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