如何在ActionListener中添加Swing组件？

Question

I have a JMenuItem called newMI, in a class that extends JFrame. I want to add Swing components to my JFrame when I click the JMenuItem. For testing purposes, I am trying to add a JPanel and setting the background color of the JPanel to red.

Here is my ActionListener:

newMI.addActionListener(new ActionListener() {
   public void actionPerformed(ActionEvent e) {
        JPanel p = new JPanel();
        p.setBackground(Color.red);
        add(p);
   }
}

However this isn't working. I can change the background color of the JPanel if I added it to the JFrame during the initialization of the other Swing components. But I can't add Swing components to the JFrame directly inside of an ActionListener. Can somebody please help?

Many thanks.

Answer 1

When you dynamically add/remove components from a visible GUI then you need to do:

panel.add(...);
panel.revalidate();
panel.repaint();

If you need more help then post your SSCCE that demonstrates the problem.

Answer 2

you need to re-layout your component -- your new panel has been added, but has a size of 0x0 px. Call layout(true) on your component after adding the panel.

In case you don't have a layout manager in your component, you must set the position and size of the added panel manually after adding it to your component.