check
public java.lang.String check(javax.swing.JComponent component)
Returns a non-null value if the check failed. Sometimes the return value
 is used (for example, it is displayed as a tooltip for menu-items);
 sometimes it is not (for example, toolbar buttons don't do anything with
 the return value). An advanced use of an EnableCheck is simply to change
 some property of a menu item (such as the text), as it is called when
 menu items are displayed.
- Specified by:
 
check in interface EnableCheck 
- Returns:
 - an error message if the check failed, or null if the check
              passed