Not really sure what you are asking. A combo box is not designed to select text. It contains a finite number of items. You select an item, not the partial text of the item.
Read the Swing tutorial on [url http://java.sun.com/docs/books/tutorial/uiswing/components/combobox.html]How to Use Combo Boxes[/url].
If you have your combo box has strings as items (which i assume, because you said you want to select text...) you simply write an ActionListener like you would do for a JButton. in actionPerformed you cast the source to JComboBox and getSelectedIdem() then returns your String (cast to string)...like this:
public void actionPerformed(ActionEvent aE) {
JComboBox combo = (JComboBox)eA.getSource();
String YourString = (String)combo .getSelectedItem();
//pass YourString to method or something
}
In that case you should think about using an inputVerifier
http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/InputVerifier.html
which has a method that gets fired when someone changes the combo...
in that method you can cast the source to JComboBox and then use setSelectedIdem(obj) to select the item of your choice...
JComboBox api:
http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/JComboBox.html