Comparison of OO Comparison of OO Programming LanguagesProgramming Languages

© Jason Voegele, 2003

C-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

C-S 546 3

OO Languages - Comparison

Properties Eiffel Smalltalk

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

C-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

C-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

C-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

C-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

C-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

C-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++

C-S 546 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

C-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

C-S 546 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

C-S 546 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

C-S 546 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.

C-S 546 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

C-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.

C-S 546 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