Every once in a while there is a video card that has compatibility problems with java ... I would recommend that you make sure you have the latest JVM (www.java.com) and then you can try running
java -Dsun.java2d.noddraw=true -jar jemplode.jar
rather than running it like you normally do ... unfortunately, you'll take a substantial performance hit doing this -- that causes java to disable all directx acceleration. This at least was the fix that helped the last person who had this problem ...