Quiz 6: Inheritance 2

1. It is usually considered a bad design to make KansasCity a subclass of City because:

  • It violates the rule of polymorphism
  • The classification for use of inheritance is premature as KansasCity is an instance of City
  • It violates the Law of Demeter
  • It violates the rule of change

Correct answer: The classification for use of inheritance is premature as KansasCity is an instance of City

Explanation:

Inheritance shouldn't be used when a simple instance would suffice.

2. Defining CarOwner as a subclass of Car violates the:

  • change rule of inheritance
  • rule of reuse
  • polymorphism rule of inheritance
  • rule of modularity

Correct answer: polymorphism rule of inheritance

Explanation:

A car owner is not themselves a car.

3. Given the following code in Java, which statement about methods getAnimalForAdoption and putAnimal in class CatShelter is correct?

//Cat extends Animal

class AnimalShelter {
    Animal getAnimalForAdoption() {}
    void putAnimal(Animal animal) {}
}

class CatShelter extends AnimalShelter {
    Cat getAnimalForAdoption() {}
    void putAnimal(Object animal) {}
}
  • Neither of them is an overriding method
  • Both are overriding methods
  • Neither of them is an overloading method
  • getAnimalForAdoption() is an overriding method while putAnimal() is an overloading method

Correct answer: getAnimalForAdoption() is an overriding method while putAnimal() is an overloading method

Explanation:

With getAnimalForAdoption(), CatShelter promises to return a more specific type of Animal. This follows a covariance relationship.

With putAnimal(), CatShelter agrees to allow a more general argument that may or may not be an Animal. This follows a contravariance relationship.

Note: some languages (including Java/C++) do not allow or put heavy restrictions on use of contravariance.1

4. Subcontracting refers to:

  • inheritance with overriding and dynamic binding, where the overriding rule is followed
  • instance variables of a class are the subcontractors of the class
  • the module view of inheritance
  • a class delegates its task to its clients

Correct answer: inheritance with overriding and dynamic binding, where the overriding rule is followed.

Explanation:

Subcontracting is a form of design by contract that makes certain guarantees about the pre/postcondition behavior of class descendants.

Recall the Subcontracting Rule of Inheritance listed in section 5.3.

Footnotes