UML22 — Section: 7

The property isLeaf as inherited by Class from RedefinableElement deals with the concept of redefinition in the context of a classifier. The concept of "this class cannot be subclassed" is missing from UML 2.0 and the current version of UML 2.1. In UML 1.4, the isLeaf property is present in two contexts: Operation and GeneralizableElement. The former refers to the concept of redefinition while the later refers to the concept of subclassing. In UML 2.1, isLeaf from RedefinableElement corresponds to the former. There is nothing corressponding to the later. It is clear from the UML 2.1 specification that redefinition of Classes is related to nesting. In the association class->nestedClassifier between Class and Classifier in Figure 7.12, the source end subsets redefinitionContext. The current constraints for RedefinableElement, Classifier and Class give the following interpretation. Let A be a class with nested class A1, B a class with nested class B1, and B be a subclass of A. Then B1 can redefine A1 as long as A1 has isLeaf = false and A1's visibility is not private. B1 can subclass A1 regardless of the the value of isLeaf on A1. In short, subclassing and redefinition are two separate, orthogonal concepts. The concept of isLeaf for subclassing is not present in UML 2.1.

In UML 1.5, isLeaf is used in 3 contexts, not two:
UML 1.5's Operation::isLeaf and Reception::isLeaf in UML 2 correspond to the concept of a redefinable element that cannot be further redefined.
UML 1.5's GeneralizableElement::isLeaf in UML 2 corresponds to the concept of a classifier that cannot be further specialized in a generalization hierarchy. There are several options to add this capability in UML 2 and the two that are least disruptive to the UML 2 specification are:
a) Rename RedefinableElement::isLeaf to RedefinableElement::isFinal
Add Classifier::isLeaf
b) Keep RedefinableElement::isLeaf
Add Classifier::isFinal
c) Keep RedefinableElement::isLeaf
Add Classifier::isFinalSpecialization
Option (a) would break compatibility with UML 2.2 in a really bad way because the original meaning of "isLeaf" is now "isFinal" and there is a completely different meaning assigned to "isLeaf".
Option (b) preserves the UML 2 meaning of "isLeaf" but adds support for the UML 1.x notion of a classifier that cannot be specialized in a generalization hierarchy. However, option (b) creates possible confusion for end users in distinguishing the purpose of isLeaf vs. isFinal.
Option (c) provides the same advantages as option (b) in addition to providing end users a clue about the role of isLeaf vs. isFinalSpecialization.
Since option (b,c) represent an upwardly compatible change w.r.t. UML2.2, it is preferred to option (a) which would not only break compatibility with UML 2.2 but also create a lot of confusion in comparing UML 2.2 vs. UML 2.3 models. The rest of this resolution follows option (c).
Add a property 'isFinalSpecialization' to a Classifier which is the basis for expressing taxonomic relationships among general and specific classifiers.
Specify a package merge transformation for merging Classifier::isFinalSpecialization according to the principle that a resulting classifier is final if either matching classifier is final.