A design pattern is a general, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into code but rather a description or template for how to solve a problem that can be used in many different situations. Design patterns help to speed up the development process by providing tested, proven development paradigms.
Key points about design patterns:
- Reusability: They promote code reuse and reduce redundancy.
- Best Practices: They encapsulate best practices and the experiences of developers.
- Efficiency: They help in writing well-structured and maintainable code, leading to better performance and easier debugging.
1. Creational Design Patterns: These patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.
| S.No | Pattern | Problem | Solution |
|---|---|---|---|
| 1 | Singleton | Need only one instance of a class (e.g., config, logging). | Ensures only one instance exists and provides global access to it. |
| 2 | Factory Method | Don't want to specify the exact class to instantiate. | Defines a method for creating objects in a subclass, leaving the choice of which object to instantiate to the subclass. |
| 3 | Abstract Factory | Need families of related objects but want to switch between them easily. | Provides an interface to create related objects without knowing their concrete classes. |
| 4 | Builder | Object construction is complex with many optional parameters. | Separates object construction from its representation, making it easy to create complex objects step by step. |
| 5 | Prototype | Creating objects is costly (e.g., large or complex objects). | Creates new objects by copying existing ones, reducing the cost of creation. |
2. Structural Design Patterns: These patterns deal with object composition or how classes and objects can be composed to form larger structures.
| S.No | Pattern | Problem | Solution |
|---|---|---|---|
| 1 | Adapter | Two incompatible interfaces need to work together. | Converts one interface into another so incompatible classes can work together. |
| 2 | Bridge | Tight coupling between abstraction and implementation. | Decouples abstraction from implementation, allowing both to vary independently. |
| 3 | Composite | Need to treat individual objects and groups uniformly (e.g., file systems). | Composes objects into tree structures to represent part-whole hierarchies, treating both individual and composite objects uniformly. |
| 4 | Decorator | Need to add functionality dynamically without modifying the original code. | Wraps the object with additional behavior dynamically while keeping the original object's interface intact. |
| 5 | Facade | Complex subsystems with too many interfaces make usage hard. | Provides a simplified interface to a larger body of code, making it easier to use. |
| 6 | Flyweight | Many similar objects consume too much memory (e.g., rendering text). | Shares common parts of objects to save memory while maintaining the unique parts. |
| 7 | Proxy | Need controlled access to an object (e.g., lazy loading, security). | Provides a placeholder for another object to control access, reduce cost, or add functionality. |
3. Behavioral Design Patterns: These patterns deal with object interaction and responsibility. They help in defining communication between objects and the flow of control in a program.
| S.No | Pattern | Problem | Solution |
|---|---|---|---|
| 1 | Strategy | Need to switch between different algorithms easily. | Encapsulates algorithms in separate classes, allowing them to be swapped at runtime without altering the client code. |
| 2 | Observer | Want to notify multiple objects when one object changes its state. | Allows an object (subject) to notify a group of observer objects automatically when its state changes. |
| 3 | Command | Need to encapsulate requests as objects to handle undo, logging, etc. | Encapsulates a request as an object, allowing parameterization, queuing, and undoable operations. |
| 4 | Chain of Responsibility | Multiple objects could handle a request, but which one should? | Passes the request along a chain of handlers, allowing each handler to process or pass it further. |
| 5 | Mediator | Too many direct interactions between objects lead to tight coupling. | Introduces a mediator to centralize communication between objects, reducing coupling. |
| 6 | Memento | Need to restore an object to a previous state (e.g., undo functionality). | Captures the object’s state in a memento so it can be restored later. |
| 7 | State | Object behavior changes based on its state. | Encapsulates state-specific behavior into separate classes, allowing the object to change behavior at runtime by changing its state. |
| 8 | Template Method | Common algorithm structure, but steps may vary. | Defines the skeleton of an algorithm in a base class, allowing subclasses to define or override specific steps. |
| 9 | Visitor | Need to perform new operations on a set of objects without modifying them. | Separates an algorithm from the object structure, allowing new operations to be added without changing the objects. |
| 10 | Interpreter | Need to interpret or evaluate language grammar (e.g., SQL, regex). | Defines a grammar and provides a way to evaluate sentences in that grammar, encapsulating the logic for interpreting expressions. |
Each design pattern provides a solution to a problem in a specific context and can be adapted to fit the needs of the situation at hand.