code: public static boolean isMixedCase(final CharSequence cs) {
          if (isEmpty(cs) || cs.length() == 1) {
              return false;
          }
          boolean containsUppercase = false;
          boolean containsLowercase = false;
          final int sz = cs.length();
          for (int i = 0; i < sz; i++) {
              if (containsUppercase && containsLowercase) {
                  return true;
              } else if (Character.isUpperCase(cs.charAt(i))) {
                  containsUppercase = true;
              } else if (Character.isLowerCase(cs.charAt(i))) {
                  containsLowercase = true;
              }
          }
          return containsUppercase && containsLowercase;
      }
docs: <p>Checks if the CharSequence contains mixed casing of both uppercase and lowercase characters.</p>
  
  <p>{@code null} will return {@code false}. An empty CharSequence ({@code length()=0}) will return
  {@code false}.</p>
  
  <pre>
  StringUtils.isMixedCase(null)    = false
  StringUtils.isMixedCase("")      = false
  StringUtils.isMixedCase("ABC")   = false
  StringUtils.isMixedCase("abc")   = false
  StringUtils.isMixedCase("aBc")   = true
  StringUtils.isMixedCase("A c")   = true
  StringUtils.isMixedCase("A1c")   = true
  StringUtils.isMixedCase("a/C")   = true
  StringUtils.isMixedCase("aC\t")  = true
  </pre>
  
  @param cs the CharSequence to check, may be null
  @return {@code true} if the CharSequence contains both uppercase and lowercase characters
  @since 3.5