I am using te fist code you posted there are 2 syntax errors that need fixing

1. in line 15 g.is3D should be g.is3D() since it is a method
2. in line 32 you are extending a non-existant class Defomer should be Deformer

Even then it won't work because you cannot use the static modifier inside an inner class. Converting the class to a top-level class by making it static class Deformer only causes different problems. There are 2 possible solutions

1. make these Deformer classes top-level by defining them in there own (.java) tabs or
2. move the createDefender method outside of the class.

In the code below I went for option (2) but even after all that we still have the error ' the function printMe() does not exist'

The reason for this is that def is declared of type Deformer and the method does not exist in this class. In the code below I have declared an abstract method in Deformer (making the class abstract). The method wil now be recognised and Java will execute the correct version of printMe depending on the actual object class def is refering to (this is called polymorphism)

1. Deformer def;
2. void setup() {
3.   size(200, 200, P3D);
4.   def = createDeformer();
5.   def.printMe();
6. }
7. 
   
8. Deformer createDeformer() {
9.   if (g.is3D()) {
10.     return new Deformer3D();
11.   }
12.   return new Deformer2D();
13. }
14. 
    
15. // . . . . . . . . . . . . . . . . . .
16. 
    
17. abstract class Deformer {
18.   abstract void printMe();
19. }
20. 
    
21. // . . . . . . . . . . . . . . . . . .
22. 
    
23. class Deformer2D extends Deformer {
24. 
    
25.   void printMe() {
26.     println("2d");
27.   }
28. }
29. 
    
30. // . . . . . . . . . . . . . . . . . .
31. 
    
32. class Deformer3D extends Deformer {
33. 
    
34.   void printMe() {
35.     println("3d");
36.   }
37. }

thanks, i'm only more interested in method 1. where the Deformer classes are in a own .java file.

How does processing do this with PGraphics and PGraphics2D and PGraphics3D?

They are created with a call to size() or createGraphcis() by indicating which renderer to use.

If each Java class is in its own tab and are name correctly then the only chane is to add the printMe method to Deformer so the matching method in the child class becomes accessable (polymorphism).

1. import processing.core.PApplet;
2. 
   
3. class Deformer {
4.    
5.   static Deformer createDeformer(PApplet p) {
6.     if (p.g.is3D()) {
7.       return new Deformer3D();
8.     }
9.     return new Deformer2D();
10.   }
11.   
12.   // Empty method to enable polymorphism
    
13.   void printMe() { }
14. }
15. 
    

Alright thanks, i will give it a try.