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.
Explicitly importing a class forces the current compilation process to treat that class as already defined. Importing a whole package just allows the compilation process to "see" those classes, if required. So trying to redefine String will be an error.