It is a convention - not a law. NO ONE can quarantee that others won't use it. However, since the industry standard is to use package names with an inverted domain address that you have registered, no one else would be able to register the SAME domain name. Therefore if everyone follows this standard there will be no clashes. So all packages belonging to GM would be in packages named com.gm.whatever etc.
"JavaRanch, where the deer and the Certified play" - David O'Meara
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop