In a class hierarchy, which rule applies to sub constructors?

In the context of class hierarchies in ABAP, the principle that the constructor of the super class must be invoked before using instance components in the child class is crucial for maintaining the integrity and expected behavior of the object. When you define a subclass that inherits from a superclass, the subclass can have its own additional attributes and methods, but it is important that the initialization logic defined in the superclass's constructor executes first.

When the superclass constructor is called, it initializes any instance variables and performs any setup required for the character and behavior of the superclass. Skipping this step could lead to situations where the subclass operates on uninitialized components, causing inconsistent behavior or runtime errors.

The requirement to invoke the superclass constructor ensures that the inheritance chain is respected, and that the object can fully rely on the method implementations and state established in the super class. This practice leads to a well-structured object-oriented design in ABAP, allowing for better maintainability and functionality across the class hierarchy.

Once the superclass constructor has been executed, the subclass can safely proceed with its own initialization logic. Thus, the answer correctly identifies the fundamental rule that governs the usage of instance components within subclasses in relation to superclass constructors.