I would say that testing a graphical application with JUnit (or TestNG or whatever unit test framework you want) isn't easy, or even doable...
Unit tests are more aimed at black boxes: input this, get that at the other end... And they might be overkill for your average small sketch...

Now, if you aim at making an ambitious sketch, you will want to modularize it, using functions, classes, etc. Some, perhaps most of these functions or classes might even have no graphical result at all. Eg. a function checking if two rectangles intersect, a Voronoi class, etc.
For such entities, unit tests can be useful, and are even recommended.
Idem if you are making a library.

Thank you for your answer.

There are tools like FEST to test Swing applications, but not too sure about them working with Processing (which is an AWT application, and an applet).
It is a bit beyond unit tests, which only check small portions of behavior.

OK thanks

You have to create a mock PApplet. The difficulty is that PApplet has no interface, but good mocking libraries (perhaps Mockito or jMock) might overcome this problem.
The mock PApplet can just provide the methods needed in your World class, and you can ensure they are called, perhaps with the right parameter values, etc.

OK, thank you