The sample "OK/Cancel Dialog" defines the constants RET_CANCEL and RET_OK while
it should use the standard constants of JOptionPane that are defined for the
same purpose JOptionPane.CANCEL_OPTION and JOptionPane.OK_OPTION.

Moreover, the method closeDialog(...) should return JOptionPane.CLOSED_OPTION
instead of RET_CANCEL and so does the default value of "returnStatus".

The use of the constants of JOptionPane is preferred so the users of this class
can use the same code for for both "standard" JOptionPane and the sample dialog:
<code>
if (result == JOptionPane.OK_OPTION) {
    ....
}
</code>