code: public void setPerMillString(String perMillString) {
          if (perMillString == null) {
              throw new NullPointerException("The input permille string is null");
          }
          this.perMillString = perMillString;
          if (perMillString.length() == 1) {
              this.perMill = perMillString.charAt(0);
          } else {
              // Use the default permille character as fallback
              this.perMill = DEF_PERMILL;
          }
      }
docs: <strong>[icu]</strong> Sets the string used for permille sign.
  <p>
  <b>Note:</b> When the input permille String is represented
  by multiple Java chars, then {@link #getPerMill()} will
  return the default permille character ('&#x2030;').
  
  @param perMillString the permille string
  @throws NullPointerException if <code>perMillString</code> is null.
  @see #getPerMillString()
  @hide draft / provisional / internal are hidden on Android