Both TextArea and TextField can install a
TextListener that catches TextEvents.
TextComponents fire TextEvents every time
the text of the component changes. This is more or less every time
the user hits a key in the component.
The java.awt.event.TextListener interface defines a
single method, textValueChanged():
public void textValueChanged(TextEvent te)
You register a TextListener with a
TextComponent by calling the component's
addTextListener() method. For example,
TextArea password = new TextArea(24)
password.addTextListener(new PasswordChecker());
However, most of the time it's sufficient to just get and set the text as you need it. It's really quite rare that you need to process it character by character.
A TextListener can be removed by calling
removeTextListener().
public void removeTextListener(TextListener tl)