Inheritance and method definition

Question 1

What are the 9 forms of inheritance?

Answer 1

Question 2

What is specialization?

Answer 2

Question 3

What is specification?

Answer 3

Question 4

What is construction?

Answer 4

Question 5

What is generalization?

Answer 5

Question 6

What is extension?

Answer 6

Question 7

What is limitation?

Answer 7

Behaviour of subclass is smaller or more restrictive than parent (excludes some operations)

• Often occurs when working with base classes that can’t be modified (similar to generalization in that sense)
• e.g. taking a list and making other data structures out of it via inheritance (e.g. a queue, a stack) – Need to turn OFF features we don’t want used

Question 8

What is containment?

Answer 8

• Specialization (#1): is-a
• Containment: has-a
• Suppose UnprintableCharAtom has a CharAtom x as a data member.
• x can be private.
• We just don’t allow access to print() method

Question 9

What is variance?

Answer 9

Question 10

What is combination?

Answer 10

Question 11

What does the stack frames look like for (instance of B is a subclass of A):

• Instance of A
  
  • x = 55
  • y = “Hello”
• Instance of B
  
  • x = 77
  • y = “Pizza”
  • z = 1.135

Answer 11

Question 12

Pointer-based objects (C++/Java) - someA is a pointer-to-A, someB is a pointer-to-B. Draw the the stack frame of what happens when we make someA point to a B instance.

Answer 12

Question 13

In Stack-Based or Static Objects (C++ only), what happens if we try someA = SomeB?

Answer 13

Question 14

For Stack-Based or Static objects(C++ only)

What happens if we try to make someB = someA?

Answer 14

Question 15

In the following code, what does the following code snippets do?:

1. private: int x;
2. :Base(initX)
3. What does Base::print()

Answer 15

1. making int x private means it is Base’s job to manipulate it
2. the :Base(initX) causes Base’s constructor to be invoked directly from Derived’s
3. Base::print() invokes Base’s print method from within Derived’s

Question 16

What does the following code output?

Answer 16

Question 17

In the following code what do the following lines of code mean?:

1. private int x;
2. super(initX)
3. super.print()

Answer 17

1. private means it’s Parent’s job to manipulate that variable
2. sends the variable to the super constructor
3. uses the super classes’ print method

Question 18

What is the output of the following code?:

Answer 18

Question 19

In general, what are the 4 things subclasses can do?

Answer 19

Question 20

What is the difference between replacement and inheritance?

Answer 20

Inheritance: If a subclass recieves a message for which it has no definition, the message is passed to the superclass.

Replacement (see below):

Question 21

What is Shadowing?

Answer 21

• There are subtle differentces between these
• In shadowing, the signatures are the same in parent and child methods, but the method is NOT virtual(no polymorphism)
• Similar to the concept of shadowing data members, which exists in many languages, like in Java.

Question 22

For the following classes, what does the attached code print and what is happening?

class Parent {

public int x = 12;

public void doIt( ) { System.out.println(“In Parent!”);}

}

class Child extends Parent{

public int x = 6;

public void doIt( ) { System.out.println(“In Child!”);}

}

Answer 22

Data members are shadowed: Child’s x shadows the parent’s x. Can bypass child’s shadowing data by referring to object throught he parent type. Done at compile time - compiler “knows” which x to refer to by pointer type being used.

p.doIt( ) still prints “in child” because it’s polymorphic, java is always looking at the dynamic type (the variable’s data) and routing the message there - you can’t turn this off!

Question 23

What does the following code do and why?

If we made the methods virtual, would they be polymorphic?

Answer 23

• Only if we were using heap-based variables - then we would print “Child”, even when accessing via parent variable P, as we saw in the Java example.
• However, then we’d be doing OVERRIDING - where the type signatures in parent and child are the same but the method IS virtual.
  
  • Java methods are always overridden rather than shadowed, because there is always polymorphism
• remember “virtual” in C++ does not amke an abstract class - only means we can use polymorphism via pointer-based variables
  
  • i.e. I can still make a direct instance of Parent that would use Parent’s method

Question 24

How do you force or forbid overriding?

Answer 24

• Recall that virtual in C++ permits overring (as opposed to shadowing)
  
  • Unless it’s pure virtual though, it’s not an abstract method: it doesn’t force the coder to properly override everything
• Abstract in Java forces overriding
• Forbiding overriding:
  
  • Java:
    
    • public final void aMethod(int{…}//in the parent class prevents implemenation in the child class
      
      • a class itself can be made final by final class Parent{…
  • C++ (C++ 11 is required):
    
    • virtual void aMethod() final {…}
    • class Parent final { //attempting to extend the Parent class is illegal

Question 25

What is redefinition?

Answer 25

• If the type signatre of a given method name differs from parent to child, the we’re doing REDEFINITION
  
  • Same method because it’s the same name, but obviously not the same definintion because the arguments don’t match

Question 26

What are the 3 types of replacement? What are the differences between them?

Answer 26

Shadowing, Overriding and redefinition

Question 27

Provide examples how there can be both good and poor types of inheritance.

Answer 27

Question 28

What is a covariant argument?

Answer 28

Question 29

What is contravariant argument?

Answer 29

Question 30

Show an example of Covariance and Contravariance:

Animal class has a method isBigger(Animal a)

Dog (subclass of Animal) isBigger(<state> )</state>

Answer 30

Question 31

Explain covariance and contravariance with the following return types:

Answer 31

Question 32

What is co/contravariance technically?

Answer 32

redefinition (because it’s a different signature)

Question 33

In general how would you implement a comparison function in an abstract class?

Answer 33

You cannot redefine these method implementations in the subclasses, your subclass methods must also take superclass type to compare to.

Question 34

What does C++ allow with regard to covariance that Java does not?

Answer 34

C++ allows differing return types, however, the subclass return type needs to be a subset of those produced by the parent.

Question 35

In general, when should you use covariant return types when overriding methods?

Answer 35

In general you should always try to follow conventions of nonvariance, even if a languages allows otherwise

• mostly for understanability

Question 36

What does the following code do? How are the subclasses working with the constructor of the parent class?

Answer 36

Question 37

In C++ what is the syntax for invoking a method from the superclass?

Answer 37

Question 38

What does inheriting publicly mean in C++?

Answer 38

Question 39

What responsibility does an object have with regard to the data members it defines?

Answer 39

Question 40

How are superclass constructors and methods called in Java?

Answer 40

Question 41

What is the syntax for self-reference in Java and C++?

Answer 41

Java: this.x = x

C++: this->x = x

Question 42

What should method names start with?

Answer 42

a descriptive verb:

insertData(), print()

Question 43

If you are outside of a class trying to refer to a static member, what is the syntax?

Answer 43

Java: ClassName.memberName

C++: ClassName::memberName

Question 44

Since Java cannot have global variables, what is the only method for creating an equivalent to a global variable/constant in a non-oo programming language?

Answer 44

ClassName.memberName

Question 45

How do you define a static variable and method in Java?

Answer 45

Question 46

What is the differnce between shadowing and overriding

Answer 46

Shadowing overrides based on scope, so if there’s a parent and child class with x and we call c.x or p.x it will print that scopes x. However if we make p = c and then call p.x it will not print c.x, it will print p.x instead. Overriding is a method that runs in the subclass instead of the parent class because we wrote the same method in the subclass as well.

Question 47

In general what is a static variable or method

Answer 47

Question 48

How do you create a static data member in C++

Answer 48

Question 49

How do you refer to a static variable in C++?

Answer 49

Question 50

What happens if you fail to initialize the static variable in C++?

Answer 50