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 {

  public Husband(String name) {
    super(name);
  }

  @Override
  public void accept(Friend friend) { // 2
    friend.greet(this); // 3
  }

}

The salient points of the preceding example are as follows:

• Here, we have an accept method, accepting all friends. The FamilyMember hierarchy opens up its doors for Friends...