code: public static String toCamelCase(String str) {
          return new WordTokenizer() {
              @Override
              protected void startSentence(StringBuilder buffer, char ch) {
                  buffer.append(Character.toLowerCase(ch));
              }
  
              @Override
              protected void startWord(StringBuilder buffer, char ch) {
                  if (!isDelimiter(buffer.charAt(buffer.length() - 1))) {
                      buffer.append(Character.toUpperCase(ch));
                  } else {
                      buffer.append(Character.toLowerCase(ch));
                  }
              }
  
              @Override
              protected void inWord(StringBuilder buffer, char ch) {
                  buffer.append(Character.toLowerCase(ch));
              }
  
              @Override
              protected void startDigitSentence(StringBuilder buffer, char ch) {
                  buffer.append(ch);
              }
  
              @Override
              protected void startDigitWord(StringBuilder buffer, char ch) {
                  buffer.append(ch);
              }
  
              @Override
              protected void inDigitWord(StringBuilder buffer, char ch) {
                  buffer.append(ch);
              }
  
              @Override
              protected void inDelimiter(StringBuilder buffer, char ch) {
                  if (ch != UNDERSCORE) {
                      buffer.append(ch);
                  }
              }
          }.parse(str);
      }
docs: 将字符串转换成camel case。
  <p>
  如果字符串是<code>null</code>则返回<code>null</code>。
  <p/>
  <p/>
  <pre>
  StringUtil.toCamelCase(null)  = null
  StringUtil.toCamelCase("")    = ""
  StringUtil.toCamelCase("aBc") = "aBc"
  StringUtil.toCamelCase("aBc def") = "aBcDef"
  StringUtil.toCamelCase("aBc def_ghi") = "aBcDefGhi"
  StringUtil.toCamelCase("aBc def_ghi 123") = "aBcDefGhi123"
  </pre>
  <p/>
  </p>
  <p>
  此方法会保留除了下划线和空白以外的所有分隔符。
  </p>
  
  @param str 要转换的字符串
  
  @return camel case字符串，如果原字符串为<code>null</code>，则返回<code>null</code>