Bernard Bernoulli If you have trouble building, make sure you use the 1.8 JDK to compile, build and make the JAR. Latest Eclipse will use the latest JDK by default (which is the proper practice) so use a project-specific fallback to 1.8.
Eclipse (latest): Right-click project, Properties, Java Compiler, Enable project-specific settings, Compiler compliance level: 1.8 (anything too high should remind you that this uses 1.8 and building it will go south), keep default compliance settings, Apply, Apply and Close.
Don't download a deprecated Java IDE or even a deprecated JAR builder just to build Inno's shit, this isn't a good idea at all. As long as you can pick what to build with, it will build just fine.