an interface that must be implemented by caller to
provide context information that is necessary to
perform conversion from value object to the XML representation.
get namespace prefix for the given namespace URI.
this method is used to convert QName.
It is a responsibility of the callee to ensure that
the returned prefix is properly declared.
Returns:
prefix for this namespace URI.
return null to indicate that this namespace URI is the
default name space. In this case, QNames are converted into
unqualified names (just the local names only as NCNames).