1. Chapter 12 Categories of languages that support OOP:
1. OOP support is added to an existing language
- C++ (also supports procedural and data-
oriented programming)
- Ada 95 (also supports procedural and data-
- CLOS (also supports functional programming)
- Scheme (also supports functional programming)

2. Chapter 11 Categories of languages that support OOP:
2. Support OOP, but have the same appearance
and use the basic structure of earlier imperative
languages
- Eiffel (not based directly on any previous
language)
- Java (based on C++)
3. Pure OOP languages
- Smalltalk

3. Paradigm Evolution
1. Procedural s-1970s (procedural
abstraction)
2. Data-Oriented - early 1980s (data-oriented)
3. OOP - late 1980s (Inheritance and dynamic
binding)

4. Origins of Inheritance
Observations of the mid-late 1980s :
- Productivity increases can come from reuse
Unfortunately,
- ADTs are difficult to reuse--never quite
right
- All ADTs are independent and at the same
level
Inheritance solves both--reuse ADTs after minor
changes and define classes in a hierarchy

5. OOP Definitions:
- ADTs are called classes
- Class instances are called objects
- A class that inherits is a derived class or a
subclass
- The class from which another class inherits is a
parent class or superclass
- Subprograms that define operations on objects
are called methods

6. Continue OOP Definitions:
- The entire collection of methods of an object is
called its message protocol or message interface
- Messages have two parts--a method name and the
destination object
- In the simplest case, a class inherits all of the
entities of its parent

7. controls to encapsulated entities
- Inheritance can be complicated by access
controls to encapsulated entities
- A class can hide entities from its subclasses
( protected in C++ for unhiding )
- A class can hide entities from its clients
- Besides inheriting methods as is, a class can
modify an inherited method
- The new one overrides the inherited one
- The method in the parent is overriden

8. - There are two kinds of variables in a class:
1. Class variables - one/class
2. Instance variables - one/object
- There are two kinds of methods in a class:
1. Class methods - messages to the class
2. Instance methods - messages to objects
- Single vs. Multiple Inheritance

9. - One disadvantage of inheritance for reuse:
- Creates interdependencies among classes that
complicate maintenance

10. Polymorphism in OOPLs
- A polymorphic variable can be defined in a class
that is able to reference (or point to) objects of
the class and objects of any of its descendants
- When a class hierarchy includes classes that
override methods and such methods are called
through a polymorphic variable, the binding to
the correct method MUST be dynamic
- This polymorphism simplifies the addition of
new methods
- A virtual method is one that does not include a
definition (it only defines a protocol)

11. - A virtual class is one that includes at least one
virtual method
- A virtual class cannot be instantiated

12. Design Issues for OOPLs
1. The Exclusivity of Objects
a. Everything is an object
Advantage - elegance and purity
Disadvantage - slow operations on simple
objects (e.g., float)
b. Add objects to a complete typing system
Advantage - fast operations on simple objects
Disadvantage - results in a confusing type system
c. Include an imperative-style typing system for
primitives but make everything else objects
and a relatively small typing system
Disadvantage - still some confusion because
of the two type systems

13. Design Issues for OOPLs
2. Are Subclasses Subtypes?
- Does an is-a relationship hold between a parent
class object and an object of the subclass?
An “is-a” relationship would guarantee that a
variable of the parent class type was legal.

14. 3. Implementation and Interface Inheritance
- If only the interface of the parent class is visible
to the subclass, it is interface inheritance
Disadvantage - can result in inefficiencies
- If both the interface and the implementation of
the parent class is visible to the subclass, it is
implementation inheritance
Disadvantage - changes to the parent class
require recompilation of
subclasses, and sometimes even
modification of subclasses

15. 4. Type Checking and Polymorphism
- Polymorphism may require dynamic type
checking of parameters and the return value
- Dynamic type checking is costly and delays
error detection
- If overriding methods are restricted to having the
same parameter types and return type, the
checking can be static

16. 5. Single and Multiple Inheritance
- Disadvantages of multiple inheritance:
- Language and implementation complexity
- Potential inefficiency - dynamic binding costs
more with multiple inheritance (but not much)
- Advantage:
- Sometimes it is extremely convenient and
valuable

17. 6. Allocation and Deallocation of Objects
- From where are objects allocated?
- If they all live in the heap, references to them
are uniform
- Is deallocation explicit or implicit?

18. 7. Dynamic and Static Binding
- Should ALL binding of messages to methods be
dynamic?
- If none are, you lose the advantages of
dynamic binding
- If all are, it is inefficient

19. Overview of Smalltalk
- Smalltalk is a pure OOP language
- Everything is an object
- All computation is through objects sending
messages to objects
- It adopts none of the appearance of imperative
languages
- The Smalltalk Environment
- The first complete GUI system
- A complete system for software development
- All of the system source code is available to
the user, who can modify it if he/she wants

20. Introduction to Smalltalk
- Expressions
- Four kinds:
1. Literals (numbers, strings, and keywords)
2. Variable names (all variables are references)
3. Message expressions (see below)
4. Block expressions (see below)

21. - Message expressions
- Two parts: the receiver object and the message
itself
- The message part specifies the method and
possibly some parameters
- Replies to messages are objects

22. - Messages can be of three forms:
1. Unary (no parameters)
e.g., myAngle sin
(sends a message to the sin method of the
myAngle object)
2. Binary (one parameter, an object)
e.g.,
(sends the message “+ 17” to the object 12;
the object parameter is “17” and the method
is “+”)
3. Keyword (use keywords to organize the
parameters)
e.g., myArray at: 1 put: 5
(sends the objects “1” and “5” to the at:put:
method of the object myArray)

23. - Multiple messages to the same object can be
strung together, separated by semicolons

24. Methods
- General form:
message_pattern [| temps |] statements
- A message pattern is like the formal parameters
of a subprogram
- For a unary message, it is just the name
- For others, it lists keywords and formal names
- temps are just names--Smalltalk is typeless!

25. Assignments
- Simplest Form:
name1 <- name2
- It is simply a pointer assignment
- RHS can be a message expression
e.g., index <- index + 1

26. Blocks
- A sequence of statements, separated by periods,
delimited by brackets
e.g.,
[index <- index + 1. sum <- sum + index]
- A block specifies something, but doesn’t do it
- To request the execution of a block, send it the
unary message, value
e.g., […] value
- Blocks can have parameters, as in
[:x :y | statements]
- If a block contains a relational expression, it
returns a Boolean object, true or false

27. Iteration
- The objects true and false have methods for
building control constructs
- The method WhileTrue: from Block is used for
pretest logical loops. It is defined for all blocks
that return Boolean objects.
e.g.,
[count <= 20]
whileTrue [sum <- sum + count.
count <- count + 1]

28. - timesRepeat: is defined for integers and can be
used to build counting loops
e.g.,
xCube <- 1.3 timesRepeat: [xCube <- xCube * x]

29. Selection
- The Boolean objects have the method
ifTrue:ifFalse: , which can be used to build
selection
e.g.,
total = 0
ifTrue: […]
ifFalse: […]

30. Large-Scale Features of Smalltalk
- Type Checking and Polymorphism
- All bindings of messages to methods is dynamic
- The process is to search the object to which
the message is sent for the method; if not
found, search the superclass, etc.
- Because all variables are typeless, methods are
all polymorphic

31. - Inheritance
- All subclasses are subtypes (nothing can be
hidden)
- All inheritance is implementation inheritance
- No multiple inheritance
- Methods can be redefined, but the two are not
related

32. C++ - General Characteristics: - Mixed typing system
- Constructors and destructors
- Elaborate access controls to class entities

33. - Inheritance
- A class need not be subclasses of any class
- Access controls for members are
1. Private (visible only in the class and friends)
(disallows subclasses from being subtypes)
2. Public (visible in subclasses and clients)

34. b. Public derivation public and protected
- Inheritance
3. Protected (visible in the class and in
subclasses, but not clients)
- In addition, the subclassing process can be
declared with access controls (private or
public), which define potential changes in
access by subclasses
a. Private derivation - inherited public and
protected members are private in the
subclasses
b. Public derivation public and protected
members are also public and protected in

35. Example (book, p. 468)

36. class base_class {
private:
int a;
float x;
protected:
int b;
float y;
public:
int c;
float z; };
class subclass_1 : public base_class { … };
// - In this one, b and y are protected and
// c and z are public
class subclass_2 : private base_class { … };
// - In this one, b, y, c, and z are private,
// and no derived class has access to any
// member of base_class

37. - Reexportation
A member that is not accessible in a subclass
(because of private derivation) can be declared
to be visible there using the scope resolution
operator (::)
e.g.,
class subclass_3 : private base_class {
base_class :: c; … }

38. One motivation for using private derivation:
- A class provides members that must be visible,
so they are defined to be public members;
a derived class adds some new members, but
does not want its clients to see the members
of the parent class, even though they had to be
public in the parent class definition
- Multiple inheritance is supported
- If there are two inherited members with the
same name, they can both be reference
using the scope resolution operator

39. - Dynamic Binding
- A method can be defined to be virtual, which
means that they can be called through
polymorphic variables and dynamically bound
to messages
- A pure virtual function has no definition at all
- A class that has at least one pure virtual
function is an abstract class

40. - Evaluation
- C++ provides extensive access control (unlike
Smalltalk)
- C++ provides multiple inheritance
- In C++, the programmer must decide at design
time which methods will be statically bound
and which must be dynamically bound
- Static binding is faster!
- Smalltalk type checking is dynamic (flexible,
but somewhat unsafe)

41. Java - General Characteristics
- All data are objects except the primitive types
- All primitive types have wrapper classes that
store one data value
- All objects are heap-dynamic, are referenced
through reference variables, and most are
allocated with new

42. - Inheritance - Single inheritance only, but there is an abstract
class category that provides some of the
benefits of multiple inheritance (interface)
- An interface can include only method
declarations and named constants
e.g.,
public class Clock extends Applet
implements Runnable
- Methods can be final (cannot be overriden)

43. - In Java, all messages are dynamically bound to
- Dynamic Binding
- In Java, all messages are dynamically bound to
methods, unless the method is final (mean it
cannot be overriden; therefore, dynamic
binding serves no purpose)

44. - Encapsulation
- Two constructs, classes and packages
- Packages provide a container for classes that
are related (can be named or unamed)
- Entities defined without an scope (access) modifier
have package scope, which makes them visible
throughout the package in which they are defined
- they go in the unnamed package
- Every class in a package is a friend to the
package scope entities elsewhere in the
package
- So, package scope is an alternative to the
friends of C++

45. Ada 95 - General Characteristics
- OOP was one of the most important extensions
to Ada 83
- Encapsulation container is a package that
defines a tagged type
- A tagged type is one in which every object
includes a tag to indicate during execution its
type
- Tagged types can be either private types or
records
- No constructors or destructors are implicitly
called

46. - Inheritance
- Subclasses are derived from tagged types
- New entities in a subclass are added in a record

47. Example (of a tagged type)

48. Package PERSON_PKG is
type PERSON is tagged private;
procedure DISPLAY(P : in out PERSON);
private
type PERSON is tagged
record
NAME : STRING(1..30);
ADDRESS : STRING(1..30);
AGE : INTEGER;
end record;
end PERSON_PKG;

49. with PERSON_PKG; use PERSON_PKG;
package STUDENT_PKG is
type STUDENT is new PERSON with
record
GRADE_POINT_AVERAGE : FLOAT;
GRADE_LEVEL : INTEGER;
end record;
procedure DISPLAY (ST: in STUDENT);
end STUDENT_PKG;
- DISPLAY is being overriden from PERSON_PKG

50. - Inheritance (more)
- All subclasses are subtypes
- Single inheritance only, except through generics
- Dynamic Binding
- Dynamic binding is done using polymorphic
variables called classwide types
e.g., for the tagged type PERSON, the classwide
type is PERSON’class
- Other bindings are static
- Any method may be dynamically bound

51. Eiffel - General Characteristics - Has primitive types and objects
- All objects get three operations, copy, clone,
and equal
- Methods are called routines
- Instance variables are called attributes
- The routines and attributes of a class are
together called its features

52. Eiffel - Object creation is done with an operator (!!)
- Constructors are defined in a creation clause,
and are explicitly called in the statement in
which an object is created
- Inheritance
- The parent of a class is specified with the
inherit clause

53. - feature clauses specify access control to the
entities defined in them
- Without a modifier, the entities in a feature
clause are visible to both subclasses and
clients
- With the name of the class as a modifier,
entities are hidden from clients but are
visible to subclasses
- With the none modifier, entities are hidden
from both clients and subclasses

54. - Inherited features can be hidden from
subclasses with undefine
- Abstract classes can be defined by including the
deferred modifier on the class definition
- Dynamic Binding
- Nearly all message binding is dynamic
- An overriding method must have parameters
that are assignment compatible with those of
the overriden method
- All overriding features must be defined in a
redefine clause
- Access to overriden features is possible by
putting their names in a rename clause

55. - Evaluation
- Similar to Java in that procedural programming
is not supported and nearly all message
binding is dynamic
- Elegant and clean design of support for OOP

56. Implementing OO Constructs
- Class instance records (CIRs) store the state of
an object
- If a class has a parent, the subclass instance
variables are added to the parent CIR
- Virtual Method Tables (VMTs) are used for
dynamic binding

57. Review Questions solutions
Q1: What are the three characteristic features
of object-oriented languages?
1- Abstract Data Types
2- Inheritance
3- Polymorphism and dynamic binding

58. Q2: What is the difference between a class variable
and an instance variable ?
Class variable belong to the class rather than to
the instant.
Instance variable is the instance of the class
( the state of the class instance variables )

59. Q3: What is an overriding method ?
In inheritance, the base and the derived class
may have the same method ( the same name).
The new method is said to override the inherited
version, which is then called an overriden method

60. Q4: Describe a situation where dynamic binding is
a great advantage over its absence ?
Suppose you have a number of objects of different
classes but you want to put them all on a list and
perform a particular operation on them using the
same function call. For example, suppose a
graphics program includes several different
shapes: a triangle, a ball, a square, and so on.
Each of these classes has a member function
draw ( ) that causes the object to be drawn on the
screen.

61. Q5: What is a virtual method ?
It is a method that does not really exit but
nevertheless appears real to some parts of the
program. Or
The method that exists in the base class without
the body ( just the protocol ). Once it is declared
a virtual, it remains virtual all the way down the
inheritance hierarchy from that point even if it is
not declared virtual when a class overrides it.

62. Q6: Describe briefly the seven design issues used
in this chapter for object-oriented languages ?
1- The exclusivity of objects
2- Are subclasses Subtypes
3- Implementation and interference inheritance
4- Type checking and polymorphism
5- Single and multiple inheritance
6- Allocation and deallocation of objects
7- Dynamic and static binding

63. Q7: What is the message protocol for a method ?
I think the author did an error here. He mean the
message protocol for a class. This mean the
entire collection of methods of an object is called.
However, the question can be solved by saying:
It is the method that exists in the base class without
the body

64. Q8 : Why is that the classes of Smalltalk can respond
to messages ?
Because all classes in Smalltalk are themselves
objects. This allow class to receive messages

65. Q9: Explain the actions of the Smalltalk statement
results <--- first * second
first is an object. This object is has the operation *
applied on it. The passing parameter is second. The
variable results is set to refer to the multiplication of
objects: first and second.

66. Q10: What are the 4 parts of a smalltalk class
definition ?
1- The class name
2- The superclass name
3- Declarations of the instance variables
4- Declarations of the instance and class methods

67. - The message has two parts: the receiver object
Q11: Explain how smalltalk messages are bound to
methods. When does this take place?
- The message has two parts: the receiver object
and the message itself
- The message part specifies the method and
possibly some parameters
- Replies to messages are objects
- Method general form is:
message_pattern [| temps |] statements
- A message pattern is like the formal parameters
of a subprogram
- temps are just names--Smalltalk is typeless!
- All the computation are accomplished through
messages.
Binding of messages to methods is dynamic

68. It operate as follows: A message to an object causes
the class to which the object belongs to be searched
for a corresponding method. If the search fails, it is
continued in the superclass of that class, and so
forth, up to the system class, Object, which has no
supercalss. Object is the root of the class derivation
tree on which every class is a node. If no method
is found anywhere in that chain, an error occurs

69. Q12: What type checking is done in smalltalk. When
does it take place.
Dynamic. It takes place when the message is sent.
Smalltalk does not, under any circumstances,
bind messages to methods statically.

70. Q13: What kind of inheritance does smalltalk support?
Single inheritance, it does not allow multiple
inheritance.

71. Q14: What are the two primary effects that smalltalk
has had on computing ?
1- The integrated use of windows, mouse-pointing
devices, and pop-up or pull-down menus.
2- The advancement of object-oriented programming

72. Q15: What purpose does the Smalltalk pseudovariable
super serve ?
It causes the method search to begin in the superclass
rather than locally.

73. Q16: In essence, all Smalltalk variables are of a single
type. What is that type ?
All Smalltalk variables are references; they can
only refer to objects or classes. They are typeless;
any variable can point to any object.

74. Q17: How many parameters are there in a Smalltalk
binary message ?
One parameter. It is object.

75. Q18: Explain the precedence rules of Smalltalk
expressions ?
Unary operators, followed by binary expression,
followed by keyword expressions. Both unary and
binary expressions are associate left to right.
Expressions can be parenthesized to force any order
of operator evaluation.

76. To request the execution of a block, send it the unary message, value
Q19: How one can force a Smalltalk block to be
executed ?
To request the execution of a block, send it the
unary message, value
e.g., [sum <- sume + index] value

77. Q20: What purpose does the Smalltalk pseudovariable
self serve ?
It is an object name that refers to the object in which
it appears. Therefore, self is used for recursive
messages, or messages to the object itself.

78. Q21: From where can C++ objects be allocated ?
Either stack-dynamically or heap.

79. Q22: How are C++ heap-allocated objected
deallocated ?
By the destructor.

80. Q23: Are all C++ subclasses subtypes ?
No, not necessary.

81. Q24: Under what circumstances is a C++ method
call statically bound to a method ?
If it is not a virtual method.

82. Q25: What drawback is there to allow designers to
specify which method can be statically bound?
The original design must include these decision, which
may have to be changed later.

83. Q26: Explain the differences between the two uses of
private in C++ ?
Private is visible only in the class and friends.
It disallows subclasses from being subtypes.
The differences is between the subtypes and the
derived types that are not subtypes. This is done
by using private access. The complete example
is in text page 470.

84. Q27: What is a friend function in C++ ?
A function that is granted access to the private data.

85. Q28: How is the type system of Java different from
that of C++ ?
Java has no enumeration or record types, and its
arrays are objects. Only values of primitive
scalar types(Boolean, char, and numeric types)
are not objects.

86. Q29: From where can Java objects be allocated ?
From the heap.

87. Q30 : How Java objects are deallocated ?
There is no explicit object deallocation operation.
The garbage collector takes care of this.

88. Q31: Are all Java subclasses subtypes ?

89. Q32: Under what circumstances is a Java method call
statically bound to a method ?
When it is final.

90. Q33: Are all Ada 95 subclasses subtypes ?
Yes

91. Dynamic binding is done using polymorphic
Q34: How is a call to a subprogram in Ada 95
specified to be dynamically bound to a subprogram
definition ? When is this decision made ?
Dynamic binding is done using polymorphic
variables called classwide types. This is done
dynamically.

92. Q35 : What purpose does a creation clause have in
Eiffel ?
It is a constructor that is called when objects are
created.

93. With the name of the class as a modifier. Look
Q36: How is an Eiffel feature defined to be visible in
subclasses but not clients
With the name of the class as a modifier. Look
for the 3 cases in the text page 482.

94. Q37: What does the { none } qualifier do when it
appear on an Eiffel feature ?
The details are hidden from both the clients and
the subclasses.

95. Q38: How are Eiffel heap-allocated objects deallocated?
There is no explicit deallocation operation. The
garbage collection process reclaims its storage

96. Q39 : Under what circumstances is an Eiffel method
call statically bound to a method ?

97. Q40: How can an Eiffel subclass be defined not to be
a subtype ?