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docs/principles.md

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+[← Design patterns index](README.md)
+
+# Principles these examples keep demonstrating
+
+These seven patterns all apply the same handful of ideas from _Dive Into Design
+Patterns_' design-principles chapter:
+
+- **Encapsulate what varies.** Each pattern isolates the one thing that changes:
+  the platform (Bridge), the added behavior (Decorator), the subsystem wiring
+  (Facade), the collaboration rules (Mediator), the saved state (Memento), the
+  reaction to a change (Observer), or the state-specific behavior (State).
+  Everything else stays untouched.
+
+- **Program to an interface, not an implementation.** `IMessageSender`,
+  `IShape`, `IObserver`, `IMemento`, `IOriginator`, and `State` all exist so
+  that a context or client depends on a contract instead of a concrete type.
+  `Subject.Notify()` calls `observer.Update()` through `IObserver` alone, which
+  is exactly why `Data` never needs to know that `PieChart` or `BarGraph`
+  exists.
+
+- **Favor composition over inheritance.** Bridge, Decorator, Mediator, and State
+  all answer "my class hierarchy is exploding" the same way: give the object a
+  _reference_ to another object instead of subclassing again. Shvets: _"with
+  composition you can easily substitute the linked 'helper' object with another,
+  changing the behavior of the container at runtime."_
+
+- **Single Responsibility.** _"A class should have just one reason to change."_
+  Visible in `ShapeDecorator` subclasses (one behavior each), the `State`
+  subclasses (one state each), and `Facade` (all engine sequencing in one
+  place).
+
+- **Open/Closed.** _"Classes should be open for extension but closed for
+  modification."_ The concrete test: adding a new `IObserver`, a new
+  `ShapeDecorator`, a new `IMessageSender`, or a new `State` requires **zero**
+  edits to existing classes. Adding a state to `BadCarState` requires editing
+  four separate `switch` statements — that's the principle failing, made
+  visible.