public int indexOf(CstString string) {
    if (string == null) {
      throw new NullPointerException("string == null");
    }

    throwIfNotPrepared();

    StringIdItem s = strings.get(string);

    if (s == null) {
      throw new IllegalArgumentException("not found");
    }

    return s.getIndex();
  }
Gets the index of the given string, which must have been added to this instance.

@param string {@code non-null;} the string to look up
@return {@code >= 0;} the string's index
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/file/StringIdsSection.java