Validating a JTextField When Permanently Losing the Focus
This example demonstrates a text field that validates its contents
when it receives a permanent focus-lost event.
If the contents are invalid, it displays a modal
dialog with an error message and regains the focus.
JTextField component = new JTextField(10);
component.addFocusListener(new MyFocusListener());
public class MyFocusListener extends FocusAdapter {
boolean showingDialog = false;
public void focusGained(FocusEvent evt) {
final JTextComponent c = (JTextComponent)evt.getSource();
String s = c.getText();
// Position the caret at the 1st non-digit character
for (int i=0; i<s.length(); i++) {
// Ensure validity
if (!Character.isDigit(s.charAt(i))) {
c.setSelectionStart(i);
c.setSelectionEnd(i);
break;
}
}
}
public void focusLost(FocusEvent evt) {
final JTextComponent c = (JTextComponent)evt.getSource();
String s = c.getText();
if (evt.isTemporary()) {
return;
}
for (int i=0; i<s.length(); i++) {
// Ensure validity
if (!Character.isDigit(s.charAt(i))) {
// Find top-level window
Component par = c;
while (par.getParent() != null) {
par = par.getParent();
}
final Frame frame = (Frame)par;
// Create and display an error message
JOptionPane optionPane = new JOptionPane("The value must only contain digits",
JOptionPane.ERROR_MESSAGE, JOptionPane.DEFAULT_OPTION);
optionPane.createDialog(frame, null).show();
// Regain the focus
c.requestFocus();
break;
}
}
}
}
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