如何从其父JFrame外部重绘JPanel？

Question

I can add/remove elements to/from a panel and repaint it when the method used to fill the panel is called by one of its parent JFrame events, but I can not repaint it by events from other classes even if their sources have been added to it, or that is how I understand the problem for now.

I want to understand what is going on here, Thank you.

Main Class

public class Principal extends JFrame implements ActionListener{
private static Principal instPrincipal = null;    
private SubClass subClassInst =new SubClass();
public JPanel panelPrincipal;

public static Principal getInstance() {
    if (instPrincipal != null)
        return instPrincipal ;
    else {
        instPrincipal = new Principal ();
        return instPrincipal ;
    }
}

public void actionPerformed(ActionEvent event) {
    Object source = event.getSource();
    try {
        if(source == btnSub)
        {
          subClassInst.fillPanelPrincipal();
        }
        }
     catch (Exception e) {
        // TODO: handle exception
        e.printStackTrace();
     }
    }
 }

Sub Classes Example

public class SubClass implements ActionListener {

private JPanel tempPanel;
private JButton btnSave; 
private Principal instPrincipal; 

public void fillPanelPrincipal() {
    instPrincipal = Principal.getInstance();
    instPrincipal.panelPrincipal.removeAll();
    //Start adding elements..
    tempPanel = new JPanel();
    instPrincipal.panelPrincipal.add(tempPanel);
    btnSave = new JButton("Save");
    btnSave.addActionListener(this);
    tempPanel.add(btnSave);
    //End. 
    instPrincipal.panelPrincipal.repaint();
  }

public void actionPerformed(ActionEvent event) {
    instPrincipal = Principal.getInstance();
    Object source = event.getSource();
    if (source == btnSave) {
    // modify local data, Database .. ; //work but need to be repainted on panelPrincipal
    instPrincipal.panelPrincipal.repaint();//does not work
    }
   }

}

Update

To clarify the problem more, I have one single JPanel on a JFrame and there are different classes to fill it for multiple functionalities, I call their methods using JMenuItems on the main frame, these Classes implement ActionListener , passing the panel didn't work, and also the method I am trying here. I thought about changing the design to use CardLayout , but it was very difficult.

Answer 1

You are calling Principal as a static reference, so how is it supposed to know what frame to repaint? You should pass the instance of the JFrame through the constructor of the subclass. Like so:

private SubClass subClassInst = new SubClass(this);

And create the constructor like this

private JFrame parent;
public SubClass(JFrame parent) { this.parent = parent; }

You can then use it like so

this.parent.repaint();