为什么我的GUI中没有变量打印？

Question

I am working on a pretty simple GUI at the moment as I am new to Java. I was wondering why my variable won't show up in message .

Works

...
  private void createContents() {
    JLabel numberPrompt = new JLabel("What's your random number (min will be 1)?");
    numberBox = new JTextField(15);
    maxNumberString = numberBox.getText();
    greeting = new JLabel();
    add(numberPrompt);
    add(numberBox);
    add(greeting);
    numberBox.addActionListener(new Listener());
  }

  private class Listener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String message;
      message = "Glad to meet you, " + numberBox.getText(); + "!";
      numberBox.setText("");
      greeting.setText(message);
    }
...

Doesn't work

...
  private void createContents() {
    JLabel numberPrompt = new JLabel("What's your random number (min will be 1)?");
    numberBox = new JTextField(15);
    maxNumberString = numberBox.getText();
    greeting = new JLabel();
    add(numberPrompt);
    add(numberBox);
    add(greeting);
    numberBox.addActionListener(new Listener());
  }

  private class Listener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String message;
      message = "Glad to meet you, " + maxNumberString + "!";
      numberBox.setText("");
      greeting.setText(message);
    }
...

Why is it that the first code works with numberBox.getText() but when I replace it with maxNumberString it doesn't work even though the variable has the value of numberBox.getText() ?

Full Code

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Greeting extends JFrame {
  private static final int WIDTH = 400;
  private static final int HEIGHT = 150;
  private JTextField numberBox;
  private JLabel greeting;
  private String maxNumberString;
  private Integer maxNumber;
  private String randomNumbeString;
  private Integer randomNumber;

  public Greeting() {
    setTitle("Random Number Generator");
    setSize(WIDTH, HEIGHT);
    setLayout(new FlowLayout());
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    createContents();
    setVisible(true);
  }

  private void createContents() {
    JLabel numberPrompt = new JLabel("What's your random number (min will be 1)?");
    numberBox = new JTextField(15);
    maxNumberString = numberBox.getText();
    greeting = new JLabel();
    add(numberPrompt);
    add(numberBox);
    add(greeting);
    numberBox.addActionListener(new Listener());
  }

  private class Listener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String message;
      message = "Glad to meet you, " + maxNumberString + "!";
      numberBox.setText("");
      greeting.setText(message);
    }
  }

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

Answer 1

numberBox = new JTextField(15);
maxNumberString = numberBox.getText();

At the time the above statement is executed, the text field is empty. If fact the frame isn't even visible so the user has had no chance to enter text into the text field.

There is no automatic updating of the variable with the text!

If you want the variable to be automatically updated, then you need to add logic to update the variable every time an event is generated. For example you could add a DocumentListener to the Document of the text field. An event is generated every time text is added (or removed) from the text field. Then you can get the current value of the text field.

Read the section from the Swing tutorial on How to Write a DocumentListner for more information and an example to get you started.