This describes how to develop and compile JOSM using ​IntelliJ IDEA

1. Getting the source Source → Download#Source
2. Run ant in the source directory to obtain a josm-custom.jar → DevelopersGuide/Compiling#CompilingusingAnt
3. In IntelliJ: File → New → Project from Existing Sources
   1. Navigate to the JOSM source code
      
   2. 
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   5. 
4. File → Project Structure
   • Modules → Sources: Exclude the directory src/org/apache
     
   • Modules → Dependencies: Make sure that josm-custom (i.e., the file generated by Ant is listed)
   • Modules → Dependencies: Add Groovy (you might need to configure that at "Global Libraries")
     
5. Run → Edit Configuration
   • Add an "Application" configuration
     

Whenever presets, images, styles, or parser definitions change, re-run Ant in order to have those modifications included in josm-custom.jar.

This is a bit hackish, but works nicely. ☻