Final

Final.member
marks a member that is not changeable in a subclass. A constructor is always implicitly Final.
Final.Class[type] := definitions
specifies a class that cannot be a superclass.
This loads the package.
In[1]:=
Click for copyable input
First, we define a class that contains a final member.
In[2]:=
Click for copyable input
The attempt to redefine the final member is rejected. Override is used to signal a replacement of functionality of a parent class.
In[3]:=
Click for copyable input
Out[3]=
A whole class may also be sealed. Individual members then need not to be marked Final.
In[4]:=
Click for copyable input
Any trial to subclass this class is rejected.
In[5]:=
Click for copyable input
Out[5]=