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 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.

One example of where Interpreter pattern is used is in compilers, where expressions in a programming language need to be parsed and evaluated. The Interpreter pattern can be used to evaluate the syntax and semantics of a programming language, and generate code or perform other actions based on the results of the evaluation.

CQRS is a pattern that separates the responsibilities of querying data (read) from modifying data (write) into separate models, improving scalability and maintainability of the application.

The Null Object pattern is a design pattern in which a special object, called the null object, is used to represent the absence of a value, avoiding the need for explicit checks for null values. The Null Object pattern is used to provide a default implementation of an interface, reducing the amount of error-prone null checks in the code. The Null Object pattern can also be used to simplify the code and reduce coupling by eliminating the need for explicit checks for null values.

The Decorator pattern is used to dynamically add or override behavior of an object, while the Flyweight pattern is used to share objects that are costly to create.

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.