simple lejos question

Hey this is my first time using rcx or lejos, ive figured out how to upload things to the rcx brick. My question is, every time i send
something to the rcx does it delete everything on it, or can I send more then one class to the rcx by simply uploading the classes.If everything gets erased everytime i uplaod something, then how would i go about uploading more then one class. thnx for your help
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