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

03-behavioral/visitor/PerimeterCalculator.java

@@ -0,0 +1,28 @@
+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);
+    }
+}