Discussion of Activity 3.6 - Writing a subclass that overrides a superclass method

The object of this activity was to extend the Member class by a class CommitteeMember, and override the toString method defined by Member. 

The GroupRecord class begun in Activity 3.5 is included in this project and two methods have been added to it, giving it slightly more functionality than before:

1. getHeadCount returns the number of members in the group record.
2. getMemberAt returns the Member reference stored at a particular index in a GroupRecord object.

Here follows the completed class CommitteeMember:

public class CommitteeMember extends Member
{
   private String position;

   public CommitteeMember(String nam, String ema, String adr, String pos)
   {
      super(nam, ema, adr);
      position = pos;
   }

   public String getPosition()
   {
      return position;
   }

   /* We can make use of the parent toString method.
      This technique works even if there are no public getter methods
      in the parent for any variables we wish to access,
      but the parent toString method may not return exactly what we want */
   public String toString()
   {
      String text;

      text = super.toString();              //use the parent toString method
      text = text + "(" + position + ")";   //add locally defined information

      return text;
   }

   /* An alternative solution illustrated below is to use available parent
      getter methods such as getName. This gives extra flexibility in this method.
      We have chosen to use the same form of output as the parent class.
      We add the 'position' information defined in this class
   */

   /*
   public String toString()
   {
      return getName() + ", email address " + getEmailAddress()
             + ", home address " + getHomeAddress() + "(" + getPosition() + ")";
   }
   */
}

Notes

Think about the following points:

• m3 and m4 were declared to be of type Member rather than of type CommitteeMember.  This is allowed because CommitteeMember (when completed correctly) extends Member, and so a CommitteeMember is-a Member.  Notice, however, that when we invoke the toString method on these references, the method invoked depends on the type of the object referenced.  When the object is of type CommitteeMember, we are able to see the office held by the member, because the toString method defined by a CommitteeMember object displays this information.
• the addMember method of GroupRecord was able to add references of both Member and CommitteeMember types to the listOfMembers array. 
• the method getMemberAt, which returns a Member reference, serves our purpose whether the references it returns are of type Member or CommitteeMember. 
• Both addMember and getMemberAt (the one taking an argument of type Member, and the other returning a value of type Member) will continue to work with other classes extending Member or any of its subclasses!