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dc38: Email management system

Imagine you are designing an email management system that needs to manage email addresses. There are two types of email address: an individual address, which identifies a single email inbox, and a group address which acts as a container for other addresses. A group can be a member of other groups; furthermore, a (group or single) address can be a member of several groups.

Write a set of Java classes that models individual and group email addresses. Your classes should fulfil the following requirements:

• Every email address (individual or group) should have an associated string identifier
• Two email addresses should be regarded as equal if they have the same identifier. Don't forget that if you override equals(), you must also override hashCode()
• On construction, a group email address should initially have no members
• It should be possible to add an email address to a group email address
• All email addresses should support a method, getTargets(). When invoked on email address a, this method should return the set of individual addresses to which an email addressed to a must be sent

First, assume that all group memberships are acyclic; that is, no group can contain itself (directly or indirectly) as a member.

Once you have a working solution for the acyclic scenario, write an extended version that will work even in the presence of cycles.

You should also write a demonstration program to show your classes in action.