Using IDE’s like Eclipse or IntelliJ Idea helps a lot with development of projects containing multiple files and libraries. Those IDEs make it very easy to move code around, rename classes and variables, re-use code across projects, include amazing tooltips, autocomplete and suggestions to make your code better.
BUT the code needs to be adapted. For prototyping simple ideas I still use the Processing IDE (one-file experiments). But for anything that has 2 or more classes, I switch to a different IDE. The main things that need to be adapted are: float numbers need an f behind them (3.0f), colors can not be specified using the # character (use 0xFFFFFFFF, not #FFFFFF), and the scope in your classes is not the PApplet, which means you can not just call line() in your class, you need to pass the scope into the class, and then do something like p5.line(...).
These changes feel like an overkill for small projects and for people with less experience. But for medium to large projects I think the extra effort is totally worth it.