Behavioral: Strategy, Observer, Command, State, Template

Observer notifies, Command packages actions, Iterator walks collections, State switches behaviour, Template Method fixes a skeleton, Visitor adds operations, Mediator centralises talk, and Memento snapshots state.

A request walks down a line of handlers, and the first one capable of dealing with it does so: the sender never picks the handler.

Closed means try normally, Open means fail fast and do not call, Half-Open means cautiously test if the dependency has recovered.

Use a guarded enum for simple lifecycles and the State pattern for rich ones, prefer composition over inheritance for variants, and let entities own their transitions while the service stays thin.

One login endpoint delegates to an AuthenticationStrategy chosen by a type field, so each method is a plug-in class and adding a method never touches the endpoint or handler.

Command turns an action into an object with execute and undo, so the invoker can queue, log, replay, undo, and combine actions without knowing the receiver.

Observer notifies many listeners when the subject changes, and its memory leak is the lapsed listener: observers that never unsubscribe are held forever by the subject.

The Chain of Responsibility passes a request through a line of handlers until one handles it or all have run.

The Command pattern turns an action into an object that can be executed, queued, logged, and undone.

The Observer pattern lets a subject broadcast state changes to many subscribers without knowing who they are.

The State pattern makes each state a class that knows its own allowed actions and transitions, so behaviour changes when the state changes.

The Strategy pattern makes an algorithm a pluggable object chosen at runtime behind a common interface.