Pure Java? Keep in mind that the Blackberry JVM is still based on J2ME, which is kind of a subset of java.
To include external JAR follow the Knowledge Base Entry
How To - Compile a jar file into a BlackBerry Library
Another way is to extract your jar file (change the file ending to .zip instead of .jar and extract the source files) and using the preverify option to check your java classes.
preverify.exe [-d] output -classpath directory input; directory
Good Luck,
rAyt
Edit in response to your comment
In C:\Program Files\Research In Motion\BlackBerry JDE 4.7\bin
try
preverify -classpath "C:\Program Files\Research In Motion\BlackBerry JDE 4.7\lib\net_rim_api.jar" "d:\myjar.jar"
and look in the output folder.