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.
You can't, because it is impossible to import the class in the default package in the source file that contains the class that's in a package. Therefore you should not put classes in the default package.
(It used to be possible to import classes from the default package by writing import HelloWorld; but Sun removed that feature since Java 5, I think).