Most of the time JAXB's mapping of XML Schema types to Java datatypes will meet your expectations, and the standard conversion of values between binary and string is just what you need. In those rare cases where this is not adequate, it is possible to customize the datatype binding. We'll illustrate this by a simple example where a xsd:simpleType for roman numbers is defined like this:

<xsd:simpleType name="RomanNumberType">
  <xsd:restriction base="xsd:string">
    <xsd:pattern value="M*D?C{,4}L?X{,4}V?I{,4}"/>
    <xsd:minLength value="1"/>
  </xsd:restriction>
</xsd:simpleType>

package util.roman;

import java.util.HashMap;
import java.util.Map;

public class RomanNumberConverter {
    private static Map<Character,Integer> rom2int =
        new HashMap<Character,Integer>();
    private static Map<Integer,Character> int2rom =
        new HashMap<Integer,Character>();
    private static int[] digits = new int[]{ 1000, 500, 100, 50, 10, 5, 1 };

    static {
        rom2int.put( 'I',    1 );
        rom2int.put( 'V',    5 );
        rom2int.put( 'X',   10 );
        rom2int.put( 'L',   50 );
        rom2int.put( 'C',  100 );
        rom2int.put( 'D',  500 );
        rom2int.put( 'M', 1000 );
        for( Map.Entry<Character,Integer> entry: rom2int.entrySet() ){
	    int2rom.put( entry.getValue(), entry.getKey() );
	}
    }

    public static int parseStringToInt( String value ){
        int result = 0;
        for( int i = 0; i < value.length(); i++ ){
	    result += rom2int.get( value.charAt( i ) );
	}
        return result;
    }

    public static String printIntToString( int value ){
	StringBuilder sb = new StringBuilder();
        for( int d: digits ){
	    while( value > d ){
		value -= d;
                sb.append( int2rom.get( d ) );
	    }
	}
        return sb.toString();
    }
}