Add all 23 GoF design pattern implementations

03-behavioral/visitor/AreaCalculator.java

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+package visitor;
+
+/**
+ * Concrete Visitor 1 — calculates area for each shape type.
+ * Area formulas live here, not scattered across shape classes.
+ */
+public class AreaCalculator implements ShapeVisitor {
+
+    @Override
+    public void visit(Circle c) {
+        double area = Math.PI * c.getRadius() * c.getRadius();
+        System.out.printf("  Area of %s = %.2f%n", c.getName(), area);
+    }
+
+    @Override
+    public void visit(Rectangle r) {
+        double area = r.getWidth() * r.getHeight();
+        System.out.printf("  Area of %s = %.2f%n", r.getName(), area);
+    }
+
+    @Override
+    public void visit(Triangle t) {
+        double area = 0.5 * t.getBase() * t.getHeight();
+        System.out.printf("  Area of %s = %.2f%n", t.getName(), area);
+    }
+}

03-behavioral/visitor/Circle.java

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+package visitor;
+
+public class Circle implements Shape {
+    private final double radius;
+    public Circle(double radius) { this.radius = radius; }
+    public double getRadius() { return radius; }
+    @Override public String getName() { return "Circle(r=" + radius + ")"; }
+    @Override public void accept(ShapeVisitor visitor) { visitor.visit(this); }
+}

03-behavioral/visitor/Main.java

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+package visitor;
+
+import java.util.List;
+
+/**
+ * Visitor Design Pattern — Runnable Demo
+ * Run: javac visitor/*.java -d out/visitor && java -cp out/visitor visitor.Main
+ * Article: https://ankurm.com/visitor-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Visitor Design Pattern Demo ===\n");
+
+        List<Shape> shapes = List.of(
+            new Circle(5),
+            new Rectangle(4, 6),
+            new Triangle(3, 8)
+        );
+
+        System.out.println("-- Area Calculator Visitor --");
+        ShapeVisitor areaCalc = new AreaCalculator();
+        for (Shape shape : shapes) {
+            shape.accept(areaCalc);
+        }
+
+        System.out.println("\n-- Perimeter Calculator Visitor --");
+        ShapeVisitor perimCalc = new PerimeterCalculator();
+        for (Shape shape : shapes) {
+            shape.accept(perimCalc);
+        }
+
+        System.out.println("\n-- Key insight --");
+        System.out.println("Added PerimeterCalculator without touching Shape, Circle, Rectangle, Triangle.");
+        System.out.println("To add another operation: write one new Visitor class.");
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}

03-behavioral/visitor/PerimeterCalculator.java

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+package visitor;
+
+/**
+ * Concrete Visitor 2 — calculates perimeter.
+ * Adding this visitor added zero code to Shape, Circle, Rectangle, Triangle.
+ */
+public class PerimeterCalculator implements ShapeVisitor {
+
+    @Override
+    public void visit(Circle c) {
+        double p = 2 * Math.PI * c.getRadius();
+        System.out.printf("  Perimeter of %s = %.2f%n", c.getName(), p);
+    }
+
+    @Override
+    public void visit(Rectangle r) {
+        double p = 2 * (r.getWidth() + r.getHeight());
+        System.out.printf("  Perimeter of %s = %.2f%n", r.getName(), p);
+    }
+
+    @Override
+    public void visit(Triangle t) {
+        // Simplified: assume isoceles — base + 2 equal sides approximated
+        double side = Math.sqrt(Math.pow(t.getBase() / 2, 2) + Math.pow(t.getHeight(), 2));
+        double p = t.getBase() + 2 * side;
+        System.out.printf("  Perimeter of %s = %.2f%n", t.getName(), p);
+    }
+}

03-behavioral/visitor/README.md

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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/

03-behavioral/visitor/Rectangle.java

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+package visitor;
+
+public class Rectangle implements Shape {
+    private final double width, height;
+    public Rectangle(double width, double height) { this.width = width; this.height = height; }
+    public double getWidth()  { return width; }
+    public double getHeight() { return height; }
+    @Override public String getName() { return "Rectangle(" + width + "x" + height + ")"; }
+    @Override public void accept(ShapeVisitor visitor) { visitor.visit(this); }
+}

03-behavioral/visitor/Shape.java

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+package visitor;
+
+/** Element interface — accepts a visitor */
+public interface Shape {
+    void accept(ShapeVisitor visitor);
+    String getName();
+}

03-behavioral/visitor/ShapeVisitor.java

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+package visitor;
+
+/** Visitor interface — one visit() method per concrete element type */
+public interface ShapeVisitor {
+    void visit(Circle circle);
+    void visit(Rectangle rectangle);
+    void visit(Triangle triangle);
+}

03-behavioral/visitor/Triangle.java

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+package visitor;
+
+public class Triangle implements Shape {
+    private final double base, height;
+    public Triangle(double base, double height) { this.base = base; this.height = height; }
+    public double getBase()   { return base; }
+    public double getHeight() { return height; }
+    @Override public String getName() { return "Triangle(b=" + base + ",h=" + height + ")"; }
+    @Override public void accept(ShapeVisitor visitor) { visitor.visit(this); }
+}