code: @Override
      public int getElementCount(final String elementType) {
          if (elementType == null) {
              throw new IllegalArgumentException("elementType must not be null.");
          }
          Integer count = elementCounts.get(elementType);
          if (count == null) {
              throw new IllegalArgumentException(
                      "Type does not exist in this file.");
          } else {
              return count;
          }
      }
docs: Gets the number of elements for a given element type.
  @param elementType Name of the element type.
  @return Number of elements of the given type.
  @throws NullPointerException if {@code elementType} is {@code null}.
  @throws IllegalArgumentException if there is no such type in this
  file.