The best preparation for the essay is to keep (or have kept) a written journal of your assignment work. You had to make some choices and forsake others, e.g., whether to store the database in memory or regularly update a file for persistence. You'll be asked, in short, what choices you considered, and why you chose what you did -- what were the benefits, what are the potential drawbacks.
The concept of the developer assignment is that there are no "correct" assignments other than ones that work. There's no one solution that conquers all, and it's important to recognize that when you develop software, so the question, which limitations did you decide to live with?
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Michael Ernest, co-author of:
The Complete Java 2 Certification Study Guide