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Java design patterns are typically categorized into three main types: Creational, Structural, and Behavioral patterns. Each category serves a different purpose in software design and architecture. Here’s a brief overview of each category:
1. Creational Patterns
Creational patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. They help in controlling the instantiation process and can make the system more flexible and reusable. Common creational patterns include:
- Singleton: Ensures a class has only one instance and provides a global point of access to it.
- Factory Method: Defines an interface for creating an object but allows subclasses to alter the type of objects that will be created.
- Abstract Factory: Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
- Builder: Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
- Prototype: Creates new objects by copying an existing object, known as the prototype.
2. 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. Common structural patterns include:
- Adapter: Allows incompatible interfaces to work together by converting the interface of a class into another interface that a client expects.
- Bridge: Separates an object’s interface from its implementation, allowing the two to vary independently.
- Composite: Composes objects into tree structures to represent part-whole hierarchies, allowing clients to treat individual objects and compositions uniformly.
- Decorator: Adds new functionality to an object dynamically without altering its structure.
- Facade: Provides a simplified interface to a complex subsystem, making it easier to use.
- Flyweight: Reduces the cost of creating and manipulating a large number of similar objects by sharing common parts.
- Proxy: Provides a surrogate or placeholder for another object to control access to it.
3. Behavioral Patterns
Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. They help in defining how objects interact in a system. Common behavioral patterns include:
- Chain of Responsibility: Passes a request along a chain of handlers, allowing multiple objects to handle the request without the sender needing to know which object will handle it.
- Command: Encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations.
- Interpreter: Defines a representation for a language’s grammar and provides an interpreter to deal with the grammar.
- Iterator: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- Mediator: Defines an object that encapsulates how a set of objects interact, promoting loose coupling.
- Memento: Captures and externalizes an object’s internal state without violating encapsulation, allowing the object to be restored to this state later.
- 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, appearing to change its class.
- Strategy: Defines a family of algorithms, encapsulates each one, and makes them interchangeable, allowing the algorithm to vary independently from clients that use it.
- Template Method: Defines the skeleton of an algorithm in a method, deferring some steps to subclasses.
- Visitor: Represents an operation to be performed on the elements of an object structure, allowing you to define new operations without changing the classes of the elements.
These design patterns provide proven solutions to common problems in software design, promoting code reusability, maintainability, and scalability.