在 Java 中将组件添加到面板

Question

I'm creating an applet which consists of a class which extends JApplet, with a menubar and a class which extends a JPanel.(So there is a menubar and a JPanel shown in the applet).

In this class I add and remove some textfields to the JPanel. This all works fine. Here's where it gets tricky: it only works the first time. When I add some new textfields to the JPanel, they are added and visible in the JPanel, but the menubar in the JFrame stops working.

Since the code is too extensive I'll only post parts of it.

Here's the code where I add the JPanel to the JApplet:

public class Simulator extends JApplet implements ItemListener, ActionListener {
    Container pane = getContentPane();
    canvas = new DrawCanvas();
    pane.add(canvas, BorderLayout.LINE_END);
}

Here's the code of the JPanel:

class DrawCanvas extends JPanel {
    public void paintComponent(Graphics g) {
        if(textfield != null)
            remove(textfield);
        textfield = new JTextField();
        this.add(textfield);
    }
}

This works the first time(when nothing is removed), but the second time the menubar stops working. When I leave out the this.add(textfield); line, the menubar keeps working.

Answer 1

I believe you are running into issues with threading. Adding and removing JComponents during painting might mess up the EDT (which is calling the paint method in the first place).

Answer 2

I once had similar problems with popup menus beeing painted behind other components. Try calling static JPopupMenu.setDefaultLightWeightPopupEnabled(false); or the setLightWeightPopupEnabled on your specific submenu. This will make (all) popup menus (ie submenus) to heavy weight components that have a native peer.