code: public double[] getDoubleList(final String propertyName) {
          if (propertyName == null) {
              throw new NullPointerException("propertyName must not be null.");
          }
          Integer index = propertyMap.get(propertyName);
          if (index == null) {
              throw new IllegalArgumentException("non existent property: '"
                      + propertyName + "'.");
          }
          return data[index].clone();
      }
docs: Gets a property value list of this element.
  <p>If the property is not list-property, a list with a single
  element is returned.</p>
  @param propertyName Name of the property.
  @return Values of the current element, each casted to {@code double}.
  @throws NullPointerException if {@code propertyName} is {@code null}.
  @throws IllegalArgumentException if the element type does not have
  a property with the given name.