Value Change Listener to JTextField

Question

I want the message box to appear immediately after the user changes the value in the textfield. Currently, I need to hit the enter key to get the message box to pop out. Is

Answer 1

The usual answer to this is "use a DocumentListener". However, I always find that interface cumbersome. Truthfully the interface is over-engineered. It has three methods, for insertion, removal, and replacement of text, when it only needs one method: replacement. (An insertion can be viewed as a replacement of no text with some text, and a removal can be viewed as a replacement of some text with no text.)

Usually all you want is to know is when the text in the box has changed, so a typical DocumentListener implementation has the three methods calling one method.

Therefore I made the following utility method, which lets you use a simpler ChangeListener rather than a DocumentListener. (It uses Java 8's lambda syntax, but you can adapt it for old Java if needed.)

/**
 * 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);
}

Unlike with adding a listener directly to the document, this handles the (uncommon) case that you install a new document object on a text component. Additionally, it works around the problem mentioned in Jean-Marc Astesana's answer, where the document sometimes fires more events than it needs to.

Anyway, this method lets you replace annoying code which looks like this:

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();
    }
});

With:

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

Code released to public domain. Have fun!