Yes, all Processing sketches are pre-processed and converted to pure java source files. JavaBuild.java handles this. Refer to the function processing.mode.java.JavaBuild.build() , at line 134. The .java files are created in the temp folder of the system. On a PC, that would be C: \[Your Username] \ AppData \ Local \ Temp \ [Sketch Name + random suffix] \ [Sketch Name + random suffix].java. The temp directory would be mostly full of such folders, so it's a better option to sort it by Date Modified to find your source file easily.
P.S - In my Sketch Outline tool, I was earlier using JavaBuild's functions to generate a java file for building the tree, but later I wrote my own simplified pre-process function which did the trick for me.
