Scala Functional Programming Patterns

The Visitor pattern

We have a FamilyMember hierarchy, as shown in the following figure, which is pretty stable. We have another hierarchy, Friends, which is pretty open ended. So now, a FamilyMember needs to accept a Friend.

Figure 7.2: A UML diagram—Visitors

When we have a hierarchy and we need to add a new method to it without modifying it, we use the visitor pattern. Here is the code snippet for this example:

public abstract class FamilyMember { . private String name; public FamilyMember(String name) { this.name = name; } public String getName() { return name; } public abstract void accept(Friend friend); // 1 } public class Husband extends FamilyMember ...