code: public void setMonetaryGroupingSeparatorString(String sep) {
          if (sep == null) {
              throw new NullPointerException("The input monetary grouping separator is null");
          }
          this.monetaryGroupingSeparatorString = sep;
          if (sep.length() == 1) {
              this.monetaryGroupingSeparator = sep.charAt(0);
          } else {
              // Use default grouping separator character as fallback
              this.monetaryGroupingSeparator = DEF_GROUPING_SEPARATOR;
          }
      }
docs: <strong>[icu]</strong> Sets the monetary grouping separator string.
  <p>
  <b>Note:</b> When the input grouping separator String is represented
  by multiple Java chars, then {@link #getMonetaryGroupingSeparator()} will
  return the default monetary grouping separator character (',').
  
  @param sep the monetary grouping separator string
  @throws NullPointerException if <code>sep</code> is null.
  @see #getMonetaryGroupingSeparatorString()
  @hide draft / provisional / internal are hidden on Android