Chapter 12 - OO Concepts

Question 1

which PL serves as the originator of OO programming

Answer 1

SIMULA 67

Question 2

3 benefits of inheritance

Answer 2

• a new ADT can “inherit” some or all of its data and behavior from an existing type and modify that type without affecting it
• allows for code reuse without harming original
• is hierarchical in nature and well suited to modeling descendant relationships

Question 3

what is a derived class?

Answer 3

the class defined using inheritance (subclass is not entirely accurate)

Question 4

what is a parent class?

Answer 4

the class being inherited from (superclass isn’t entirely accurate)

Question 5

what is a message protocol or message interface

Answer 5

the entire collection of methods of an object

Question 6

message passing vs subroutine call

Answer 6

message passing implies a request to an object to execute one of its methods that partially entails operating on the object itself

subroutines typically process data sent by the caller

Question 7

class variables/methods vs instance variables/methods

Answer 7

class variables/methods only have one copy per class and are shared by all instance of the class

instance variables/methods have one copy for each instance of the class and are only used by the instance to which they belong

Question 8

1 disadvantage of inheritance

Answer 8

creates interdependencies among classes that complicate maintenance

Question 9

what is dynamic dispatch?

Answer 9

the dynamic binding of messages to method definitions. AKA the messages are statically coded but the receiving object is determined at runtime.

This creates polymorphism of sorts

Question 10

What is the way in which dynamic dispatch is most often implemented?

Answer 10

through abstract methods (pure virtual methods in C++)

Question 11

8 design issues for OO PLs

Answer 11

• the exclusivity of objects
• are subclasses subtypes
• type checking and polymorphism
• single or multiple inheritance allowed
• allocation and deallocation of objects
• dynamic and static binding
• nested classes
• object initialization

Question 12

What does “exclusivity of objects” mean

Answer 12

it refers to how objects are used in the PL and what kinds of data are represented by objects

Question 13

3 approaches to the exclusivity of objects

Answer 13

• everything is an object (Ruby)
• add objects to a complete typing system (C++)
• include an imperative-style typing system for primitives but make everything else objects (Java)

Question 14

1 advantage and 1 disadvantage to everything as an object exclusivity approach

Answer 14

Adv: elegance and purity

dis: slow operations on simple objects

Question 15

1 advantage and 1 disadvantage of adding objects to complete typing system exclusivity approach

Answer 15

adv: fast operations on simple objects
dis: results in a confusing type system

Question 16

1 advantage and 1 disadvantage of an imperative style system for primitives and objects for everything else exclusivity approach

Answer 16

adv: fast operations on simple objects and relatively small typing system
dis: still some confusion because of two type systems

Question 17

4 things that must hold for a derived class to be a subtype

Answer 17

• the “is-a” relationship holds between the base class object and an object of the derived class
• if the derived class is also a parent class, then objects of the derived class must expose all of the members that are exposed by objects of the parent class
• at a minimum, an “is-a” relationship should guarantee that in a client a variable of the derived class type could appear anywhere a variable of the parent class type was legal (syntactically equivalent)
• the derived class objects should be behaviorally equivalent to the parent class objects when used as a parent class object (Liskov substitution principle - aka semantically equivalent)

Question 18

What must be done to allow overridden method to be type checked statically

Answer 18

must have the same parameter types and return type

Question 19

2 design issues with multiple inheritance

Answer 19

• complexity = two base classes could name the same variable or have a common ancestor
• efficiency = more of a perceived issue than a real issue

Question 20

2 advantages of allocating objects on the heap?

Answer 20

• creates a uniform method of creation and access through pointer or reference variables
• simplifies assignment and implicit dereferencing

Question 21

What is object slicing?

Answer 21

some information is lost about an object that is allocated on the stack

this occurs when an object on the stack adds a data field and then a child class is assigned as a reference to its base and that data is no longer reachable

Question 22

What is the purpose of a nested class

Answer 22

allows information to be hidden in one class and be visible to another (nested) class

-kind of equivalent to friends in C++

Question 23

2 design issues of nested classes

Answer 23

• what bits of the nesting class should be visible in the nested class
• should any of the nested class be visible in the nesting class

Question 24

3 design issues with object initialization

Answer 24

• should an object be initialized manually (aka by the constructor only)?
• should objects be implicitly initialized
• when creating a derived class object, should the base be initialized implicitly or should the programmer explicitly manage that

Question 25

4 general characteristics of C++ in regard to OO concepts

Answer 25

• mixed typing system (primitive types and objects)
• constructors and destructors
• elaborate access controls to class entities
• allows nested classes

Question 26

3 inheritance characteristics of C++

Answer 26

• a class need not be subclasses of any type
• has private, protected, public access controls for inherited members
• supports multiple inheritance

Question 27

C++ private vs public inheritance

Answer 27

private - in derived class any public or protected members from parent class are now private
(these now private members and methods need to be accessed by scope resolution operator ::)

public - in derived class any protected or public members from parent class are public or protected respectively

Question 28

what is boxing?

Answer 28

the coercing of primitive types to their corresponding wrapper class types. Most notably in Java

Question 29

5 characteristics of Java interfaces in their interaction with classes

Answer 29

• can only contain constants and method declarations
• a class can implement any number of interfaces
• interfaces can be used to simulate multiple inheritance
• no code reuse
• two interfaces with methods of the same name cannot be implemented in the same class

Question 30

What is the Ada equivalent of a class? what are 3 properties?

Answer 30

• tagged types (tagged referring to a construct that expresses the type of the entity at the system level)
• can be records or private types
• no implicit constructor calls
• defined in packages and can thus be separately compiled

Question 31

2 characteristics of inheritance in Ada

Answer 31

• only tagged types can be inherited from

- no private/protected inheritance, the derived class gets everything

Question 32

4 OO characteristics in Eiffel PL

Answer 32

• has primitive types and objects
• all objects get three operations: copy, clone, and equal
• object creation is done with ‘!!’ operator

constructors are defined in a creation clause and are explicitly called

Question 33

what are methods called in Eiffel

Answer 33

routines

Question 34

what are instance variables called in Eiffel

Answer 34

attributes

Question 35

What are “features” in Eiffel

Answer 35

the routines and attributes of a class together

Question 36

7 inheritance characteristics of Eiffel

Answer 36

• supports multiple inheritance
• parent of a class is specified with the “inherit” clause
• with no access modifier, entities are essentially public
• using the name of the class as an access modifier = protected
• the “none” access modifier = private
• the “undefine” clause can be used to hide features from subclasses
• abstract classes can be created by using the “deferred” modifier on the class definition

Question 37

what is the “redefine” clause in Eiffel

Answer 37

defines an overridden feature

Question 38

what is the “rename” clause in Eiffel

Answer 38

same as :: scope modifier for accessing the parent version of an overridden feature

Question 39

what is “design by contract” in Eiffel

Answer 39

formal properties of a class that are enforced by assertions expressed as:

• routine pre-conditions
• routine post-conditions
• class invariants