Comparison of OO

Comparison of OO
Programming Languages


© Jason Voegele, 2003

C
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S 546

2

Characteristics of an OO Programming
Language

•
Encapsulation / Information Hiding

•
Inheritance

•
Polymorphism / Dynamic Binding

•
All pre
-
defined types are objects

•
All operations performed by sending messages to
objects

•
All user
-
defined types are objects

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S 546

3

OO Languages
-

Comparison

Properties

Eiffel

Smallta
lk

Ruby

Java

C#

C++

Visual
Basic

1

Yes

Yes

Yes

Yes

Yes

Yes

Yes?

2

Yes

Yes

Yes

Yes

Yes

Yes

No

3

Yes

Yes

Yes

Yes

Yes

Yes

Yes?

4

Yes

Yes

Yes

No

No

No

No

5

Yes

Yes

Yes

No

No

No

No

6

Yes

Yes

Yes

Yes

Yes

No

No

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S 546

4

Observations

•
Eiffel, Smalltalk and Ruby are the pure OO
languages

•
Java and C# fail to meet properties 4 and 5; many
OO designers still accept that it is not a serious
issue.

•
C++ does not satisfy properties 4, 5 and 6. Lack of
property 6 leads to inconsistencies in the set of
objects created in the program.

•
VB is simply a procedural language with some
OO features
–

should not be considered to
implement an OO design

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S 546

5

Static Vs Dynamic Binding

•
Static binding

–
Every variable has a fixed, permanent type attached to
the variable at compile time

•
Dynamic binding

–
A variable has no fixed type; its type changes
dynamically as the program executes

•
Hybrid binding

–
A variable has a fixed type assigned at compile time,
but it may assume other types during run
-
time through
polymorphic substitutions

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S 546

6

Static and Dynamic Binding (continued)

•
Smalltalk and Ruby are the only languages that
support dynamic binding

•
Eiffel, Java, C# and C++ support hybrid binding

•
Visual Basic supports only static binding

•
When a language uses generic classes (templates),
it supports hybrid binding

–
Eiffel, C# and C++

•
Future version of Java is expected to support
generic classes

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S 546

7

Inheritance

•
Almost all OO languages, except VB, support
inheritance

•
Multiple inheritance is supported by Eiffel and
C++

–
Jason Voegele argues that Eiffel’s mechanisms to
handle conflicts in multiple inheritance is neat and
clear than those in C++

•
Java does not support multiple inheritance
directly, but can be realized through interfaces

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S 546

8

Class based inheritance Vs Object based
inheritance

•
Most OO languages support class base inheritance

–
A class designated as the subclass of a superclass

–
Objects of subclasses can be substituted for objects of
superclass (polymorphism)

–
Subtype inheritance

•
Some languages support object based inheritance

–
Objects can inherit features from other objects during run
-
time

•
SELF and Javascript support this feature

•
Ruby supports both class based and object based
inheritance

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S 546

9

Method overloading and operator
overloading

•
Method overloading

–
Same name for more than one method

–
Differentiated by the number and/or type of
parameters

–
Supported by Java and C++ only

•
Operator overloading

–
Overloading the meaning of one or more of the
language symbols such as “+” and “*”

–
Supported by Eiffel, Ruby, C++

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10

Operator overloading
-

expectations

•
All operations in the system must be messages to
objects

–
Executions with operators are considered to be
equivalent to message calls

•
Operators must have equivalent functional forms

–
1 + 2 is the same as 1. + (2)

•
Smalltalk, Ruby and Eiffel support both of these
features

•
C++ supports neither, even though it supports
operator overloading

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S 546

11

Higher order functions

•
Functions whose arguments are functions
themselves

–
Example: calculate_tax (find year(), tax_rate)

•
The scope of the functions that are passed as
parameters must also be visible or computable

•
Smalltalk and Ruby support higher order
functions; recent version of Eiffel supports too.

•
Java, C# and C++ do not support this feature;
neither does VB.

Another function

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12

Garbage Collection

•
Mechanism to free memory of unused objects

•
Mark and sweep method

–
One phase marks the unused objects and another phase cleans
them up

–
Both phases are done at unspecified time (programmer does not
know)

–
Smalltalk, Eiffel, Ruby and Java support this mechanism

•
Reference count mechanism

–
Used by Visual Basic; has its own problems

•
C++ does not support garbage collection

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13

Class variables

•
Variables that are declared as static in a class and are
accessible to all instances of that class

–
Only one copy of each class variable exists and is shared by all
instances

•
Smalltalk, Ruby, Java, C# and C++ support this feature
though the implementation mechanisms are slightly
different

•
Smalltalk also supports additional classifications such as
“pool variables” that are shared by only a group of
classes

–
An extended mechanism of class variables

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14

Access Control

•
Access to various features of a class are generally
declared as “public” or “private” in most OO languages

•
Java, C# and C++ support “protected” by which private
features of a class can be accessed by its subclasses

–
In these languages, a private feature can be accessed by any
member of the same object or objects of the same class as well

–
C++ supports “friend” mechanism by which another class B can
access the private features of a class A as long as B is declared
as a friend of A


this violates the encapsulation property

•
Eiffel supports selective export, much similar to
“protected”

•
In Smalltalk, methods can be “public” or “private” but
attributes are strictly “private” only.

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15

Design by Contract

•
A notion introduced by Bertrand Meyer

•
Each method is designed with a contract described
by “pre” and “post
-
conditions”, and “invariants”

•
Eiffel is the only language that supports this
feature

•
Java and Ruby has libraries that tend to support
the design by contract feature, but it is not at the
same level as in Eiffel

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S 546

16

Concurrency

•
Concurrency is generally supported in OO
languages by threads of execution control

•
Almost all languages support multi
-
threading

•
It is explicit in Java, C# and C++ but somewhat
indirect(?) in Smalltalk

•
VB supports it through timers

•
Ruby supports threading not only in its syntax but
a separate built
-
in interpreter, and hence does not
rely on OS support for its implementation.

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17

Pointer Arithmetic

•
Allows a program to directly manipulate memory
addressed through “pointers”.

–
Goal is to improve run
-
time efficiency

•
C++ is the only language that supports pointers

•
Other OO languages tend not to support pointer
arithmetic because

–
Pointers are not OO concepts/principles

–
Misuse of (or improper use of) pointers will lead to a
lot of programming problems that are not part of the
application domain