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);
        }
    }

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());
    }
});

通常的答案是“使用DocumentListener”。然而，我总是觉得这个界面很麻烦。说实话，这个界面被过度设计了。它有三种方法，用于插入、删除和替换文本，而它只需要一种方法:替换。(插入可以看作是用一些文本替换了没有文本，删除可以看作是用没有文本替换了一些文本。)

通常，您只需要知道框中的文本何时发生了更改，因此典型的DocumentListener实现有三个方法调用一个方法。

因此，我创建了以下实用程序方法，它允许您使用更简单的ChangeListener而不是DocumentListener。(它使用Java 8的lambda语法，但如果需要，您可以根据旧Java对其进行调整。)

/**
 * Installs a listener to receive notification when the text of any
 * {@code JTextComponent} is changed. Internally, it installs a
 * {@link DocumentListener} on the text component's {@link Document},
 * and a {@link PropertyChangeListener} on the text component to detect
 * if the {@code Document} itself is replaced.
 * 
 * @param text any text component, such as a {@link JTextField}
 *        or {@link JTextArea}
 * @param changeListener a listener to receieve {@link ChangeEvent}s
 *        when the text is changed; the source object for the events
 *        will be the text component
 * @throws NullPointerException if either parameter is null
 */
public static void addChangeListener(JTextComponent text, ChangeListener changeListener) {
    Objects.requireNonNull(text);
    Objects.requireNonNull(changeListener);
    DocumentListener dl = new DocumentListener() {
        private int lastChange = 0, lastNotifiedChange = 0;

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

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

        @Override
        public void changedUpdate(DocumentEvent e) {
            lastChange++;
            SwingUtilities.invokeLater(() -> {
                if (lastNotifiedChange != lastChange) {
                    lastNotifiedChange = lastChange;
                    changeListener.stateChanged(new ChangeEvent(text));
                }
            });
        }
    };
    text.addPropertyChangeListener("document", (PropertyChangeEvent e) -> {
        Document d1 = (Document)e.getOldValue();
        Document d2 = (Document)e.getNewValue();
        if (d1 != null) d1.removeDocumentListener(dl);
        if (d2 != null) d2.addDocumentListener(dl);
        dl.changedUpdate(null);
    });
    Document d = text.getDocument();
    if (d != null) d.addDocumentListener(dl);
}

与直接向文档添加侦听器不同，这可以处理在文本组件上安装新文档对象的情况(不常见)。此外，它还解决了Jean-Marc Astesana回答中提到的问题，即文档有时会触发超出所需的事件。

不管怎样，这个方法可以让你替换看起来像这样烦人的代码:

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

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

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

:

addChangeListener(someTextBox, e -> doSomething());

代码发布到公共领域。玩得开心!

向底层Document中添加侦听器，底层Document是自动为您创建的。

// 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 (Integer.parseInt(textField.getText())<=0){
       JOptionPane.showMessageDialog(null,
          "Error: Please enter number bigger than 0", "Error Message",
          JOptionPane.ERROR_MESSAGE);
     }
  }
});

一种优雅的方法是将侦听器添加到插入符号位置，因为每次键入/删除内容时它都会更改，然后只需将旧值与当前值进行比较。

String oldVal = ""; // empty string or default value
JTextField tf = new JTextField(oldVal);

tf.addCaretListener(e -> {
    String currentVal = tf.getText();

    if(!currentVal.equals(oldVal)) {
        oldVal = currentVal;
        System.out.println("Change"); // do something
    }
});

(此事件也被触发每次用户只是点击进入TextField)。

只需创建一个扩展DocumentListener并实现所有DocumentListener方法的接口:

@FunctionalInterface
public interface SimpleDocumentListener extends DocumentListener {
    void update(DocumentEvent e);

    @Override
    default void insertUpdate(DocumentEvent e) {
        update(e);
    }
    @Override
    default void removeUpdate(DocumentEvent e) {
        update(e);
    }
    @Override
    default void changedUpdate(DocumentEvent e) {
        update(e);
    }
}

然后:

jTextField.getDocument().addDocumentListener(new SimpleDocumentListener() {
    @Override
    public void update(DocumentEvent e) {
        // Your code here
    }
});

或者你甚至可以使用lambda表达式:

jTextField.getDocument().addDocumentListener((SimpleDocumentListener) e -> {
    // Your code here
});