code: @Override
      public void validateDefault(InputProblemReporter rep, boolean normalize)
          throws XMLStreamException
      {
          String defValue = mDefValue.getValue();
          int len = defValue.length();
  
          // Then code similar to actual value validation:
          StringBuilder sb = null;
          int count = 0;
          int start = 0;
  
          main_loop:
          while (start < len) {
              char c = defValue.charAt(start);
  
              // Ok, any white space to skip?
              while (true) {
                  if (!WstxInputData.isSpaceChar(c)) {
                      break;
                  }
                  if (++start >= len) {
                      break main_loop;
                  }
                  c = defValue.charAt(start);
              }
  
              int i = start+1;
  
              do {
                  if (++i >= len) {
                      break;
                  }
                  c = defValue.charAt(i);
              } while (!WstxInputData.isSpaceChar(c));
              ++count;
              String token = defValue.substring(start, i);
              int illegalIx = WstxInputData.findIllegalNmtokenChar(token, mCfgNsAware, mCfgXml11);
              if (illegalIx >= 0) {
                  reportValidationProblem(rep, "Invalid default value '"+defValue
                                          +"'; character #"+illegalIx+" ("
                                          +WstxInputData.getCharDesc(defValue.charAt(illegalIx))
                                          +") not a valid NMTOKENS character");
              }
  
              if (normalize) {
                  if (sb == null) {
                      sb = new StringBuilder(i - start + 32);
                  } else {
                      sb.append(' ');
                  }
                  sb.append(token);
              }
              start = i+1;
          }
  
          if (count == 0) {
              reportValidationProblem(rep, "Invalid default value '"+defValue
                                      +"'; empty String is not a valid NMTOKENS value");
              return;
          }
  
          if (normalize) {
              mDefValue.setValue(sb.toString());
          }
      }
docs: Method called by the validator
  to ask attribute to verify that the default it has (if any) is
  valid for such type.