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I'm not sure what happened but now when I boot up my Mac I get this message that pops up every time:

"The Java class file "myClass.class" could not be launched.
Check the Console for possible error messages."

This class file is part of a larger program I'm developing.

I checked the console and found the message with sender [0x0-0x1b01b].com.apple.JarLauncher and the message says java.lang.NoClassDefFoundError: ....

I failed to find the Jar Launcher or anything Java related in /Library/StartupItems and ~/Library/StartupItems but that's all I thought to do.

Any ideas on why I'm getting this message and what to do to get rid of it?

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1) com.apple.JarLauncher or the program you're developing may be started at login if they saved state: In the Finder press Cmd+Shift+G and type ~/Library/Saved Application State. Now look for folders named com.apple.JarLauncher and after your program's identifier. Delete them. 2) Boot in safe mode (support.apple.com/kb/ht1564). If no message pops up look for jarLauncher and myClass.class entries in /Library/LaunchAgents and ~/Library/LaunchAgents. 3) Unlikely, but check whether a login hook is defined: defaults read com.apple.loginwindow LoginHook. Try sudo defaults ... also. –  jaume Jun 21 '13 at 10:37
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