I've got a sketch with a some class instances I'd like to export (basically, it's an evolution-simulation with a population of boids). All the involved classes implement Serializable, and java.io.* is imported for the entire sketch.
However, I get the following error:

java.io.NotSerializableException: processing.core.PApplet$RegisteredMethods

(I got a different notSerializable-exception when I used OpenGL, but OpenGL isn't really necessary in this situation [neither is P3D]).
I've got the following code (all in seperate try-catch blocks):

f_out= new FileOutputStream("winner.data");
ObjectOutputStream obj_out = new ObjectOutputStream (f_out);
obj_out.writeObject ( boid[0] ); // the exception above is thrown here

the boid[0]-instance is the boid with the best fitness after x generations, and that class instances some other classes of my own making, but as I mentioned, they all implement Serializable.

Soooo what do I do?