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 there anything wrong with my code?

textField.addActionListener(new java.awt.event.ActionListener() {     public void actionPerformed(java.awt.event.ActionEvent e) {          if (Integer.parseInt(textField.getText())<=0){             JOptionPane.showMessageDialog(null,                     "Error: Please enter number bigger than 0", "Error Message",                     JOptionPane.ERROR_MESSAGE);         }            } } 

Any help would be appreciated!

Answer 1

Add a listener to the underlying Document, which is automatically created for you.

// 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);      }   } }); 

Answer 2

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!