The Singleton pattern and the Factory pattern are both creational design patterns but serve different purposes.
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The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. The primary intent of the Singleton pattern is to restrict the instantiation of a class to a single object, which can be useful in scenarios where having multiple instances would cause issues or is unnecessary. The Singleton pattern typically involves the following elements:
- A private constructor to prevent direct instantiation of the class.
- A static method or property that provides access to a single instance of the class.
- A static variable that holds the single instance.
Example: A logger class that needs to have only one instance throughout the application.
The Factory pattern provides an interface for creating objects but delegates the actual creation logic to subclasses or methods. It encapsulates the object creation process and provides flexibility in creating different types of objects without tightly coupling the client code to the specific classes. The Factory pattern typically involves the following elements:
- An abstract or interface-based factory class that declares a method for creating objects.
- Concrete factory classes that implement the factory method to create specific types of objects.
- Concrete product classes that represent the objects being created.
Example: Consider a scenario where you have different types of shapes (e.g., Circle, Square, Triangle), and based on user input, you want to create the corresponding shape object. You can have a ShapeFactory that encapsulates the creation logic and returns the appropriate shape object based on user input.
In summary, the Singleton pattern ensures that only one instance of a class exists, while the Factory pattern provides a way to create objects without exposing the creation logic to the client code.