Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/visitor/Main.java

@@ -0,0 +1,38 @@
+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 ===");
+    }
+}