Can't add list element when calling a method from another class

Question

I am trying to call a method in another class, which contains code listmodel.addElement("text"); to add an element into a list component made in that class.
I've put in System.out.println("passed"); in the method just to make sure if the method was being called properly and it displays normally.
I can change variables in the other class by calling the method with no problem. The only thing I can't do is get listmodel.addElement("text"); to add a new element in the list component by doing it this way.
I've called that method within it's class and it added the element with no problem. Does Java have limitations about what kind of code it can run from other classes? And if that's the case I'd really like to know just why.

Answer 1

There were no errors, just the element doesnt get added to the list by doing it this way
class showpanel extends JPanel implements ActionListener, MouseMotionListener {
       framepanel fp = new framepanel();
       --omitted--
     public void actionPerformed(ActionEvent e){
              if(e.getSource() == button1){
                   fp.addLayer();
/*is in a different class file*/
class framepanel extends JPanel implements ActionListener{
        --omitted--
     public void addLayer(){
          listmodel.addElement("Layer"+numLayer);
          numLayer++;
}