Facing problem inside the implementation of ActionListener

Question

Hi, I don't understand what the problem is. Please go through the following code. The explanation of the problem is documented.
Any kind of help is appreciated.
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JOptionPane;
* Class Test is intended for receiving a String input from the user and
* manipulating the input string later in some other methods. So the input
* string has to be stored for later use.
* Whenever an action takes place in JTextField or JButton (OK button) component,
* the corresponding ActionListener's actionPerformed() method assigns the
* user input to 'userInput' referene variable. This was assured when I placed
*              JOptionPane.showMessageDialog(frame, userInput);
* right after the assignment statement. But when I cut and pasted the above
* statement right after the getUserInput() method call (in constructor's body)
* it showed null string.
* WHY & HOW this happens?
* I could have implemented this application with the help of JOptionPane's
* showInputDialog() method. But I just want to know what the problem is if
* I have tried in this way.
public class Test {
     * String that the user types
    private String userInput;
     * Text field that contains the user input.
    private JTextField inputTextField;
     * Frame that contains all other components
    private JFrame frame;
    public Test() {
        prepareFrame();
        getUserInput();
        // shows null, why?????
        JOptionPane.showMessageDialog(frame, userInput);
        // some other methods goes here that uses the input string....
        // methodA();
        // methodB();
     * Creates the frame and sets some of its properties.
    private void prepareFrame() {
        frame = new JFrame("Experiment with ActionListener");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 300);
        frame.setVisible(false);
     * Receives user input and stores the reference to user input in userInput.
    private void getUserInput() {
        // Create sub-components
        JLabel promptLabel = new JLabel("Enter a string: ");
        inputTextField = new JTextField(20);
        JButton okButton = new JButton("OK");
        JButton cancelButton = new JButton("Cancel");
        // Add listeners
        inputTextField.addActionListener(new TextFieldHandler());
        okButton.addActionListener(new OkButtonHandler());
        cancelButton.addActionListener(new CancelButtonHandler());
        // Create a panel to contain the sub-components
        JPanel panel = new JPanel();
        // Add the sub-components to the panel
        panel.add(promptLabel);
        panel.add(inputTextField);
        panel.add(okButton);
        panel.add(cancelButton);
        frame.add(panel, BorderLayout.NORTH);
        frame.validate();
        frame.setVisible(true);
     * Event handler for text field that contains user input.
    private class TextFieldHandler
            implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            userInput = event.getActionCommand();
            // works fine if the user input is shown here.
            // JOptionPane.showMessageDialog(frame, userInput);
     * Event handler for OK button
    private class OkButtonHandler
            implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            userInput = inputTextField.getText();
            // works fine if the user input is shown here.
            // JOptionPane.showMessageDialog(frame, userInput);
     * Event handler for Cancel button
    private class CancelButtonHandler
            implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            System.exit(0);
    public static void main(String[] args) {
        new Test();
}

Answer 1

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JOptionPane;
public class T {
    private String userInput;
    private JTextField inputTextField;
    private JFrame frame;
    public T() {
        prepareFrame();
        getUserInput();
        // shows null, why?????
        // userInput is null because it has not been given a value
        // you can give it a value in the declaration as a member
        // variable above or here in your class constructor
        System.out.println("userInput = " + userInput);
        JOptionPane.showMessageDialog(frame, userInput);
        // some other methods goes here that uses the input string....
        // methodA();
        // methodB();
    private void prepareFrame() {
        frame = new JFrame("Experiment with ActionListener");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 300);
        frame.setVisible(false);
    private void getUserInput() {
        // Create sub-components
        JLabel promptLabel = new JLabel("Enter a string: ");
        inputTextField = new JTextField(20);
        JButton okButton = new JButton("OK");
        JButton cancelButton = new JButton("Cancel");
        // Add listeners
        inputTextField.addActionListener(new TextFieldHandler());
        okButton.addActionListener(new OkButtonHandler());
        cancelButton.addActionListener(new CancelButtonHandler());
        // Create a panel to contain the sub-components
        JPanel panel = new JPanel();
        // Add the sub-components to the panel
        panel.add(promptLabel);
        panel.add(inputTextField);
        panel.add(okButton);
        panel.add(cancelButton);
        frame.add(panel, BorderLayout.NORTH);
        frame.validate();
        frame.setVisible(true);
    private class TextFieldHandler implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            // userInput is given a value here so it is no longer null
            userInput = event.getActionCommand();
            // works fine if the user input is shown here.
            System.out.println("userInput = " + userInput);
            JOptionPane.showMessageDialog(frame, userInput);
    private class OkButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            // userInput is given a value here so it is no longer null
            userInput = inputTextField.getText();
            // works fine if the user input is shown here.
            System.out.println("userInput = " + userInput);
            JOptionPane.showMessageDialog(frame, userInput);
    private class CancelButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            System.exit(0);
    public static void main(String[] args) {
        new T();
}