ABAP Keyword Documentation → ABAP - Reference → Declarations → Declaration Statements → Classes and Interfaces → ABAP Objects - Overview → Inheritance
Inheritance and Instantiation
If you instantiate a subclass you instantiate all the superclasses at the same time, whereby the initialization of superclass attributes is ensured by the call of the superclass constructors, as described in Inheritance and Constructors.
For each individual class, the CREATE PUBLIC|PROTECTED|PRIVATE additions
to the CLASS statement control who can create an instance of the class or, in other words, can call its instance constructor.
This has the following consequences:
CREATE PRIVATE
addition, outside users cannot instantiate a subclass, and a subclass cannot even instantiate itself, because it has no access to the instance constructor of the superclass.
It would therefore also be useful to apply the FINAL addition to a class
that was defined using the CREATE PRIVATE addition, in order to prevent a
derivation of subclasses. Otherwise subclasses of such superclasses have the implicit CREATE NONE addition.
The only exception to this rule is if a superclass that can be privately instantiated offers its friendship to its subclasses. The direct route is rarely the case here because the superclass must know its subclasses in order for it to be possible. However, a superclass can also offer friendship to an interface which, in turn, can be implemented by its subclasses.
Conversely, you cannot create objects of subclasses in their superclass, if these are declared using CREATE PROTECTED or CREATE PRIVATE, unless they are
friends of its subclasses.
Other versions: 7.31 | 7.40 | 7.54
Overview of all cases
Superclass with no addition or CREATE PUBLIC
Whether a friend of the superclass or not, subclasses can have anyCREATE
addition. Without addition they inherit the attribute CREATE PUBLIC. The
superclass instance constructor is visible to everyone. The subclass controls the visibility of its own instance constructor, independently of the superclass.
Superclass with CREATE PROTECTED addition.
Whether a friend of the superclass or not, subclasses can have anyCREATE
addition. Without addition they inherit the attribute CREATE PROTECTED. The superclass allows its subclasses unlimited instantiation and therefore also the publishing of its
protected instance constructor.
Superclass with CREATE PRIVATE addition
Subclass is not a friend of the superclass
The subclass has the implicit addition CREATE NONE. Because nobody other
than the superclass itself can call its instance constructor, the subclass cannot be instantiated. None
of the CREATE additions is permitted, because this would always lead to the unauthorized publishing of the superclass constructor.
Subclass is a friend of the superclass
If the subclass has no addition, it inherits the attribute CREATE PRIVATE.
However, all CREATE additions are permitted. As a friend, the subclass can publish the superclass's private constructor in any form.