In object oriented design, what is an abstract class, and how does it relate to direct instances and its descendants?

Introduction / Context:
An abstract class is a core concept in object oriented programming used to define a common contract and shared behavior while preventing direct instantiation. It serves as a template for subclasses that implement or specialize the abstract behavior.

Given Data / Assumptions:

• The term abstract class refers to languages such as Java, C#, C++, and similar paradigms.
• Abstract classes may include abstract methods and concrete methods.
• Subclasses are permitted to create objects if they satisfy the abstract requirements.

Concept / Approach:
By definition, an abstract class cannot be instantiated directly. Instead, its concrete subclasses are responsible for providing implementations of abstract members and then can be instantiated. This enforces a design where common structure is centralized, but variability is left to descendants.

Step-by-Step Solution:

Verification / Alternative check:
Attempting to instantiate an abstract class in mainstream languages leads to compile time errors, confirming the rule.

Why Other Options Are Wrong:

• Options stating an abstract class has direct instances contradict the definition.
• Options denying instantiation for descendants are incorrect because the whole purpose is to allow concrete subclasses to be created.

Common Pitfalls:
Confusing abstract classes with interfaces; declaring a class abstract without a clear need; overusing inheritance instead of composition when behavior sharing is minimal.

Final Answer:
A class that has no direct instances, but whose descendants may have direct instances.