Be aware that when the user modify the field, the DocumentListener can, sometime, receive two events. For instance if the user selects the whole field content, then press a key, you'll receive a removeUpdate (all the content is remove) and an insertUpdate. In your case, I don't think it is a problem but, generally speaking, it is. Unfortunately, it seems there's no way to track the content of the textField without subclassing JTextField. Here is the code of a class that provide a "text" property :

package net.yapbam.gui.widget;

import javax.swing.JTextField;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;

/** A JTextField with a property that maps its text.
 * <br>I've found no way to track efficiently the modifications of the text of a JTextField ... so I developed this widget.
 * <br>DocumentListeners are intended to do it, unfortunately, when a text is replace in a field, the listener receive two events:<ol>
 * <li>One when the replaced text is removed.</li>
 * <li>One when the replacing text is inserted</li>
 * </ul>
 * The first event is ... simply absolutely misleading, it corresponds to a value that the text never had.
 * <br>Anoter problem with DocumentListener is that you can't modify the text into it (it throws IllegalStateException).
 * <br><br>Another way was to use KeyListeners ... but some key events are throw a long time (probably the key auto-repeat interval)
 * after the key was released. And others events (for example a click on an OK button) may occurs before the listener is informed of the change.
 * <br><br>This widget guarantees that no "ghost" property change is thrown !
 * @author Jean-Marc Astesana
 * <BR>License : GPL v3
 */

public class CoolJTextField extends JTextField {
    private static final long serialVersionUID = 1L;

    public static final String TEXT_PROPERTY = "text";

    public CoolJTextField() {
        this(0);
    }

    public CoolJTextField(int nbColumns) {
        super("", nbColumns);
        this.setDocument(new MyDocument());
    }

    @SuppressWarnings("serial")
    private class MyDocument extends PlainDocument {
        private boolean ignoreEvents = false;

        @Override
        public void replace(int offset, int length, String text, AttributeSet attrs) throws BadLocationException {
            String oldValue = CoolJTextField.this.getText();
            this.ignoreEvents = true;
            super.replace(offset, length, text, attrs);
            this.ignoreEvents = false;
            String newValue = CoolJTextField.this.getText();
            if (!oldValue.equals(newValue)) CoolJTextField.this.firePropertyChange(TEXT_PROPERTY, oldValue, newValue);
        }

        @Override
        public void remove(int offs, int len) throws BadLocationException {
            String oldValue = CoolJTextField.this.getText();
            super.remove(offs, len);
            String newValue = CoolJTextField.this.getText();
            if (!ignoreEvents && !oldValue.equals(newValue)) CoolJTextField.this.firePropertyChange(TEXT_PROPERTY, oldValue, newValue);
        }
    }

使用KeyListener(在任意键上触发)而不是ActionListener(在进入时触发)

textBoxName.getDocument().addDocumentListener(new DocumentListener() {
   @Override
   public void insertUpdate(DocumentEvent e) {
       onChange();
   }

   @Override
   public void removeUpdate(DocumentEvent e) {
      onChange();
   }

   @Override
   public void changedUpdate(DocumentEvent e) {
      onChange();
   } 
});

但我不会仅仅将用户(可能是意外)在键盘上触碰的任何东西解析为Integer。你应该捕获抛出的任何异常，并确保JTextField不是空的。

它是Codemwnci的更新版本。他的代码很好，除了错误信息外，其他方面都很好。为了避免错误，必须更改条件语句。

  // Listen for changes in the text
textField.getDocument().addDocumentListener(new DocumentListener() {
  public void changedUpdate(DocumentEvent e) {
    warn();
  }
  public void removeUpdate(DocumentEvent e) {
    warn();
  }
  public void insertUpdate(DocumentEvent e) {
    warn();
  }

  public void warn() {
     if (textField.getText().length()>0){
       JOptionPane.showMessageDialog(null,
          "Error: Please enter number bigger than 0", "Error Massage",
          JOptionPane.ERROR_MESSAGE);
     }
  }
});

如果我们使用可运行的方法SwingUtilities.invokeLater()，而使用文档侦听器应用程序有时会卡住，需要时间更新结果(根据我的实验)。我们也可以使用KeyReleased事件作为文本字段更改监听器，就像这里提到的那样。

usernameTextField.addKeyListener(new KeyAdapter() {
    public void keyReleased(KeyEvent e) {
        JTextField textField = (JTextField) e.getSource();
        String text = textField.getText();
        textField.setText(text.toUpperCase());
    }
});

这里是@Boann的答案的Kotlin移植，这是一个很好的解决方案，对我来说一直很有效。

import java.beans.*
import javax.swing.*
import javax.swing.event.*
import javax.swing.text.*

/**
 * Installs a listener to receive notification when the text of this
 * [JTextComponent] is changed. Internally, it installs a [DocumentListener] on the
 * text component's [Document], and a [PropertyChangeListener] on the text component
 * to detect if the `Document` itself is replaced.
 *
 * @param changeListener a listener to receive [ChangeEvent]s when the text is changed;
 * the source object for the events will be the text component
 */
fun JTextComponent.addChangeListener(changeListener: ChangeListener) {
    val dl: DocumentListener = object : DocumentListener {
        private var lastChange = 0
        private var lastNotifiedChange = 0
        override fun insertUpdate(e: DocumentEvent) = changedUpdate(e)
        override fun removeUpdate(e: DocumentEvent) = changedUpdate(e)
        override fun changedUpdate(e: DocumentEvent) {
            lastChange++
            SwingUtilities.invokeLater {
                if (lastNotifiedChange != lastChange) {
                    lastNotifiedChange = lastChange
                    changeListener.stateChanged(ChangeEvent(this))
                }
            }
        }
    }
    addPropertyChangeListener("document") { e: PropertyChangeEvent ->
        (e.oldValue as? Document)?.removeDocumentListener(dl)
        (e.newValue as? Document)?.addDocumentListener(dl)
        dl.changedUpdate(null)
    }
    document?.addDocumentListener(dl)
}

你可以在任何文本组件上使用它，如下所示:

myTextField.addChangeListener { event -> myEventHandler(event) }

就像他的代码一样，也是公有领域。