JComboBox给出了IllegalComponentStateException并且我不明白为什么

Question

I am writing a GUI for a simple program that mimics a block of RAM and I wanted to use a JComboBox to let the user select a command option from a drop down list. However, whenever I try to select an option from the list, it gives me an IllegalComponentStateException and says that the component must be showing on the screen to determine its location. Here is the code for creating the main screen.

  public static void createMainMenu(){
    mainFrame.getContentPane().removeAll();

    mainFrame.setSize(700 , 500);
    mainFrame.setBackground(Color.WHITE);
    mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    mainFrame.setLayout(new GridLayout(3 , 3));

    JPanel ramStatus = new JPanel();
    ramStatus.setBackground(Color.WHITE);
    ramStatus.setSize(200 , 200);

    JLabel heading = new JLabel("Please Select An Option to Continue.");

    JComboBox<String> userChoice = new JComboBox<String>();

    for(int i = 0; i < choices.length; i++)
        userChoice.addItem(choices[i]);

    userChoice.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent e){
        JComboBox combo = (JComboBox)e.getSource();

        choice = combo.getSelectedItem().toString();
    }});

    JButton enter = new JButton("Enter");
    enter.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent e){

        handleSelection(choice);
    }});

    mainFrame.add(new JPanel());
    mainFrame.add(userChoice);
    mainFrame.add(enter);

    mainFrame.add(new JPanel());
    mainFrame.add(heading);
    mainFrame.add(new JPanel());

    mainFrame.add(new JPanel());
    mainFrame.add(ramStatus);
    mainFrame.add(new JPanel());

    //Set the frame visible.
    mainFrame.setVisible(true);
}

I should mote that choice is a static String variable declared after class declaration. Can anyone explain why this is happening and how I can fix it?

** * ** * ** * **** EDIT * ** * ** * ** * ** * ** * ** * ** * ** * ** * ** * ** * ** * ***

Here is the error message generated when I select a new option from the drop down menu. The exception is thrown before I hit enter, so it has nothing to do with outside methods.

 java.awt.IllegalComponentStateException: component must be showing on the screen to determine       
 its location
    at java.awt.Component.getLocationOnScreen_NoTreeLock(Component.java:2044)
    at java.awt.Component.getLocationOnScreen(Component.java:2018)
    at sun.lwawt.macosx.CAccessibility$22.call(CAccessibility.java:390)
    at sun.lwawt.macosx.CAccessibility$22.call(CAccessibility.java:388)
    at sun.lwawt.macosx.LWCToolkit$CallableWrapper.run(LWCToolkit.java:532)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:241)
    at sun.lwawt.macosx.LWCToolkit$CPeerEvent.dispatch(LWCToolkit.java:689)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

Why do you use addActionListener to listen to ComboBox item selection? Have you tried using addItemListener instead?

Answer 2

Can anyone explain why this is happening and how I can fix it?

No, because you do not give complete information.

Since the question is not worth making a runnable example to you, I did it: I added the vars mainFrame, choices, and choice, and gave string values to the choices array, I eliminated the "String" parameter from JComboBox type (running 1.6, JComboBox is not generic), and I commented out the call to handleSelection. I start the UI, and can select values from the drop-down multiple times with no error.

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;


    public class ComboBoxPlay
    {

      private static JFrame mainFrame = null;
      private static String[] choices = {"one", "two", "three" };
      private static String choice = null;

      public static void main(String[] args)
      {
         mainFrame = new JFrame();
        createMainMenu();

      }

      public static void createMainMenu(){
        mainFrame.getContentPane().removeAll();

        mainFrame.setSize(700 , 500);
        mainFrame.setBackground(Color.WHITE);
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setLayout(new GridLayout(3 , 3));

        JPanel ramStatus = new JPanel();
        ramStatus.setBackground(Color.WHITE);
        ramStatus.setSize(200 , 200);

        JLabel heading = new JLabel("Please Select An Option to Continue.");

        JComboBox userChoice = new JComboBox();

        for(int i = 0; i < choices.length; i++)
            userChoice.addItem(choices[i]);

        userChoice.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent e){
            JComboBox combo = (JComboBox)e.getSource();

            choice = combo.getSelectedItem().toString();
        }});

        JButton enter = new JButton("Enter");
        enter.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent e){

    //        handleSelection(choice);
        }});

        mainFrame.add(new JPanel());
        mainFrame.add(userChoice);
        mainFrame.add(enter);

        mainFrame.add(new JPanel());
        mainFrame.add(heading);
        mainFrame.add(new JPanel());

        mainFrame.add(new JPanel());
        mainFrame.add(ramStatus);
        mainFrame.add(new JPanel());

        //Set the frame visible.
        mainFrame.setVisible(true);
    }
    }