34 lines
1.8 KiB
Markdown
34 lines
1.8 KiB
Markdown
# Visitor Design Pattern — Java Example
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**Pattern:** Behavioral → Visitor
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**Article:** https://ankurm.com/visitor-design-pattern-java/
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## What this example shows
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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.
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## How to run
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```bash
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javac visitor/*.java -d out/visitor
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java -cp out/visitor visitor.Main
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```
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Requires Java 25.
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## Post Section ↔ File Mapping
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| Post Section | File(s) |
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| Implementation: Shape Geometry Operations — the Visitor interface | `ShapeVisitor.java` |
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| Implementation: Shape Geometry Operations — the Element interface | `Shape.java` |
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| Implementation: Shape Geometry Operations — Circle | `Circle.java` |
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| Implementation: Shape Geometry Operations — Rectangle | `Rectangle.java` |
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| Implementation: Shape Geometry Operations — Triangle | `Triangle.java` |
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| Implementation: Shape Geometry Operations — AreaCalculator | `AreaCalculator.java` |
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| Implementation: Shape Geometry Operations — PerimeterCalculator | `PerimeterCalculator.java` |
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| Implementation: Shape Geometry Operations — wiring it together | `Main.java` |
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Article: https://ankurm.com/visitor-design-pattern-java/
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All patterns: https://ankurm.com/design-patterns-java/
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