Okay haha 
i did it like this
but how can i let the node know where it is?
class Knoten extends Zaehler{
int knotennummer;
// neuen Knoten erstellen
Knoten(){
hochzaehler();
fill(0);
display();
fill(255);
textSize(20);
text(knotennummer,mouseX,mouseY);
}
void display(){
ellipse(mouseX,mouseY,50,50);
}
void hochzaehler(){
knotennummerzaehler ++;
knotennummer = knotennummerzaehler;
}
}