code: public void setMinusSignString(String minusSignString) {
          if (minusSignString == null) {
              throw new NullPointerException("The input minus sign is null");
          }
          this.minusString = minusSignString;
          if (minusSignString.length() == 1) {
              this.minusSign = minusSignString.charAt(0);
          } else {
              // Use the default minus sign as fallback
              this.minusSign = DEF_MINUS_SIGN;
          }
      }
docs: <strong>[icu]</strong> Sets the string used to represent minus sign.
  <p>
  <b>Note:</b> When the input minus sign String is represented
  by multiple Java chars, then {@link #getMinusSign()} will
  return the default minus sign character ('-').
  
  @param minusSignString the minus sign string
  @throws NullPointerException if <code>minusSignString</code> is null.
  @see #getGroupingSeparatorString()
  @hide draft / provisional / internal are hidden on Android