"The only idea that comes to my mind is to store a reference to MmmuhApplet instance in MouseProcessing class and access x/y from there via said reference. Is it a good idea?"
You had the right idea. What you can do is pass the applet to the listener through constructor injection (or pass by reference). Then have setters for whatever fields you need. Something like
public class MmmuhApplet extends JApplet {
int x, y;
public void inti() {
addMouseListener(new MouseProcessing(MmmuhApplet.this));
}
public void setX(int x) {
this.x = x;
}
public void setY(int y) {
this.y = y;
}
}
public class MouseProcessing implements MouseListener {
private MmmuhApplet mmmuh;
public MouseProcessing(MmmuhApplet mmmuh) {
this.mmmuh = mmmuh;
}
public void mousePressed(MouseEvent e) {
Point p = e.getPoint();
mmmuh.setX(p.x);
mmmuh.setY(p.y);
}
}