Add all 23 GoF design pattern implementations

03-behavioral/visitor/AreaCalculator.java

@@ -0,0 +1,26 @@
+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);
+    }
+}