In this example you set ‘value’ in the MXML implicitly by using the setter. This way you could actually remove the not so nice violation of the interface specification in your constructor. Note, that you offer a Number parameter in your constructor, whereas your base class, the ShaderFilter, offers a Shader. If someone now would want to code against the generic ShaderFilter and use your concrete filter class, he couldn’t use the constructors value argument, because it is not specified in ShaderFilter, so it is better to offer ‘value’ via setter/getter, as your are using it in the MXML anyway.