ABAP Keyword Documentation → ABAP Programming Guidelines → Architecture → Object-Oriented Programming
Other versions: 7.31 | 7.40 | 7.54
Inheritance is the method by which subclasses are derived from a superclass while inheriting the components
of the superclass. A subclass can be made more specific by declaring new components and redefining instance
methods. ABAP Objects supports simple inheritance, in which a class can have multiple subclasses but
only one direct superclass. (Despite this, the interface concept does enable something like multiple
inheritance to take place, at least with regard to attributes and method declarations. Method implementations,
on the other hand, are not inherited when an interface is included.) This creates an inheritance hierarchy
in an inheritance tree, with a unique path running from each subclass to a root class. In ABAP Objects,
all classes are subclasses of the predefined abstract root class
Final classes (classes defined using the addition
FINAL) close the bottom of a path in the inheritance tree.
Avoid using deep inheritance hierarchies
Avoid using deep inheritance hierarchies, since they are often difficult to maintain.
Deep inheritance hierarchies are examples of successful reuse, but are also the source of maintenance problems, due to the complexity inherent in the large number of classes involved.
- The behavior of classes deep down in the inheritance hierarchy is difficult to predict, since they potentially inherit from a large number of methods.
- Classes with a lot of subclasses exert great influence on the system as a whole, making the consequences of modifications to a superclass hard to predict.
- A large number of subclasses may also indicate an unsuitable level of abstraction.
To prevent unintended reuse of your classes by inheritance, we recommend that you use final classes to close the paths of inheritance trees.
If your main aim is to exploit the possibilities of polymorphy, then interfaces are often a preferable solution to inheritance. If all you want to do is use interfaces, then method interfaces should be used instead of abstract classes. These can be used to create composite interfaces. In ABAP, on the other hand, a composite interface is impossible due to the nature of simple inheritance using classes.