* @param cronDefinition
*/
private void buildPossibleExpressions(CronDefinition cronDefinition) {
List<CronParserField> expression = new ArrayList<CronParserField>();
for(FieldDefinition fieldDefinition : cronDefinition.getFieldDefinitions()){
expression.add(new CronParserField(fieldDefinition.getFieldName(), fieldDefinition.getConstraints()));
}
Collections.sort(expression, CronParserField.createFieldTypeComparator());
expressions.put(expression.size(), expression);
if (cronDefinition.isLastFieldOptional()) {