I was unaware of this as well... All classes in Processing are nested within the main class, so that throws in some unexpected behavior... see this question on StackOverflow.

As calsign said, the game is biased in Processing, because all classes defined in .pde files are internal to the main class (named after the sketch name) and these children classes have special access rules to their parent's elements.
You can see the real Java code generated by Processing in a .java file created when you export a sketch.
If you want to see the real behavior of a separate class, put that class in a .java file in your sketch. It won't be nested.

That's why we use a somewhat relaxed coding style in Processing, rarely using private, protected or public, as most sketches are small and usually fit in one file only, with little code reusing.
Of course, libraries use a stricter style, in general.

Ok. That make sense.
I used an external .java file for my class and now Processing returns an arror when I try to call a private method in my sketch.
I'm just a newbie so I think I will not use private methods at the moment but I wanted to uderstand how do they work.
Thaks a lot!