Computer Science : Class Inheritance

Study concepts, example questions & explanations for Computer Science

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Class Inheritance

class Pet {
public:
    Pet() {}
    virtual void bar() {cout << "In pet bar(); }
};


class Cat : public Pet {
public:

    virtual void eat() {cout << "Cat eating"; }

    virtual void bar() {cout << "In Cat bar()"; }

};

Given the above classes, what would the result of:

int main(){

    Cat felix;

    Pet peeve;

    peeve = felix;

    peeve.bar();

}

Possible Answers:

Runtime error

The program runs and prints: In Cat bar() 

The program runs and prints: In Pet Bar()

None of the above

Compilation Error

Correct answer:

The program runs and prints: In Pet Bar()

Explanation:

The Cat class is inherited from the Pet class and an instance of each object is created in main. 

peeve = felix;

When felix is assigned to peeve, it loses it's atributes as a Cat and now becomes a Pet. When the bar method is called, it will call bar inside the Pet class and not the Cat class. 

If the line of code was:

felix = peeve;

Then, the program will crash. This crashes because we are trying to assign a Pet (which is the parent), to a Cat (which is a child class). Assigning a parent to a child will cause a Slicing Problem because the parent is "bigger" than the child. If you try to put something big (a Pet)  into a small container (a Cat), it will not work.

Example Question #2 : Class Inheritance

class Member{

public: 

    Member() { cout<< 1 ; }

};

 

class Base{

public:

    Base(){ cout<< 1; }

    Memeber member;

};

 

class Derived : public Base{

public:

    Derived(){ cout << 3; }

};

int main(){

  Derived der;

}

Given the code above, what is the output of the program?

Possible Answers:

123

312

321

213

132

Correct answer:

123

Explanation:

When an object is created the order of which everything is created is as follows:

1. Member variables are created first.

2. The constructor of the parent class called (if the object has a parent)

3. The object's own constructor is called.

 

Following the code, we can see that a Derived object is created in main. Since the Derived class has a parent, the call order jumps to the Base class. Now we are inside the Base class, nothing has been outputted yet. Inside the Base class, the local variables are created first. This brings us to the Member class.

In the Member class, the constructor is called and 1 is outputted.

After this, we jump back to the constructor of the Base class and 2 is outputted.

Finally, we get back to where we started, the Derived class. The constructor of derived is called and 3 is outputted. The final output is 123.

Example Question #21 : Object Oriented Program Design

Consider the following code:

public class Rectangle {

     private double width, height;

     public Rectangle(double w,double h) {

          width = w;

          height = h;

     }

    

     public double getArea() {

          return width * height;

     }

    

     public double getPerimeter() {

          return 2 * width + 2 * height;

     }

}

 

public class Square {

     private double side;

     public Square(double s) {

          side = s;

     }

     public double getArea() {

          return side * side;

     }

     public double getPerimeter() {

          return 4 * side;

     }

}

Which of the following represents a redefinition of Square that utilizes the benefits of inheritance?

Possible Answers:

public class Square implements Rectangle {

     public Square(double s) {

          super(s,s);

     }

}

public class Square extends Rectangle {

     public Square(double s) {

         new Rectangle(s,s);

     }

}

public class Square {

     private Rectangle r;

     public Square(double s) {

          r = new Rectangle(s,s);

     }

     public double getArea() {

          return r.getArea();

     }

     public double getPerimeter() {

          return r.getPerimeter();

     }

}

public class Square extends Rectangle {

     private double side;

     public Square(double s) {

          super(s,s);

     }

}

public class Square extends Rectangle {

     public Square(double s) {

          super(s,s);

     }

}

Correct answer:

public class Square extends Rectangle {

     public Square(double s) {

          super(s,s);

     }

}

Explanation:

We know that a square really is just a subclass of a rectangle, for it is merely a rectangle having four sides that are all equal. Using inheritance, you can very easily reuse much of your Rectangle code. First, you need to extend the Rectangle class:

public class Square extends Rectangle { . . .

Next, you can be rid of the field side. This allows you to alter the constructor for Square to call the Rectangle constructor. You do this using super (because Rectangle is the superclass).

After this, you can delete the getArea and getPerimeter methods, for they will be handled by the superclass. This gives you a very simple bit of code!

Example Question #15 : Class Design

Which of the following is TRUE about the Object class?

Possible Answers:

Object has one superclass, Entity.

Every class has all of Object's methods.

Object inherits a minimum of four classes.

Object is the only class that never has any descendants.

None of these answers are true.

Correct answer:

Every class has all of Object's methods.

Explanation:

Object is the most basic class which all others, even user made ones, inherit. Therefore, all classes have Object's methods. A way to think of classes is to think of a tree: the Object class is the lowest node on the tree, where all other nodes can connect back to.

Example Question #2 : Class Inheritance

True or False.

The class BetterMan inherits from the class Man.

 

public class BetterMan extends Man {

 

}

Possible Answers:

True

False

Correct answer:

True

Explanation:

The class BetterMan inherits methods from the class man. The keyword "extends" means that BetterMan will get all the methods from Man plus be able to extend the class by adding its own methods. All methods from Man can be used in BetterMan by calling the keyword better.

Example Question #21 : Object Oriented Program Design

What does the code print?

class Parent{
    final public void show() {
       System.out.println("Parent::show() called");
    }
}
  
class Child extends Parent {
    public void show() {
       System.out.println("Child::show() called");
    }
}
  
public class Main {
    public static void main(String[] args) {
        Parent parent = new Child();
        parent.show();
    }
}
Possible Answers:

Compiler Error

Parent::show() called

Runtime Error

Parent::show() called

Child::show() called

Child::show() called

 

Correct answer:

Compiler Error

Explanation:

Final methods can't be overriden. So the code won't compile because of this. Now if the final modifier were to be removed. The code would compile and run and produce:

Child::show()

Example Question #1 : Class Inheritance

class Pet {
public:
    Pet() {}
    virtual void bar() {cout << "In pet bar(); }
};


class Cat : public Pet {
public:

    virtual void eat() {cout << "Cat eating"; }

    virtual void bar() {cout << "In Cat bar()"; }

};

Given the above classes, what would the result of:

int main(){

    Pet * petPtr = new Cat();

    petPtr -> eat();

}

Possible Answers:

The program runs and prints: "Pet eating"

The program compiles but has a runtime error

The program runs and prints: "Cat eating"

None of these

Compilation error

Correct answer:

Compilation error

Explanation:

When a child class inherits the properties of a parent class, some attributes are inherited and some aren't. In C++, the constructor of the parent class is not inherited. The Cat class does not have a constructor therefore the program will not compile.

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: