This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I have two elements with corresponding complex types, student and grad_student. grad_student extends student which has an required attribute called "salary". Now if I I want to make "salary" attribute "optional" in for grad_student, how can I do that?