Have you ever come across a situation where you modify a class and when you run it you do not see the changes you just committed? Most probably in most cases the issue lies with an older .class file being loaded in from another jar which overrides your current jar. This is definitely a debugging night mare.
Fortunately in java you can find out from which location your class file is loaded from. This article provides the info you need to find out on how to do it the eclipse way or injecting it directly to your code.