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# Cross-pattern comparisons worth memorizing

These are the distinctions most likely to be tested, and the ones that actually
bite in review:

| Confusion                           | The tell                                                                                                                                                                                                                                             |
| ----------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Bridge vs. State vs. Strategy**   | Near-identical UML (context → interface, delegate). Bridge: two hierarchies designed up front, both varying. State: the delegate swaps _itself_ and drives transitions. Strategy: interchangeable, mutually-unaware algorithms chosen by the client. |
| **Decorator vs. Proxy vs. Adapter** | All wrap one object. Adapter **changes** the interface. Decorator **keeps** it and adds behavior, client-controlled and stackable. Proxy **keeps** it but controls access/lifetime and manages its own subject.                                      |
| **Decorator vs. Composite**         | Decorator has exactly one child and _adds_ responsibility; Composite has many and _aggregates_ their results.                                                                                                                                        |
| **Facade vs. Mediator**             | Direction. Facade is one-way and the subsystem doesn't know it exists. Mediator is many-way and colleagues hold references to it.                                                                                                                    |
| **Mediator vs. Observer**           | Mediator removes mutual dependencies among a known set of components via a hub. Observer creates dynamic, independently-subscribable one-way links. Often combined: mediator-as-publisher.                                                           |
| **Memento vs. Command**             | Not rivals — partners. Command = "the operation"; Memento = "the state to restore if it's undone."                                                                                                                                                   |
| **Facade vs. Abstract Factory**     | Both hide a subsystem. Abstract Factory hides only _creation_; Facade hides creation **and** call sequencing.                                                                                                                                        |

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