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Visitor: sync post code blocks to repo exactly (post used a completely different design - return-value visitors vs repo's void/print-in-place visitors, different Triangle model), split merged Circle/Rectangle/Triangle and AreaCalculator/PerimeterCalculator blocks, Java 25, H6 file labels, fix stale console output, README mapping

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# Visitor Design Pattern — Java Example
**Pattern:** Behavioral → Visitor
**Article:** https://ankurm.com/visitor-design-pattern-java/
## What this example shows
A shape hierarchy that never changes, with operations added as separate visitor classes instead of methods on each shape. `ShapeVisitor` declares one `visit()` overload per concrete type. `Shape` declares `accept(ShapeVisitor)` — the double-dispatch trick: the shape hands itself to the visitor as a concretely-typed `this`, so overload resolution picks the right `visit()` method with no `instanceof` checks anywhere. `Circle`, `Rectangle`, and `Triangle` are plain data holders that know nothing about area or perimeter. `AreaCalculator` and `PerimeterCalculator` are two concrete visitors — adding the second one required zero changes to any shape class. `Main` runs both visitors across the same three shapes.
## How to run
```bash
javac visitor/*.java -d out/visitor
java -cp out/visitor visitor.Main
```
Requires Java 25.
## Post Section ↔ File Mapping
| Post Section | File(s) |
|---|---|
| Implementation: Shape Geometry Operations — the Visitor interface | `ShapeVisitor.java` |
| Implementation: Shape Geometry Operations — the Element interface | `Shape.java` |
| Implementation: Shape Geometry Operations — Circle | `Circle.java` |
| Implementation: Shape Geometry Operations — Rectangle | `Rectangle.java` |
| Implementation: Shape Geometry Operations — Triangle | `Triangle.java` |
| Implementation: Shape Geometry Operations — AreaCalculator | `AreaCalculator.java` |
| Implementation: Shape Geometry Operations — PerimeterCalculator | `PerimeterCalculator.java` |
| Implementation: Shape Geometry Operations — wiring it together | `Main.java` |
Article: https://ankurm.com/visitor-design-pattern-java/
All patterns: https://ankurm.com/design-patterns-java/

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@@ -460,6 +460,25 @@ javac 03-behavioral/template-method/*.java -d out/template-method
java -cp out/template-method template.Main
```
### Visitor (`03-behavioral/visitor/`)
| Post Section | File(s) |
|---|---|
| Implementation: Shape Geometry Operations — the Visitor interface | `ShapeVisitor.java` |
| Implementation: Shape Geometry Operations — the Element interface | `Shape.java` |
| Implementation: Shape Geometry Operations — Circle | `Circle.java` |
| Implementation: Shape Geometry Operations — Rectangle | `Rectangle.java` |
| Implementation: Shape Geometry Operations — Triangle | `Triangle.java` |
| Implementation: Shape Geometry Operations — AreaCalculator | `AreaCalculator.java` |
| Implementation: Shape Geometry Operations — PerimeterCalculator | `PerimeterCalculator.java` |
| Implementation: Shape Geometry Operations — wiring it together | `Main.java` |
Run it:
```bash
javac 03-behavioral/visitor/*.java -d out/visitor
java -cp out/visitor visitor.Main
```
## Reference
- *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides