# 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/