Inheritance and Polymorphism

Question 1

what is ad-hoc pulymorphism?

Answer 1

• Polymorphism is over few (often, very few) shapes
• Different shapes are generated manually (or semi- )
• No unifying common ground to all shapes, other than designer’s intentions
• Uniformity is a coincidence, not a rule

two types: overloading (defined by progammer) and coercion(defined by language/compiler).

Question 2

what is universal polymorphism?

Answer 2

Polymorphism is over infinitely many types
There is a unifying, common ground to all the different shapes the polymorphic entity may take

Parametric:
template void Sort(T list) …
Relies on templates/generics

Inclusion:
Base* b = new Derived();
Relies on inheritance

Question 3

what is strict inheritance?

Answer 3

A limited form of inheritance:
extend a given class without touching its code

Question 4

what’s the connection between strict inheritance and the class parts?

Answer 4

Forge
- The derived class has a new forge
Mill
- The derived class has a new mill
Usually, must invoke the mill of the base class
Protocol
- The derived class can add protocol elements
Behavior
- The derived class implements only the new protocol elements
Structure
- The structure of the derived class is an extension of the base class

Question 5

pros and cons of strict inheritance?

Answer 5

Benefits of strict inheritance:
1. No performance penalty
   Compile-time creature
2. No conceptual penalty
    Structured path for understanding the classes
3. Conformance (AKA substitutability) 
     If a class B inherits from another class A, then the objects of B can be used wherever the objects of A are 
     used. I.e., B is an A.

Drawbacks of strict inheritance:
Not overly powerful…

Question 6

what are polymorphic variables?

Answer 6

• Pointers/references of polymorphic types
• All variables in Smalltalk are polymorphic. They may store instances of all classes.
• this is a polymorphic variable. It may point to things of different subtypes at different times.
• A pointer to an inherited type is generated whenever an inherited method is called.
• In fact, all class pointers and all class references in C++ are polymorphic

Question 7

what is up-casting?

Answer 7

casting pointers up the
inheritance hierarchy.
this pointer is up-casted implicitly whenever an inherited method is called.

Question 8

what is down-casting?

Answer 8

casting pointers and
references down the inheritance hierarchy.

must be done explicitly.

Question 9

does Java have array subtyping?

Answer 9

yes! 
in Java, if class Manager is a subtype of class Employee, then Manager[] is a subtype of Employee[]: