JPanel doesn't update when adding Component in another class

Question

I'm fairly new to Java Swing and I'm running into a few problems.

As a side question, when making a fairly large Java Swing Application, what is the best way to split up code? In my case I want to have an application that has a layout just as Microsoft Word where there is a JToolBar filled with buttons and a main JPanel where changes are made based on the buttons pressed in the Tool Bar.

So as shown in the code below, I have a JFrame and I call the MainPanel class in order to create a panel and add a ToolBar with a button. When the button is pressed it adds a button to the panel. The problem comes when you click the button nothing shows up until you resize the window(in my case I simply manually drag the screen to make it larger).

public class Main {

private static void createAndShowGUI() {
    //Create and set up the window.
    JFrame frame = new JFrame("MathMaker");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Create the menu bar.  Make it have a green background.
    //MainToolBar mainTB = new MainToolBar();
    MainPanel mainPanel = new MainPanel();

    frame.getContentPane().add(mainPanel.getGUI(), BorderLayout.CENTER);
    frame.pack();
    frame.setVisible(true);
}

public static void main(String[] args) {
    //Schedule a job for the event-dispatching thread:
    //creating and showing this application's GUI.
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}

}

public class MainPanel implements ActionListener{
JPanel mPanel;
JToolBar mToolBar;
JButton addQuestion;
    public MainPanel() {
        mPanel = new JPanel(new BorderLayout());
        mToolBar = new JToolBar();
        addQuestion = new JButton("test");

    addQuestion.addActionListener(this);

    mPanel.setLayout(new BorderLayout());
    mPanel.setBackground(new Color(248, 213, 131));
    mPanel.setPreferredSize(new Dimension(200, 180));

    mToolBar.add(addQuestion);
    mPanel.add(mToolBar, BorderLayout.PAGE_START);
}
public JComponent getGUI()
{
    return mPanel;
}

@Override
public void actionPerformed(ActionEvent e) {
     JButton temp = new JButton("temp");
     mPanel.add(temp);
}

}

..setPreferredSize(new Dimension(200, 180)); Swing GUIs might have to work on different platforms, using different PLAFs, on different screen sizes and resolutions with different default settings for font size. As such, they are not conducive to exact placement of components. Instead use layout managers, or combinations of layout managers as well as layout padding and borders for white space. — Andrew Thompson

Sergiy Medvynskyy Sergiy Medvynskyy · Accepted Answer · 2014-08-06T15:08:56

You should revalidate your panel

@Override
public void actionPerformed(ActionEvent e) {
   JButton temp = new JButton("temp");
   mPanel.add(temp);
   mPanel.revalidate();
   mPanel.repaint();
}

JPanel doesn't update when adding Component in another class

3 Answers