Mastering Design Patterns
Design patterns are reusable solutions to common software design problems that occur during the development process. They provide guidelines and best practices for designing flexible, maintainable, and scalable software systems. Design patterns are not specific to a particular programming language or technology but can be applied to various software development contexts.
Design patterns can be categorized into three main types:
- Creational Patterns: These patterns focus on object creation mechanisms, providing flexibility and decoupling between objects.
- Structural Patterns: These patterns deal with the composition of classes and objects, focusing on how they are connected and organized.
- Behavioral Patterns: These patterns focus on the interaction and communication between objects, defining how they work together to fulfill a given task.
In this document, we will explore various design patterns by building a “virtual classroom management system” as our single application. By using this single application as an example, we can demonstrate the application of each design pattern and understand how they interrelate.
The virtual classroom management system will allow administrators to manage courses, enroll students, track their progress, and facilitate communication within the classroom. Throughout the development process, we will apply different design patterns to address specific requirements and design challenges.
By using this single application, we can see how each design pattern fits into the overall architecture and how they work together to create a robust and maintainable system. This approach will provide a practical understanding of design patterns and their real-world applications.
Factory
As a requirement, we need to create different types of courses. We can apply the factory pattern to create instances of various course types.
Usage
The Factory pattern is used to create different types of courses in the virtual classroom management system. It provides a central interface, the CourseFactory, which encapsulates the creation logic for various course types.
Benefits
The Factory pattern allows the system to abstract the process of object creation, providing a flexible way to create different types of courses without tightly coupling the client code to concrete implementations. It promotes code maintainability, as adding new course types can be done by extending the factory without modifying existing client code.
Abstract Factory
Next, we have a requirement to create families of related objects for the courses. We can apply the abstract factory pattern to create different course-related objects.
Usage
The Abstract Factory pattern is used to create families of related objects in the virtual classroom management system. The CourseFactory interface is extended to include methods for creating course materials, such as lesson plans and assignments, which are specific to each course type.
Benefits
The Abstract Factory pattern provides a way to create related objects that belong to a particular family. It ensures that the created objects are compatible and consistent across the entire system. This pattern also enables easy scalability and extension of the system by adding new families of objects without modifying existing client code.
Builder
As a requirement, we want to construct complex course objects with multiple optional attributes using a builder pattern.
Usage
The Builder pattern is used to construct complex course objects with multiple optional attributes, such as course duration, instructor details, and prerequisites. The Course class utilizes a nested Builder class to provide a fluent interface for constructing the course objects.
Benefits
The Builder pattern allows the construction of complex objects step by step, providing a clear and readable way to configure optional attributes. It enhances code readability and maintainability by separating the construction process from the object’s representation. The pattern also enables the creation of different variations of objects using the same construction process.
Prototype
As a requirement, we want to create new course objects by cloning existing ones. We can apply the prototype pattern.
Usage
The Prototype pattern is used to create new course objects by cloning existing ones in the virtual classroom management system. The Course class implements the CoursePrototype interface and provides a cloning method to create copies of course objects.
Benefits
The Prototype pattern allows the creation of new objects by cloning existing ones, avoiding the costly creation process. It provides a mechanism to create objects dynamically at runtime while preserving their state. This pattern is useful when the system needs to create many similar objects with different states, improving performance and reducing memory consumption.
Singleton
As a requirement, we want a single instance of a course registry throughout the system. We can apply the singleton pattern.
Usage
The Singleton pattern is used to ensure that only one instance of the CourseRegistry class exists in the virtual classroom management system. The CourseRegistry class uses a private constructor and a static method to provide global access to a single instance.
Benefits
The Singleton pattern ensures that only one instance of a class exists throughout the system, allowing global access to it. In the virtual classroom management system, the singleton pattern can be used to provide a centralized registry for courses, ensuring consistent access and avoiding multiple instances of the registry being created. This pattern is particularly useful when there should be a single point of control or coordination in the system.
Adapter
As a requirement, we need to integrate with a third-party student enrollment service. We can apply the adapter pattern to convert the third-party interface into a common interface used within the system.
Usage
The Adapter pattern is used to integrate with a third-party student enrollment service in the virtual classroom management system. The ThirdPartyEnrollmentServiceAdapter class acts as an adapter between the system's EnrollmentService interface and the third-party service's interface.
Benefits
The Adapter pattern allows two incompatible interfaces to work together by providing a wrapper or adapter that converts the interface of one class into another interface that clients expect. In the virtual classroom management system, the adapter enables seamless integration with the third-party enrollment service without modifying the existing system’s code, promoting interoperability and maintainability.
Bridge
As a requirement, we want to separate the abstraction of courses from their implementation. We can apply the bridge pattern.
Usage
The Bridge pattern is used to decouple the abstraction of courses from their implementation in the virtual classroom management system. The Course class acts as the abstraction and the CourseType the hierarchy represents the implementation hierarchy.
Benefits
The Bridge pattern separates the abstraction and implementation, allowing them to vary independently. It provides flexibility by enabling the system to choose different course types and change their behavior dynamically. In the virtual classroom management system, the Bridge pattern allows the system to handle different course types and their specific operations while keeping the core logic of the courses decoupled from the implementation details.
Composite
As a requirement, we want to represent the virtual classroom with a hierarchical structure. We can apply the composite pattern.
Usage
The Composite pattern is used to represent the structure of the virtual classroom, where a classroom can contain both individual students and subgroups of students. The ClassroomComponent interface is implemented by both the Student class and the Classroom class, allowing them to be treated uniformly.
Benefits
The Composite pattern enables the creation of tree-like structures and treats individual objects and compositions uniformly. It provides a recursive mechanism to traverse and perform operations on the elements of the structure. In the virtual classroom management system, the Composite pattern allows treating individual students and classrooms as a single entity, simplifying operations that need to be performed in the classroom as a whole.
Decorator
As a requirement, we want to add additional functionality to courses dynamically. We can apply the decorator pattern.
Usage
The Decorator pattern is used to dynamically add additional responsibilities or behaviors to the courses in the virtual classroom management system. The CourseDecorator class wraps the Course objects and extends their functionality without modifying their original implementation.
Benefits
The Decorator pattern allows the addition of new functionalities to objects dynamically. It promotes flexibility by providing a flexible alternative to subclassing for extending functionality. In the virtual classroom management system, the Decorator pattern can be used to add features such as certificates, additional materials, or interactive components to courses without modifying their base implementations, enhancing the system’s extensibility.
Facade
As a requirement, we want to provide a simplified interface for managing various components of the virtual classroom. We can apply the facade pattern.
Usage
The Facade pattern is used to provide a simplified interface for managing various components of the virtual classroom management system, such as courses, students, and classrooms. The VirtualClassroomFacade class acts as a unified interface that encapsulates the complexities of the underlying subsystems.
Benefits
The Facade pattern provides a high-level, simplified interface to a complex subsystem, making it easier to use and understand. It promotes loose coupling by decoupling the client code from the complexities of the subsystem. In the virtual classroom management system, the Facade pattern can be used to provide a straightforward and unified interface for managing different components, simplifying interactions, and reducing dependencies.
Flyweight
As a requirement, we want to optimize memory usage by sharing common properties among multiple students. We can apply the flyweight pattern.
Usage
The Flyweight pattern is used to optimize memory usage by sharing common properties among multiple students in the virtual classroom management system. The StudentFactory class maintains a pool of shared student objects based on specific attributes.
Benefits
The Flyweight pattern allows the sharing of common, immutable parts of objects to conserve memory. It reduces the overall memory footprint and improves performance when dealing with large numbers of similar objects. In the virtual classroom management system, the Flyweight pattern can be used to avoid creating separate student objects for each occurrence and instead reuse existing objects with shared attributes, optimizing memory usage.
Proxy
As a requirement, we want to control access to student objects and add additional behavior. We can apply the proxy pattern.
Usage
The Proxy pattern is used to control access to student objects and add additional behavior in the virtual classroom management system. The StudentProxy class acts as a proxy for the RealStudent class, intercepting requests, and performing additional operations before or after delegating to the real student.
Benefits
The Proxy pattern provides a surrogate or placeholder for another object, allowing control over its access and adding extra functionality. It allows for the implementation of additional logic, such as lazy loading, caching, or access control, without modifying the original object. In the virtual classroom management system, the Proxy pattern can be used to add access control, logging, or other cross-cutting concerns to student objects, enhancing security and functionality.
Chain of Responsibility
As a requirement, we want to handle various actions or requests in the virtual classroom using a chain of handlers. We can apply the chain of responsibility pattern.
Usage
The Chain of Responsibility pattern is used to handle various actions or requests in the virtual classroom management system. Each handler in the chain has the opportunity to handle the request or pass it to the next handler in the chain.
Benefits
The Chain of Responsibility pattern promotes loose coupling between senders and receivers of requests. It allows multiple objects to handle a request without explicitly specifying the receiver, providing flexibility and extensibility. In the virtual classroom management system, the Chain of Responsibility pattern can be used to handle different actions, such as attendance tracking, homework submission, or grading, in a modular and customizable manner.
Command
As a requirement, we want to encapsulate classroom actions as objects, allowing us to parameterize and queue actions. We can apply the command pattern.
Usage
The Command pattern is used to encapsulate requests as objects in the virtual classroom management system. It decouples the sender of the request from the receiver, allowing parameterization and queuing of requests.
In the usage section, we create an Classroom object and an ClassroomActionInvoker object. We then create instances of JoinClassroomAction and LeaveClassroomAction and enqueue them in the invoker's action queue. Finally, we execute the actions by calling executeActions() on the invoker, which invokes the execute() method for each action, causing the corresponding classroom actions to be performed.
Benefits
The Command pattern provides a way to encapsulate a request as an object, allowing for parameterization, queuing, and execution at different times. It promotes loose coupling between the sender and receiver of a request, enabling extensibility and flexibility. In the virtual classroom management system, the Command pattern can be used to encapsulate operations such as participating in class, submitting assignments, or generating reports as command objects, making them easy to manage and execute.
Mediator
As a requirement, we want to facilitate communication between students in a classroom without direct coupling. We can apply the mediator pattern.
Usage
The Mediator pattern is used to facilitate communication between students in the virtual classroom management system. The ClassroomMediator acts as a central point of communication, encapsulating the interaction between students without direct coupling.
Benefits
The Mediator pattern enables loose coupling between objects by promoting indirect communication through a mediator. It simplifies complex communication patterns and reduces dependencies between objects. In the virtual classroom management system, the Mediator pattern allows students to communicate with each other without knowing the details of other students, promoting decoupling and modularity.
Memento
As a requirement, we want to save and restore the state of a student’s progress in the classroom. We can apply the memento pattern.
Usage
The Momento pattern is used to save and restore the state of a student’s progress in the virtual classroom management system. The Student class provides methods to capture and restore the internal state using a StudentMemento object.
Benefits
The Momento pattern allows the capture and restoration of an object’s internal state without violating encapsulation. It provides a way to save and revert to previous states, supporting undo/redo functionality and maintaining history. In the virtual classroom management system, the Momento pattern can be used to save and restore a student’s progress or activity, allowing them to resume from a previous state if needed.
Observer
As a requirement, we want to notify students about changes in the classroom. We can apply the observer pattern.
Usage
The Observer pattern is used to establish a one-to-many dependency between objects in the virtual classroom management system. The students act as observers, and the classroom acts as the subject, notifying the observers of any changes.
Benefits
The Observer pattern provides a way to establish loose coupling between objects, ensuring that changes in one object trigger updates in multiple dependent objects. It promotes modularity, flexibility, and maintainability. In the virtual classroom management system, the Observer pattern allows students to receive notifications about changes in the classroom, such as schedule updates or announcements.
State
As a requirement, we want to manage the behavior of students based on their current state (e.g., active, suspended). We can apply the state pattern.
Usage
The State pattern is used to manage the behavior of students based on their current state (e.g., active, suspended) in the virtual classroom management system. Each student can transition between different states and perform actions accordingly.
Benefits
The State pattern allows an object to alter its behavior when its internal state changes. It avoids conditional statements and promotes encapsulation of state-specific behavior. In the virtual classroom management system, the State pattern can be used to manage the behavior of students based on their enrollment status, such as allowing or restricting certain actions depending on their state (e.g., active, suspended, graduated).
Strategy
As a requirement, we want to provide different grading strategies for different types of assessments. We can apply the strategy pattern.
Usage
The Strategy pattern is used to provide different grading strategies for different types of assessments in the virtual classroom management system. The GradingStrategy interface defines the contract for grading, and different strategies can be implemented and assigned to assessments.
Benefits
The Strategy pattern allows the selection of an algorithm at runtime, providing flexibility and extensibility. It separates the algorithm implementation from the client code, enabling easy swapping or the addition of new strategies. In the virtual classroom management system, the Strategy pattern allows different grading strategies, such as pass/fail or numeric grading, to be applied to various assessments, providing flexibility and customization.
Template
As a requirement, we want to provide a template for creating a study plan for each student. We can apply the template pattern.
Usage
The Template pattern is used to define a skeleton of an algorithm in the virtual classroom management system. The StudyPlanTemplate provides a framework for creating study plans for students, with customizable steps for each specific plan.
Benefits
The Template pattern provides a way to define an algorithm’s structure while allowing subclasses to override specific steps. It promotes code reuse, consistency, and modularity. In the virtual classroom management system, the Template pattern can be used to define a common study plan structure and customize it for each student, ensuring a consistent approach while allowing personalization.
Visitor
As a requirement, we want to perform different operations on students without modifying their classes. We can apply the visitor pattern.
Usage
The Visitor pattern is used to perform different operations on students without modifying their classes in the virtual classroom management system. The StudentVisitor interface defines the operations, and each operation is implemented by a concrete visitor class.
Benefits
The Visitor pattern separates operations from the objects they operate on, allowing new operations to be added without modifying the object structure. It promotes extensibility and encapsulates related operations into separate visitor classes. In the virtual classroom management system, the Visitor pattern can be used to perform operations on students, such as calculating
Summary
- Creational Patterns: These patterns focus on object creation mechanisms, providing flexibility and decoupling between objects. Creational patterns include:
- Factory: Provides a central interface for creating objects, hiding the creation logic from the client.
- Abstract Factory: Provides an interface for creating families of related or dependent objects.
- Builder: Separates the construction of complex objects from their representation, allowing the same construction process to create different representations.
- Prototype: Allows the creation of new objects by cloning existing ones.
- Singleton: Ensures that only one instance of a class exists and provides a global access point to it.
2. Structural Patterns: These patterns deal with the composition of classes and objects, focusing on how they are connected and organized. Structural patterns include:
- Adapter: Converts the interface of a class into another interface that clients expect.
- Bridge: Decouples an abstraction from its implementation, allowing them to vary independently.
- Composite: Represents objects in a tree structure, allowing clients to treat individual objects and compositions uniformly.
- Decorator: Dynamically adds responsibilities to an object by wrapping it with one or more decorators.
- Facade: Provides a unified interface to a set of interfaces in a subsystem, simplifying complex operations.
- Flyweight: Shares objects to support large numbers of fine-grained objects efficiently.
- Proxy: Provides a surrogate or placeholder for another object to control its access or add additional behavior.
3. Behavioral Patterns: These patterns focus on the interaction and communication between objects, defining how they work together to fulfill a given task. Behavioral patterns include:
- Chain of Responsibility: Allows an object to pass a request along a chain of potential handlers until the request is handled.
- Command: Encapsulates a request as an object, allowing clients to parameterize and queue requests.
- Iterator: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- Mediator: Defines an object that encapsulates the interaction between a set of objects, allowing them to communicate without direct references.
- Memento: Captures and restores an object’s internal state without violating encapsulation, allowing it to be reverted to a previous state.
- Observer: Defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically.
- State: Allows an object to alter its behavior when its internal state changes, resulting in different actions based on the state.
- Strategy: Enables the selection of an algorithm at runtime, encapsulating it in a separate class, and allowing it to be changed or extended independently.
- Template: Defines the skeleton of an algorithm in a base class, allowing subclasses to redefine certain steps of the algorithm without changing its structure.
- Visitor: Separates algorithms from the objects on which they operate, allowing new operations to be added to existing objects without modifying their classes.
