Visitors

• Problem: Operation may be different for different element types
• Can't rely on polymorphism
• Polymorphism assumes fixed set of methods, defined in superclass
  
• Trick: Can use variable set of methods if set of classes is fixed
• Supply separate visitor methods:
  public interface Visitor
  {
     void visitElementType1(ElementType1 element);
     void visitElementType2(ElementType2 element);
     ...
     void visitElementTypen(ElementTypen element);
  }