that allows duplicating shapes with specific properties.

3. Structural Patterns
Structural patterns focus on how classes and objects are composed to form larger structures. They help ensure that if one part of a system changes, the entire system doesn’t need to change. Here are some essential structural patterns:

3.1 Adapter Pattern
The Adapter pattern allows incompatible interfaces to work together. It acts as a bridge between two incompatible interfaces, making them compatible.

Example: A legacy system that needs to interact with a new system. An adapter can convert the old system’s interface to the new one.

3.2 Composite Pattern
The Composite pattern lets you compose objects into tree structures to represent part-whole hierarchies. This pattern treats individual objects and compositions of objects uniformly.

Example: A graphic design application where you can group shapes (e.g., circles, squares) and treat them as a single unit.

3.3 Decorator Pattern
The Decorator pattern adds new functionality to an object dynamically without altering its structure. It is used to extend the behavior of individual objects.

Example: Adding scrollbars to a window dynamically at runtime, where the base window object remains unchanged.

3.4 Facade Pattern
The Facade pattern provides a simplified interface to a complex subsystem. It helps to reduce the complexity of interacting with the subsystem by providing a unified interface.

Example: A home theater system with multiple components (e.g., DVD player, projector, speakers). A facade can provide a simple interface to control the entire system.

3.5 Proxy Pattern
The Proxy pattern provides a surrogate or placeholder for another object to control access to it. This pattern can be used to implement lazy initialization, access control, and logging.

Example: A virtual proxy for loading a large image only when it is needed rather than at the start.

4. Behavioral Patterns
Behavioral patterns focus on communication between objects, what goes on between objects and how they operate together. Here are some key behavioral patterns:

4.1 Chain of Responsibility Pattern
The Chain of Responsibility pattern allows a request to be passed along a chain of handlers. Each handler can either process the request or pass it to the next handler in the chain.

Example: An approval system where requests for approval are passed through different levels of management until one approves or rejects it.

4.2 Command Pattern
The Command pattern encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations. It also supports undoable operations.

Example: A remote control system where different commands (e.g., turn on TV, change channel) are encapsulated as command objects.

4.3 Interpreter Pattern
The Interpreter pattern defines a grammar for a language and provides an interpreter to interpret sentences in that language. It is used for designing languages or expressions.

Example: A simple arithmetic expression evaluator where expressions like “2 + 3” are parsed and evaluated.

4.4 Iterator Pattern
The Iterator pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Example: Iterating over elements of a collection such as a list or set.

4.5 Mediator Pattern
The Mediator pattern defines an object that encapsulates how a set of objects interact. This pattern promotes loose coupling by preventing objects from referring to each other explicitly.

Example: A chat room where users communicate through a mediator rather than directly with each other.

4.6 Memento Pattern
The Memento pattern captures and externalizes an object’s internal state without violating encapsulation, allowing the object to be restored to that state later.

Example: A text editor with undo functionality that allows reverting to previous states of the document.

4.7 Observer Pattern
The Observer pattern defines a dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Example: A stock market application where multiple clients are notified of changes in stock prices.

4.8 State Pattern
The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example: A state machine for a vending machine where its behavior changes based on whether it is in the idle, accepting money, or dispensing product state.

4.9 Strategy Pattern
The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. The algorithm can vary independently from the clients that use it.

Example: A sorting algorithm that can be switched between quicksort and mergesort based on performance needs.

4.10 Template Method Pattern
The Template Method pattern defines the skeleton of an algorithm in a base class but lets subclasses redefine certain steps of the algorithm without changing its structure.

Example: A data processing pipeline where the core processing steps are defined in a base class, but specific details can be customized by subclasses.

4.11 Visitor Pattern
The Visitor pattern represents an operation to be performed on elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example: A tax calculation system where the operation of calculating tax is separated from the object structure of the items being taxed.

5. Conclusion
Design patterns are valuable tools that help developers create more flexible and maintainable software. By understanding and applying these patterns, developers can tackle common design challenges and improve the robustness of their applications. This cheat sheet provides a quick reference to some of the most essential patterns, helping to guide design decisions and enhance overall software architecture.

6. References

• Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides
• Head First Design Patterns by Eric Freeman and Bert Bates
• Design Patterns Explained: A New Perspective on Object-Oriented Design by Alan Shalloway and James R. Trott