code: public static String lowerCase(final String str, final Locale locale) {
          if (str == null) {
              return null;
          }
          return str.toLowerCase(locale);
      }
docs: <p>Converts a String to lower case as per {@link String#toLowerCase(Locale)}.</p>
  
  <p>A {@code null} input String returns {@code null}.</p>
  
  <pre>
  StringUtils.lowerCase(null, Locale.ENGLISH)  = null
  StringUtils.lowerCase("", Locale.ENGLISH)    = ""
  StringUtils.lowerCase("aBc", Locale.ENGLISH) = "abc"
  </pre>
  
  @param str  the String to lower case, may be null
  @param locale  the locale that defines the case transformation rules, must not be null
  @return the lower cased String, {@code null} if null String input
  @since 2.5