Why JOptionPane prevents ActionListener on JButton?

PreventingButtonListener.java

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class PreventingButtonListener extends JFrame implements ActionListener{

    JTextField fld = new JTextField(5) ;

    PreventingButtonListener(){
        super("PreventingButtonListener") ;
        setSize(300, 250) ;
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE) ;
        setResizable(false) ;

        fld.addFocusListener(
        new FocusListener(){

            public void focusGained(FocusEvent event) {}

            public void focusLost(FocusEvent event) {
                int i = (int) (Math.random() * 3) ;

                switch(i){
                    case 0 :
                        System.out.println("random value : " + i);
                        fld.requestFocus() ;
                        break ;

                    case 1 :
                        System.out.println("random value : " + i);

                        i = (int) (Math.random() * 2) ;

                        if(i == 0){
                            System.out.println("JOptionPane not displayed, followed with button action.");
                        }else{
                            System.out.println("JOptionPane displaye开发者_C百科d, but button action not performed.");
                            JOptionPane.showMessageDialog(null, "Hello") ;
                        }

                        break ;

                    default :
                        System.out.println("random value : " + i);
                        System.out.println("Nothing to be done...");
                        break ;
                }
            }

        }) ;

        add(fld, "North") ;

        JButton btn = new JButton("Hit me Repeatedly considering Note 1") ;

        btn.addFocusListener(new FocusListener(){

            public void focusGained(FocusEvent arg0) {
                System.out.println("focus gained on button.");              
            }

            public void focusLost(FocusEvent arg0) {
                System.out.println("focus removed on button.");             
            }

        }) ;

        btn.addActionListener(this) ;
        add(btn, "South") ;

        String str = "Note 1 : Whenever, u click on button, and if the focus" + "\n" +
                     "after clicking is not in the textfield, then, plz" + "\n" +
                     "explicitly click in the JTextField." + "\n\n" +
                     "Note 2 : Kindly notice the output on cmd " + "\n" +
                     "and related focuses of GUI." + "\n\n" ;

        JTextArea area = new JTextArea(str) ;
        area.setEditable(false) ;
        area.setWrapStyleWord(true) ;
        area.setBackground(Color.LIGHT_GRAY) ;
        add(new JScrollPane(area)) ;


        setVisible(true) ;
    }

    public void actionPerformed(ActionEvent arg0) {
        System.out.println("data of txtfld : >" + fld.getText() + "<");
    }

    public static void main(String[] args) {
        new PreventingButtonListener() ;
    }
}

the problem lies in the execution of the attached program???

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The JOptionPane that you bring up will be modal, and therefore action will not be processed for any component in your app outside of that JOptionPane.

from the New Modality API article from Sun:

A dialog box can be either modeless or modal. A modal dialog box is one that blocks input to some other top-level windows in the application, except for any windows created with the dialog box as their owner. The modal dialog box captures the window focus until it is closed, usually in response to a button press. A modeless dialog box, on the other hand, sits off on the side and allows you to change its state while other windows have focus. The latter is often used for a toolbar window, such as what you might find in an image-editing program.

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JOptionPane methods won't return until the dialog is closed so the action code can be written after the call.