The Observer pattern is a design pattern in which an object, called the subject, maintains a list of its dependents and notifies them automatically of any changes to its state. The Observer pattern is used to implement a publish/subscribe mechanism, allowing the subject to be decoupled from its dependents, and allowing multiple dependents to be notified of changes to the subject's state. The Observer pattern is used to implement event-driven systems.

A static class can have multiple instances, while a singleton class can only have one instance. A static class is a class that is declared as static and can only access static variables and methods. A singleton class is a class that ensures that only one instance of the class exists in the JVM.

The Decorator design pattern is used to attach additional responsibilities to an object dynamically. This allows for greater flexibility and modularity in the design of a system, as well as the ability to add or remove responsibilities as needed.

This means that when designing software, the focus should be on defining the interfaces between different components, rather than the specific implementations of those components. This makes the code more flexible and easier to maintain, as it reduces the amount of coupling between components.

The Null Object pattern is a behavioral design pattern that provides a default object as a representative of "null" in the context of design patterns.

Command pattern is a design pattern that converts a request into a standalone object, allowing the client to parametrize objects with different requests, queue a request, or log requests. The pattern involves creating a command class that encapsulates the request as an object, and passes the request to the invoker object. The invoker object then executes the command. This pattern can be useful in situations where you want to decouple the client from the receiver of the request, and where you want to provide a unified interface for undo and redo operations.

The State pattern allows an object to alter its behavior when its internal state changes, while the Strategy pattern allows an object to change its behavior based on the context it is in

A behavioral design pattern that separates an algorithm from an object structure on which it operates. The pattern allows you to define a new operation without changing the classes of the objects on which it operates.

Inversion of Control is a design principle in which the flow of control in a system is inverted. Instead of a system calling components to perform tasks, components are notified when tasks are ready and perform them. This leads to a more flexible and decoupled system, allowing for easier maintenance and testing.

The actor classes used in the Memento pattern are: Originator, Memento, and Caretaker. The Originator class is responsible for creating the memento and restoring its state. The Memento class is responsible for storing the internal state of the Originator. The Caretaker class is responsible for maintaining a list of mementos and providing the Originator with the appropriate memento at the appropriate time.