code: @SuppressWarnings("unchecked")
      public <T> T getExtraPropertyValue(@Nonnull final ExtProp pExtraPropName)
      {
          ExtraPropertiesExtension extraProps = project.getExtensions().getByType(ExtraPropertiesExtension.class);
          if (extraProps.has(pExtraPropName.getPropertyName())) {
              return (T) extraProps.get(pExtraPropName.getPropertyName());
          }
          throw new GradleException(
              "Reference to non-existent project extra property '" + pExtraPropName.getPropertyName() + "'");
      }
docs: Read the value of an extra property of the project.
  
  @param pExtraPropName the reference to the extra property name
  @param <T> type of the property value
  @return the property's value