1. Object-Oriented Programming
Seif Haridi
Peter Van Roy
Copyright by S. Haridi and P. Van Roy

2. Object-oriented programming
We present a rich style in program structure based on a collection of stateful entities (abstract data types)
Most popular current representatives are C++, and Java
Most popular design model is UML, an object-oriented design model
Principle programming techniques
Relation to other models (higher-order programming, component based programming, functional)
Case-study in object-oriented language (based on Mozart/Oz)
Copyright by S. Haridi and P. Van Roy

3. Component based programming
Supports
Encapsulation
Compositionality
Instantiation
Copyright by S. Haridi and P. Van Roy

4. Object-oriented programming
Supports
Encapsulation
Compositionality
Instantiation
Plus
Inheritance
Copyright by S. Haridi and P. Van Roy

5. Copyright by S. Haridi and P. Van Roy
Inheritance
Programs can be built in hierarchical structure from ADT’s that depend on other ADT’s (Components)
Object-oriented programming (inheritance) is based on the idea that ADT has so much in common
Example, sequences (stacks, lists, queues)
Object oriented programming builds ADT incrementally, this is done by inheritance
An ADT can be defined to ”inherit” from another abstract datatype, have substantially the same functionality of the other abstract datatype
Only the difference between an abstract datatype and its ancestor has to be specified
Copyright by S. Haridi and P. Van Roy

6. What is object-oriented programming?
OOP (Object-oriented programming) = encapsulated state + inheritance
Object
An entity with unique identity that encapsulates state
State can be accessed in a controlled way from outside
The access is provided by means of methods (procedures that can directly access the internal state)
Class
A specification of objects in an incremental way
Incrementality is achieved inheriting from other classes
and by specifying how its objects (instances) differ from the objects of the inherited classes
Copyright by S. Haridi and P. Van Roy

7. Copyright by S. Haridi and P. Van Roy
Instances (objects)
Interface (what methods are available)
State (attributes)
procedures (methods)
Copyright by S. Haridi and P. Van Roy

8. Classes as complete spec of ADT
We start our case study
elements of a class (members)
attributes (mutable instance variables)
features (stateless info about objects)
methods
Copyright by S. Haridi and P. Van Roy

9. Classes (syntax simple)
A class is a statement
class ClassVariable
attr
AttrName1 :
AttrNameN
meth Pattern1 Statement end
meth PatternN Statement end
end
Copyright by S. Haridi and P. Van Roy

10. Classes (syntax simplified)
A class is also a value that can be in an expression position
class $
attr
AttrName1 :
AttrNamen
meth Pattern Statement end
end
Copyright by S. Haridi and P. Van Roy

11. Copyright by S. Haridi and P. Van Roy
Classes in Oz
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
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12. Copyright by S. Haridi and P. Van Roy
Attributes of Classes
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
val is an attribute
a modifiable cell
that is access by the
atom val
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13. Copyright by S. Haridi and P. Van Roy
Attributes of classes
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
the attribute val
is accessed by the
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14. Copyright by S. Haridi and P. Van Roy
Attributes of classes
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
the attribute val
is assigned by the
operator 
as val  ...
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15. Copyright by S. Haridi and P. Van Roy
Methods of classes
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
methods
are statements
method head is a
record (tuple) pattern
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16. Copyright by S. Haridi and P. Van Roy
Classes in Oz
The class Counter has the syntactic form
class Counter attr val meth browse end meth inc(Value) val + Value end meth init(Value) val  Value end end
Copyright by S. Haridi and P. Van Roy

17. Copyright by S. Haridi and P. Van Roy
Example
The following shows how an object is created from a class using the procedure New/3, whose first argument is the class, the second is the initial method, and the result is the object.
New/3 is a generic procedure for creating objects from classes.
declare C = {New Counter init(0)} {C browse} {C inc(1)} {C browse}
Copyright by S. Haridi and P. Van Roy

18. The procedure-based approach
fun {Counter} X
S = {Record.toDictionary state(val:X)} % new variable X
proc {Inc inc(Value)} S.val := S.val + Value end
proc {Display browse} {Browse S.val} end
proc {Init init(Value)} S.val := Value end
D = o(inc:Inc browse:Display init:Init)
in proc{$ M} {D.{Label M} M} end
end
Copyright by S. Haridi and P. Van Roy

19. The procedure-based approach
fun {Counter} X
S = {Record.toDictionary state(val:X)} % new variable X
...
D = o(inc:Inc browse:Display init:Init)
in proc{$ M} {D.{Label M} M} end
end
fun {New Class InitialMethod}
O = {Class}
in {O InitialMethod} O end
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20. Copyright by S. Haridi and P. Van Roy
Example
The following shows how an object is created from a class using the procedure New/3, whose first argument is the class, the second is the initial method, and the result is the object.
New/3 is a generic procedure for creating objects from classes.
declare C = {New Counter init(0)} {C browse} {C inc(1)} {C browse}
Object interface is as a procedure
with one argument (see procedure
dispatch method Chapter 8)
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21. Copyright by S. Haridi and P. Van Roy
Summary
A class X is defined by:
class X ... end
Attributes are defined using the attribute-declaration part before the method-declaration part:
attr A1 ... AN
Then follows the method declarations, each has the form:
meth E S end
The expression E evaluates to a method head, which is a record whose label is the method name.
Copyright by S. Haridi and P. Van Roy

22. Copyright by S. Haridi and P. Van Roy
Summary
An attribute A is accessed
An attribute is assigned a value using A  E
A class can be defined as a value:
X = class $ ... end
Copyright by S. Haridi and P. Van Roy

23. Attribute Initialization
Stateful (may be updated by )
Initialized at object creation time, all instances have the initial balance = 0
class Account
attr balance:0
meth … end …
end
In general the initial value
of an attribute could be any
legal value (including
classes and objects)
Copyright by S. Haridi and P. Van Roy

24. Attribute Initialization
Initialization by instance
class Account
attr balance
meth init(X) balance  X end …
end
C1 = {New Account init(100)}
C1 = {New Account init(50)}
Copyright by S. Haridi and P. Van Roy

25. Attribute Initialization
Initialization by brand
declare L=linux
class RedHat
attr ostype:L
meth get(X) X end
end
class SuSE
class Debian
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26. Copyright by S. Haridi and P. Van Roy
Example
class Queue
attr front back count
meth init
Q in
front  Q back  Q count  0
end
meth put(X)
@back = X|Q
back  Q
count + 1
...
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27. Copyright by S. Haridi and P. Van Roy
Example
class Queue
attr front back count
meth init
Q in
front  Q back  Q count  0
end
meth put(X)
@back = X|Q
back  Q
count + 1
...
front Q0
back
put(a)
front
a | Q1
back
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28. Copyright by S. Haridi and P. Van Roy
Example
class Queue
attr front back count
...
meth get(?X)
Q in
X|Q
front  Q
count - 1
end
meth count(X) X end
front
a | Q1
back X
front
a | Q1
back X
Copyright by S. Haridi and P. Van Roy

29. Classes as incremental ADTs
Object-oriented programming allows allows us to define a class by extending existing classes
Three things have to be introduced
How to express inheritance, and what does it mean?
How to access particular methods in the new class and in preexisting classes
Visibility – what part of the program can see the attributes and methods of a class
The notion of delegation as a substitute for inheritance
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30. Copyright by S. Haridi and P. Van Roy
Inheritance
Inheritance should be used as a way to specialize a class while retaining the relationship between methods
In this way it is a just an extension of an ADT
The other view is inheritance is just a (lazy) way to construct new abstract data types !
No relationships are preserved
general
class
specialized
class
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31. Copyright by S. Haridi and P. Van Roy
Inheritance
class Account
attr balance:0
meth transfer(Amount)
balance
end
meth getBal(B) B
A={New Account transfer(100)}
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32. Copyright by S. Haridi and P. Van Roy
Inheritance II
The class VerboseAccount has the methods:
transfer, getBal, and verboseTransfer
Conservative extension
class VerboseAccount
from Account
meth verboseTransfer(Amount)
...
end
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33. Copyright by S. Haridi and P. Van Roy
Inheritance II
The class AccountWithFee has the mothods:
transfer, getBal, and verboseTransfer
The method transfer has been redefined (overridden) with another definition
Non-Conservative extension
class AccountWithFee
from VerboseAccount
attr fee:5
meth transfer(Amount)
...
end
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34. Copyright by S. Haridi and P. Van Roy
Inheritance II
Non-Conservative extension
class AccountWithFee
from VerboseAccount
attr fee:5
meth transfer(Amount)
...
end
Account
VerboseAccount
AccountWithFee
Copyright by S. Haridi and P. Van Roy

35. Static and dynamic binding
Inside an object O we want to invoke a method M
This is written as {self M}, and chooses the method visible in the current object (M of D)
class C
meth M
class D
a subclass of C
meth M O
an instance
of D
Copyright by S. Haridi and P. Van Roy

36. Static and dynamic binding
Static binding
Inside an object O we want to invoke a method M in a specific (super) class
This is written as C, M and chooses the method visible in the super class C (M of C)
class C
meth M
class D
a subclass of C
meth M O
an instance
of D
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37. Copyright by S. Haridi and P. Van Roy
Static method calls
Given a class and a method head m(…), a static method-call has the following form: C, m(…)
Invokes the method defined in the class argument.
A static method call can only be used inside class definitions.
The method call takes the current object denoted by self as implicit argument.
The method m could be defined in the class C, or inherited from a super class.
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38. Copyright by S. Haridi and P. Van Roy
Inheritance
class Account
attr balance:0
meth transfer(Amount)
balance
end
meth getBal(B) B
A={New Account transfer(100)}
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39. Copyright by S. Haridi and P. Van Roy
Inheritance II
The class VerboseAccount has the methods:
transfer, getBal, and verboseTransfer
Conservative extension
class VerboseAccount
from Account
meth verboseTransfer(Amount)
{self transfer(Amount)}
end
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40. Copyright by S. Haridi and P. Van Roy
Inheritance II
The class AccountWithFee has the mothods:
transfer, getBal, and verboseTransfer
The method transfer has been redefined (overridden) with another definition
Non-Conservative extension
class AccountWithFee
from VerboseAccount
attr fee:5
meth transfer(Amount)
VerboseAccount, transfer(Amount
end
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41. Copyright by S. Haridi and P. Van Roy
Inheritance II
Non-Conservative inheritance is dangerous
because it might change the relationship between methods and the invariants the programmer depends on
Non-Conservative extension
class AccountWithFee
from VerboseAccount
attr fee:5
meth transfer(Amount)
VerboseAccount, transfer(Amount
end
Account
getBalance(B); transfer(S); getBalance(B1) => B1 = B-S
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42. Copyright by S. Haridi and P. Van Roy
Inheritance II
Non-Conservative inheritance is dangerous
because it might change the relationship between methods and the invariants the programmer depends on
Non-Conservative extension
class AccountWithFee
from VerboseAccount
attr fee:5
meth transfer(Amount)
VerboseAccount, transfer(Amount
end
AccountWithFree
getBalance(B); transfer(S) iff
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43. Copyright by S. Haridi and P. Van Roy
Inheritance III
Classes may inherit from one or several classes appearing after the keyword: from.
A class B is a superclass of a class A if:
B appears in the from declaration of A, or
B is a superclass of a class appearing in the from declaration of A.
The methods (attributes and features) available in a class C (i.e. visible) are defined through a precedence relation on the methods that appear in the class hierarchy based on the overriding relation:
A method in a class C overrides any method, with the same label, in any super class of C.
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SuperClass relation
SuperClass relation is directed and acyclic. C
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45. Copyright by S. Haridi and P. Van Roy
SuperClass relation
SuperClass relation is directed and acyclic.
After striking out all overridden methods each remaining method should have a unique label and is defined only in one class in the hierarchy. C
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46. Copyright by S. Haridi and P. Van Roy
Inheritance relation m m
A (valid hierarchy) m C
(invalid hierarchy)
Copyright by S. Haridi and P. Van Roy

47. Multiple Inheritance Example
class Account
attr balance:0
meth transfer(Amount)
balance
end
meth getBal(?B) B end
class Customer
attr name
meth init(N) name  N end
class CustomerAccount from Customer Account end
A={New CustomerAccount init}
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48. Copyright by S. Haridi and P. Van Roy
Illegal inheritance
class Account
attr balance
meth init(Amount)
balance  Amount
end
meth transfer(Amount)
balance
meth getBal(B) B end
class Customer
attr name
meth init(N) name  N end
class CustomerAccount from Customer Account end
There are two init methods
visible for CustomerAccount
This is illegal
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49. Copyright by S. Haridi and P. Van Roy
Legal inheritance
class Account
attr balance
meth init(Amount) balance  Amount end
meth transfer(Amount) balance end
meth getBal(B) B end
end
class Customer
attr name
meth init(N) name  N end
class CustomerAccount from Customer Account
meth init(N A)
Customer, init(N)
Account, init(A)
CustomerAccount has
attributes balance and name
methods init, transfer and getBalance
This overriding is not
harmful it does not
change relationships
in super classes
Copyright by S. Haridi and P. Van Roy

50. Controlling visibility
Visibility is the control given to the user to limit access to members of a class (attributes, methods and features)
Each member is defined with a scope (part of program text that the member can be accessed by name)
Programming languages uses words like public, private and protected to define visibility
Unfortunately different languages use these keywords to define different scopes
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51. Public and private scopes in ADTs
A private member is one which is only visible in the object instance (it is used for implementing the ADT)
The object instance can see all the private members in its class and its super classes
A public member is visible anywhere in the program
It is part of the interface of the ADT
In Oz (and Smalltalk) attributes are private and methods are public (the default rule)
In Java and C++ private has another meaning
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52. Copyright by S. Haridi and P. Van Roy
The meaning of Private C
Class Hierarchy
SubC
SubSubC I1 I2 I3 I4
Instances
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53. Copyright by S. Haridi and P. Van Roy
The meaning of Private C
Class Hierarchy
According to
Smalltalk and Oz
SubC
All private memebers
in this region are visible
to I3
SubSubC I1 I2 I3 I4
Instances
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54. Copyright by S. Haridi and P. Van Roy
The meaning of Private C
Class Hierarchy
According to
C++ and Java
All private memebers
in this region are visible
to I3
SubC
SubSubC I1 I2 I3 I4
Instances
Copyright by S. Haridi and P. Van Roy

55. Public and private scopes in ADTs
In Oz (and Smalltalk) attributes are private and methods are public
It is possible in Oz to make a method private within a class
Using a variable identifier as a method head will make the method local to the class
The variable is automatically bound to a unique name
class C
meth A(...) ... end
....
end
Copyright by S. Haridi and P. Van Roy

56. Public and private scopes in ADTs
class C
meth A(...) ... end
....
end
In Oz (and Smalltalk) attributes are private and methods are public
It is possible in Oz to make a method private within a class
Using a variable identifier as a method head will make the method local to the class
The variable is automatically bound to a unique name
! is an escape character, !A means escape the class scope
local A = {NewName} in
class C
meth !A(...) ... end
....
end
Copyright by S. Haridi and P. Van Roy

57. Programming techniques
First class messages (higher order programming)
Parameterized classes
Use of multiple inheritance
Copyright by S. Haridi and P. Van Roy

58. Techniques of Higher order programming
Control abstractions
class HigherOrderControl
meth forAll(ListObjects M)
for O in ListObjects do {O M} end
end
...
This technique allows messages as parameters
Copyright by S. Haridi and P. Van Roy

59. Techniques of Higher order programming
Control abstractions
class HigherOrderControl
...
meth nil skip end
meth ’|’(M Ms)
{self M} {self Ms}
end
C = {New class $ from HigherOrderControl Counter end init(0)}
{C [inc(2) browse inc(3) browse]}
Copyright by S. Haridi and P. Van Roy

60. Patemeterized classes
Classes are values like any other value
Therefore is it possible to define functions that return new classes as output
fun {MakeClassAcountWithFee Fee}
class $ %% Fee is in context environment
from Account
meth init(Amount)
Account, init(Amount-Fee)
end
Account={MakeClassAccountWithFee 100}
{New Account init(1000)}
Copyright by S. Haridi and P. Van Roy

61. Classes as First Class Values
fun {MakeClassAcountWithFee Fee}
class $ %% Fee is in closure
from Account
meth init(Amount)
Account,init(Amount-Fee)
end
Account={MakeClassAccountWithFee 100}
{New Account init(1000)}
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62. Copyright by S. Haridi and P. Van Roy
Delegation
Some object systems do not use inheritance (SELF)
They use a notion known as delegation
Every class is an object
Inheritance is implemented by forwarding
messages the object cannot handle to a ”delegate”
which behaves as a super-class
More dynamic, inheritance can be dynamic
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63. Copyright by S. Haridi and P. Van Roy
Delegation
Account = {New class $
attr balance
meth init balance  0 end
meth transfer(Amount)
balance
end
meth getBal(B) B end
init}
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64. Programming techniques with multiple inheritance
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65. Copyright by S. Haridi and P. Van Roy
Delegation
VerboseAccount =
{New
class $ from BaseObject
attr delegate:Account
meth verboseTransfer(Amount)
B in
{self transfer(Amount)} {self getBalance(B)}
{Show B}
end
meth otherwise(M) M} end
init }
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66. Copyright by S. Haridi and P. Van Roy
Multiple inheritance
Multiple Inheritance is useful when an object has to be two different things in the same program (mixin inheritance)
Example, we have graphical objects
Line, circle, etc
Composite graphical objects (groups)
We use multiple inheritance to add the ability of grouping figures
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67. Copyright by S. Haridi and P. Van Roy
Class diagrams
LinkList
Figure
elem, next
init, add(O),
forall(P)
Line
Circle
CompositeFigure
canvas
x1, x2, y1, y2
canvas
x, y, r
init, move(X Y),
display
init, move(X Y),
display
init, move(X Y),
display
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68. Copyright by S. Haridi and P. Van Roy
LinkedList Class
class LinkedList
attr items
meth init items  nil end
meth add(E) items  end
meth forall(M)
for O do {O M} end
end
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69. LinkedList (pure object) Java style
class LinkedList
attr item next
meth init(item:E<=null next:N<=null)
item  E next  N
end
meth add(E)
next {New LinkedList init(item:E
meth forall(M)
then M} end
then forall(M)} end
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70. Copyright by S. Haridi and P. Van Roy
Line
class Line from Figure
attr canvas x1 y1 x2 y2
meth init(Can X1 Y1 X2 Y2)
canvas  Can
x1  X1 y1  Y1
x2  X2 y2  Y2
end
meth move(X Y)
x1 y1
x2 y2
meth display
@y2)}
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71. Copyright by S. Haridi and P. Van Roy
CompositeFigure
class CompositeFigure from Figure LinkedList
meth init
LinkedList, init
end
meth move(X Y)
{self forall(move(X Y))}
meth display
{self forall(display)}
Copyright by S. Haridi and P. Van Roy

72. Use of single inheritance
Figure
association 1 1
CompositeFigure
LinkList
elem, next
linkList
init, add(O),
forall(P)
init, move(X Y),
display, add(O)
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73. Copyright by S. Haridi and P. Van Roy
CompositeFigure
class CompositeFigure from Figure
attr linkList
meth init
figlist  {New LinkedList init}
end
meth add(F)
add(F)}
meth move(X Y)
forall(move(X Y))}
meth display
forall(display)}
Copyright by S. Haridi and P. Van Roy

74. Multiple vs. Single inheritance
With multiple inheritance a composite figure is also a linked list.
In general use multiple inheritence in this case if you want all operations of linked list to be available
With single inheritance a composite figure completely hides the linked list
Use single inheritance if you want to hide the linked list functionality
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75. Rules for using inheritance
Do not violate the substitution property. Programs working on objects of a given class should work on all the objects of its subclasses
Do not use subclassing to fix small problem. That is to say do not patch up a class by making a subclass. The class hierarchy tends to get large and the program slower
Copyright by S. Haridi and P. Van Roy

76. When does multiple inheritance work?
Multiple inheritance works well when combining two completely independent abstractions
Multiple inheritance does not work when abstractions have something in common
Mutiple inheritance does not work when their is a shared class with mutable attributes
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77. Copyright by S. Haridi and P. Van Roy
When it does not work?
Mutiple inheritance does not work when there is a shared class with mutable attributes
Creating a BHistoryPoint object could replicate the operations on Point though
HistroyPoint and BoundedPoint
Known as the implementation sharing problem
Point
HistoryPoint
BoundedPoint
BHistoryPoint
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78. Copyright by S. Haridi and P. Van Roy
HOP vs.OOP
We show how to get some of the flexibility of higher order programming in OOP
proc {NewSort Order ?SortRoutine}
proc {SortRoutine InL ?OutL}
... {Order X Y Z}
end
class SortRoutineClass
attr ord
meth init(Order)
ord  Order
end
meth sort(InL ?OutL)
... order(X Y Z)}
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79. Copyright by S. Haridi and P. Van Roy
HOP vs.OOP
We show how to get some of the flexibility of higher order programming in OOP
class Proc
attr x y
meth init(X Y)
x  X y  Y
end
meth apply
Some statement
X ... Y
P = proc{$}
Some Statement with free X Y
end
.... {P}
X ... Y
P = {New Proc init(X Y)}
.... {P apply}
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80. Copyright by S. Haridi and P. Van Roy
HOP vs.OOP
We show how to get some of the flexibility of higher order programming in OOP
A lot of the higher order functionality can be coded
meth map(Xs O Ys)
.... {O apply(X Y)}
Ys = Y|Yr
map(Xr O Yr)
proc {Map Xs P Ys}
.... {P X Y}
Ys = Y|Yr
{Map Xr P Yr}
Copyright by S. Haridi and P. Van Roy