Questions: Inheritance and Subclasses

Question 1

Which pair of classes is the best fit for an inheritance (superclass/subclass) relationship?

• A. Car and Engine
• B. Library and Book
• C. Shape and Circle
• D. Student and Classroom

Answer: (C)

Inheritance models an 'is-a' relationship. A Circle is a Shape, so Circle can extend Shape. A Car has an Engine, a Library has Books, and a Classroom has Students; those are 'has-a' relationships, which are modeled by storing one object inside another (composition), not by inheritance.

Question 2

In Python, class Animal has an __init__ that sets self.name. Class Dog(Animal) defines its own __init__(self, name, breed) that sets only self.breed and never calls super().__init__(name). What happens when code later reads my_dog.name?

• A. It returns the name, because Dog inherits Animal's attributes automatically
• B. It raises an AttributeError, because Animal's __init__ never ran, so name was never set
• C. It returns None, because inherited attributes default to None
• D. It returns the breed, because Dog's __init__ replaces name with breed

Answer: (B)

Defining __init__ in Dog replaces Animal's __init__ for Dog objects. Attributes in Python are created when code assigns them, so if Animal's __init__ never runs, self.name is never assigned. Calling super().__init__(name) inside Dog's __init__ runs the superclass setup and fixes the problem.

Question 3

In Java, a subclass can directly read and assign a private instance variable declared in its superclass.

• T. True
• F. False

Answer: False

Private members are visible only inside the class that declares them. A subclass object still contains that data, but subclass code must reach it through the superclass's public or protected methods (or a protected field), not by name.

Question 4

Two classes, Teacher and Student, both store a name and an email address and both have an identical send_email method. Explain how inheritance improves this design.

Model answer: Create a superclass such as Person that holds name, email and send_email. Teacher and Student then extend Person and add only what is specific to each. The shared code exists once, so a bug fix or change to send_email happens in one place and applies to both subclasses.

Removing duplicated code is the main practical benefit of inheritance. It also passes the 'is-a' test: a Teacher is a Person and a Student is a Person.