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