GRASP (General Responsibility Assignment Software Principles) patterns are a set of principles aimed at guiding the assignment of responsibilities to classes and objects in object-oriented design. These patterns help developers create flexible, maintainable, and cohesive software systems. Below is an overview of the key GRASP patterns:

1. Information Expert

• Definition: Assign responsibility to the class that has the necessary information to fulfill it.
• Purpose: Promotes low coupling and high cohesion by ensuring that methods are allocated to classes that have the necessary data.
• Example: A Customer class should handle tasks related to customer data, such as validating a customer’s credit because it owns that data.

2. Creator

• Definition: Assign the responsibility of creating an instance of a class to a class that aggregates, contains, or closely uses the instance.
• Purpose: Helps reduce dependencies among classes and fosters more intuitive designs by ensuring that classes responsible for an object also create it.
• Example: A ShoppingCart class should create CartItem instances, as it contains and manages cart items.

3. Controller

• Definition: Assign the responsibility of handling system events to a controller class that represents the overall system or a use case scenario.
• Purpose: Delegates responsibility from user interface components to a dedicated controller, facilitating the separation of concerns.
• Example: In a GUI application, the MainController might handle events like user input, delegating functionality to model and view components.

4. Low Coupling

• Definition: Aim to reduce dependencies between classes, promoting loose coupling.
• Purpose: Ensures that changes in one class minimally affect others, enhancing maintainability and flexibility.
• Example: Using interfaces to allow a PaymentProcessor to switch between different payment methods without modifying the underlying logic.

5. High Cohesion

• Definition: Assign related responsibilities to a single class to enhance the cohesion of that class.
• Purpose: Makes classes easier to understand and maintain by focusing on a single set of related tasks.
• Example: A ReportGenerator class should exclusively handle tasks related to report generation, avoiding unrelated functionalities.

6. Polymorphism

• Definition: Use polymorphic methods to handle variation in behavior, often via interfaces or abstract classes.
• Purpose: Reduces the need for conditionals in the code and simplifies class design by leveraging the benefits of polymorphism.
• Example: A Shape interface implemented by Circle and Square classes may have a method draw(), allowing for polymorphic invocation for different shapes.

7. Pure Fabrication

• Definition: Create a class that does not represent a concept in the problem domain to achieve low coupling, high cohesion, or reuse.
• Purpose: Helps maintain a clean design by avoiding an unnecessary dependence on a real-world concept, often leading to a cleaner architecture.
• Example: A NotificationService class might handle email and SMS notifications without being tied directly to the domain model.

8. Indirection

• Definition: Assign responsibilities in a way that avoids direct coupling between classes by introducing an intermediate class.
• Purpose: Reduces the dependencies between classes and aids in decoupling.
• Example: Using a service layer as an intermediary between data access and business logic components.

9. Protected Variance

• Definition: Allow subclasses to alter behavior while keeping their interfaces stable by using encapsulation.
• Purpose: Enables safe variations in behavior through inheritance while maintaining the client interface.
• Example: An abstract Dialog class with a method createButton() can be overridden in subclasses to produce various types of buttons.

Summary

The GRASP principles provide a foundation for designing robust, maintainable, and scalable object-oriented systems. They encourage assigning responsibilities thoughtfully, balancing between cohesion and coupling, and utilizing polymorphism effectively. Applying these principles can lead to high-quality software designs that are easier to manage and evolve over time.