These design patterns are frequently applied in software development to improve code reusability, maintainability, and flexibility.
1. Singleton
Ensure a class has only one instance and provide a global point of access to it.
Managing application-wide configurations, logging services.
- Centralized configuration management.
- Database connection pooling.
- Logger instances.
- Print spooler.
2. Factory Method
Define an interface for creating an object, but let subclasses decide which class to instantiate.
Creating different UI button types based on the platform.
Use Cases:
- Creating different types of documents (PDF, HTML).
- Instantiating various database connections (MySQL, PostgreSQL).
- Generating different log file formats.
- Creating platform-specific UI elements.
3. Abstract Factory
Provide an interface for creating families of related or dependent objects.
Designing UI toolkits with consistent sets of widgets for different platforms.
Use Cases:
- Creating consistent UI widget sets (buttons, text fields) for different operating systems.
- Generating different themes for an application.
- Producing families of related database objects.
4. Builder
Separate the construction of a complex object from its representation.
Creating complex objects with many optional parameters, like document builders.
Use Cases:
- Constructing complex query objects with various clauses.
- Building different representations of a document (plain text, HTML).
- Creating user interface components with numerous options.
- Assembling complex data transfer objects (DTOs).
5. Observer
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified.
Event handling mechanisms, model-view-controller (MVC) architectures.
Use Cases:
- Implementing event listeners in UI frameworks.
- Updating views when the underlying data model changes.
- Publishing and subscribing to messages in messaging systems.
- Broadcasting state changes in reactive programming.
6. Strategy
Define a family of algorithms, encapsulate each one, and make them interchangeable.
Implementing different sorting algorithms or payment processing methods.
Use Cases:
- Selecting different sorting algorithms at runtime.
- Implementing various payment gateway integrations.
- Applying different compression algorithms to data.
- Choosing different rendering engines for graphics.
7. Decorator
Attach additional responsibilities to an object dynamically.
Adding logging, caching, or authentication to services.
Use Cases:
- Adding logging, caching, or authentication to existing service implementations.
- Extending the functionality of UI components (e.g., adding borders, scrollbars).
- Adding encryption or compression to data streams.
- Dynamically adding behavior to objects without subclassing.
8. Adapter
Convert the interface of a class into another interface clients expect.
Integrating third-party libraries with incompatible interfaces.
Use Cases:
- Integrating legacy systems with new architectures.
- Using third-party libraries with different APIs.
- Normalizing data from different sources with varying formats.
- Wrapping existing classes to provide a more convenient interface.
9. Facade
Provide a unified interface to a set of interfaces in a subsystem.
Simplifying interaction with complex subsystems like compilers.
Use Cases:
- Providing a simplified interface to a complex framework or library.
- Hiding the complexity of multiple interacting subsystems.
- Creating an entry point to a set of related functionalities.
10. Template Method
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses.
Defining the basic steps of a document processing workflow.
Use Cases:
- Defining the steps for building different types of reports.
- Implementing the lifecycle of an activity or fragment in Android development.
- Creating different database access workflows.
- Defining the structure of a build process.
11. Prototype
Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
Creating copies of complex or expensive objects, like graphical components or configurations.
Use Cases:
- Creating copies of complex UI elements efficiently.
- Implementing undo/redo functionality by cloning object states.
- Generating new objects based on pre-configured templates.
- Creating test data by copying existing sample objects.
12. Composite
Compose objects into tree structures to represent part-whole hierarchies.
Representing file systems where files and directories can be treated the same way.
Use Cases:
- Representing organizational charts.
- Building UI component trees.
- Implementing menu structures.
- Handling hierarchical data in file systems or XML structures.
13. Proxy
Provide a surrogate or placeholder for another object to control access to it.
Implementing lazy loading of expensive objects, controlling access to sensitive resources.
Use Cases:
- Lazy loading of large or rarely used objects.
- Controlling access to resources based on user permissions.
- Implementing remote procedure calls (RPC).
- Caching the results of expensive operations.
14. Command
Encapsulate a request as an object, thereby letting you parameterize clients with different requests.
Implementing undo/redo functionality, menu items as objects.
Use Cases:
- Implementing undo/redo functionality in editors.
- Representing menu items and actions in UI applications.
- Implementing transaction logging and rollback.
- Supporting asynchronous operations and task scheduling.
15. Iterator
Provide a way to access the elements of an aggregate object sequentially.
Traversing collections of objects (lists, sets, trees) in a consistent way.
Use Cases:
- Providing a standard way to iterate over collections in various programming languages.
- Implementing custom iterators for complex data structures.
- Supporting different traversal strategies for the same data structure.
16. Interpreter
Given a language, define a representation for its grammar along with an interpreter.
Implementing a simple query language or evaluating mathematical expressions.
Use Cases:
- Implementing simple scripting languages.
- Evaluating mathematical expressions.
- Parsing and executing domain-specific languages.
- Implementing rule engines.
17. Mediator
Define an object that encapsulates how a set of objects interact.
Managing complex interactions between UI components in a dialog box.
Use Cases:
- Managing interactions between UI components in complex dialogs.
- Coordinating the behavior of multiple participants in a system.
- Implementing chat rooms or message brokers.
18. Memento
Without violating encapsulation, capture and externalize an object’s internal state.
Implementing undo/redo mechanisms or saving and restoring the state of an application.
Use Cases:
- Implementing undo/redo functionality in applications.
- Saving and restoring the state of game objects.
- Implementing transaction rollback in databases.
19. State
Allow an object to alter its behavior when its internal state changes.
Implementing the different states of a finite state machine, like the states of a traffic light.
Use Cases:
- Implementing the different states of a traffic light.
- Managing the different states of an order in an e-commerce system.
- Controlling the behavior of a network connection based on its state.
20. Visitor
Represent an operation to be performed on the elements of an object structure.
Performing different operations on a complex object structure (like a parse tree).
Use Cases:
- Performing different operations on elements of a complex data structure (e.g., traversing a parse tree for code analysis, optimization, or code generation).
- Implementing different rendering algorithms for UI elements.
- Serializing objects in different formats.
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