Add all 23 GoF design pattern implementations

03-behavioral/visitor/Circle.java

@@ -0,0 +1,9 @@
+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); }
+}