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