Percy Frankland wrote:
And can I create projectSet so that its an extension of HashSet so that it can have its own properties an methods in addition to those it would inherit?
Some points to ponder before extending any class which is not in direct control of your development.
1. Favor composition over inheritance,
2. Carefully override the methods and read the docs for the same.
Read #16 and #17 from "Effective Java", Joshua Bloch explain it beautifully.