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.
However if you allow this, then you must write your code so that any time you use the net weight, you consider the possibility that it's null. If you're not careful, there's a good chance you'll get a NullPointerException as a result.