tldr

Pattern	Strong Points	Weaknesses	When to Use	Problem Solved	Comparison
Adapter	- Allows incompatible interfaces to work together. - Improves code reusability. - Provides a way to integrate third-party classes.	- Can introduce complexity if overused. - May make the code harder to maintain.	- When you need to adapt an existing class/interface to another one. - When integrating third-party libraries or legacy code.	- Solves the problem of incompatible interfaces by converting one interface to another.	- Unlike the Facade pattern (which simplifies interaction), Adapter transforms one interface into another.
Builder	- Separates the construction process from the final product. - Makes the code more readable and maintainable. - Can be used for complex objects with multiple configuration options.	- Can lead to excessive class creation if there are many variants. - Overuse may make code unnecessarily complex.	- When creating complex objects that require step-by-step construction. - When object creation involves multiple optional configurations.	- Solves the problem of constructing complex objects without exposing the construction logic.	- In contrast to Factory Method (which creates objects in one go), Builder breaks the creation process into distinct, manageable steps.
Decorator	- Adds functionality dynamically without altering the original object. - Supports the Open/Closed Principle (open for extension, closed for modification). - More flexible than subclassing.	- Can result in an excessive number of small classes, complicating maintenance. - Might introduce complexity if used extensively.	- When you need to add responsibilities or features to objects without modifying their code. - When you want to avoid subclassing and prefer flexible runtime behavior changes.	- Solves the problem of extending an object’s behavior dynamically, without affecting other objects.	- Unlike Strategy (which changes behavior by swapping algorithms), Decorator adds functionality to objects.
Facade	- Simplifies interactions with complex subsystems. - Provides a unified interface to a set of interfaces. - Reduces dependencies between client and complex subsystem.	- May obscure necessary details, making debugging harder. - Can hide flexibility of the subsystem behind a simple facade.	- When you need to provide a simplified interface to a complex system. - When you want to reduce the complexity of interacting with multiple subsystems.	- Solves the problem of complexity in a system by providing a simplified, high-level interface.	- Facade hides internal subsystem complexity, while Adapter allows compatibility between incompatible interfaces.
Factory Method	- Promotes loose coupling between client and product classes. - Encapsulates object creation logic. - Makes the code more flexible by deferring object instantiation to subclasses.	- Can result in a proliferation of factory classes. - May lead to excessive indirection.	- When you need to instantiate different types of objects, but want to defer instantiation to subclasses. - When object creation needs to be abstracted for flexibility.	- Solves the problem of object creation, allowing subclasses to decide which class to instantiate.	- Unlike Abstract Factory (which creates families of related products), Factory Method creates objects of one class hierarchy.
Observer	- Facilitates a decoupled, one-to-many relationship between objects. - Supports event-driven programming. - Allows for easy notification of state changes in an object.	- Can result in memory management issues if observers are not properly deregistered. - May create unexpected dependencies.	- When one object (the subject) needs to notify many dependent objects (observers) about changes. - When implementing event-driven systems or notification mechanisms.	- Solves the problem of notifying multiple objects when a subject changes its state.	- Observer focuses on communication between objects via events, while Strategy focuses on changing behavior by swapping algorithms.
Singleton	- Ensures that a class has only one instance. - Provides a global point of access to that instance. - Useful for managing shared resources.	- Makes unit testing difficult. - Can lead to hidden dependencies. - Makes the code harder to extend and maintain over time.	- When you need to ensure that a class has only one instance across the system. - When you need a global access point to an object.	- Solves the problem of ensuring a single, globally accessible instance of a class.	- Unlike Factory (which creates multiple instances), Singleton ensures that only one instance exists and is shared.
Strategy	- Allows algorithms to be swapped dynamically at runtime. - Promotes the Open/Closed Principle by allowing new behaviors to be added without altering the client. - Makes code easier to maintain by separating concerns.	- Can result in an increased number of classes (one for each strategy). - May introduce unnecessary complexity if not used properly.	- When you need to define a family of algorithms, and make them interchangeable. - When you need to change an object’s behavior at runtime based on specific conditions.	- Solves the problem of changing behavior or algorithms dynamically without altering the client code.	- Unlike Decorator (which adds functionality), Strategy changes the way an object behaves based on the selected algorithm.

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1. Adapter:

• Focuses on making two incompatible interfaces compatible.
• Can introduce complexity if overused but is useful when integrating third-party or legacy code.

2. Builder:

• Ideal for constructing complex objects step-by-step, especially with many optional parameters.
• Can result in too many classes if many variants are needed.

3. Decorator:

• Provides a flexible way to add new responsibilities to objects at runtime, without modifying the original object.
• Can lead to class proliferation if used excessively.

4. Facade:

• Hides the complexities of a subsystem and provides a simplified interface.
• Risk of losing flexibility as the internal subsystem becomes hidden.

5. Factory Method:

• Helps to decouple the creation of objects from their usage by defining a common interface for object creation.
• Can lead to a proliferation of factory classes.

6. Observer:

• Enables a one-to-many dependency relationship where one object notifies many others about changes in its state.
• Can cause memory management problems if observers are not deregistered properly.

7. Singleton:

• Ensures that a class has only one instance and provides a global point of access.
• Can make unit testing harder and hide dependencies.

8. Strategy:

• Defines a family of interchangeable algorithms, allowing behavior to change at runtime.
• May introduce complexity if too many strategies are defined.