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.
int  a2 = (int )o1; int  b2 = (int)o1;
try to cast o1 to two different, incompatible types. Only one of them can possibly succeed; these two types can't be interconverted. The first line succeeds, since o1 points to the original "a", which is indeed an int. Therefore the second line fails, because o1 is not an int.