This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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?