MELJUN CORTES Programming Languages Abstraction

8/8/2019 MELJUN CORTES Programming Languages Abstraction

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Abstraction

Programming Languages

* Property of STI

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The Concept of

Abstraction

Abstraction is a view or representation ofan entity that includes only the most

significant attributes.

The concept of abstraction is fundamentalin programming (and computer science)

Nearly all programming languages supportprocess abstraction

with subprograms

Nearly all programming languagesdesigned since 1980 have supported

data

abstraction

with some kind of module

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Abstraction

Abstraction is a weapon against thecomplexity of programming, it simplify the

programming.

Two fundamental kinds:

Process bstraction

Data bstraction

Floating-point as an Abstract Data Type

User-defined Abstract Data Type musthave the same characteristics as built-inabstract data types




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Introduction to Data

Abstraction

An abstract data type is a user-defineddata type that satisfies the following

conditions:

The representation of, and operations on,objects of the type are defined in a singlesyntactic unit; also, other units can createobjects of the type.

The representation of objects of the type ishidden from the program units that usethese objects, so the only operationspossible are those provided in the type'sdefinition.




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Advantages of Data

Abstraction

Advantage of the first condition:

Program organization, modifiability

(everything associated with a data structureis together), and separate compilation.

Advantage of the second condition:

Reliability--by hiding the datarepresentations, user code cannot directly

access objects of the type. User code cannotdepend on the representation, allowing therepresentation to be changed withoutaffecting user code.




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Classes An example of a class – a class

implementation of a complex number realplus the imaginary counterpart

class complex {

double re, im;};

public:

complex& operator++ (complex a) {

re+= a.re;

im+= a.im;

return *this;

}

complex& operator+= (double a) {

re+= a;

return *this;

}

complex operator+ (complex a, complex b) {

complex r= a;

complex r+= b;

}

complex operator+ (complex a, complex b) {

complex r= a;

return r+= b;

}

complex operator (double a, complex b) {

complex r= b;

return+= a;

}


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Encapsulation

Large programs have two special needs:

Some means of organization, other than

simply division into subprograms Some means of partial compilation

(compilation units that are smaller than the whole program)

Obvious solution : a grouping of

subprograms that are logically related intoa unit that can be separately compiled

These are called en psul tions




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Information Hiding

The principle of information hiding is thehiding of design decisions in a computer

program that are most likely to change,thus protecting other parts of the programfrom change if the design decision ischanged.


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Language Examples

Abstract Data Type in Ada

The encapsulation constructs are called

packages• Specification package (the interface)

• Body package (implementation of the entitiesnamed in the specification)

Information Hiding

• The specification package has two parts:public and private

• The name of the abstract type appears in thepublic part of the specification package

• The representation of type appears in a part ofthe specification called the private part

– More restricted form with limited privatetypes

• Reasons for the public/private specification

package: – The compiler should be able to see the

representation after seeing only thespecification package (it cannot see theprivate part)

– Clients should see the type name, butnot the representation (also cannot seethe private part)




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Language Examples

Abstract Data Type in C++

Based on C struct type and Simula 67

classes The class is the encapsulation device

All of the class instances of a class share asingle copy of the member functions

Each instance of a class has its own copy ofthe class data members

Instances can be static, stack dynamic, orheap dynamic

Information Hiding

• Private clause for hidden entities

• Public clause for interface entities

• Protected clause for inheritance




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Language Examples

Abstract Data Type in C++

Constructors

• Functions to initialize the data members ofinstances (they DO NOT create the objects)

• May also allocate storage if part of the objectis heap-dynamic

• Can include parameters to provideparameterization of the objects

• Implicitly called when an instance is created

• Can be explicitly called• Name is the same as the class name

Destructors

• Functions to cleanup after an instance isdestroyed; usually just to reclaim heap storage

• Implicitly called when the object’s lifetime

ends• Can be explicitly called

• Name is the class name, preceded by a tilde (~)




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Language Examples

Abstract Data Type in Java

Similar to C++, except:

• All user-defined types are classes• All objects are allocated from the heap and

accessed through reference variables

• Individual entities in classes have accesscontrol modifiers (private or public), ratherthan clauses

• Java has a second scoping mechanism,

package scope, which can be used in place offriends

– All entities in all classes in a packagethat do not have access control modifiersare visible throughout the package




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Language Examples

Abstract Data Type in C#

Based on C++ and Java

Adds two access modifiers: internal and protected internal

All class instances are heap dynamic

Default constructors are available for allclasses

Garbage collection is used for most heap

objects, so destructors are rarely used structs are lightweight classes that do not

support inheritance

Common solution to need for access to datamembers: accessor methods (getter andsetter)

C# provides properties as a way ofimplementing getters and setters withoutrequiring explicit method calls




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Get /Set Methods

Getters are methods that return the valueof the field. Prefix the name of any method

that returns a value with ‘get’. If the getter is a Boolean field, prefix it with

‘is’.

Example:

• getFirstname();

• GetAccountNumber();

• isPersistent();

• isAtEnd();

Setters are method that modify the value ofa field.

Prefix with ‘set’, regardless of the field type. Example:

• setFirstName(String aName)

• setAccountNumber(int anAccountNumber)

• setReasonableGoals(Vector newGoals)

• setPersistent(boolean isPersistent)

• setAtEnd(boolean isAtEnd)


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Constructors

Functions to initialize data members ofinstances (they DO NOT create object)

May also allocate storage if part of theobject is heap-dynamic

Can include parameters to provide

parameterization of the objects

Implicitly called when an instance iscreated

Can be explicitly called

Name is the same as the class name




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Destructors

Functions to clean up after an instance isdestroyed; usually just to reclaim heap

storage

Implicitly called when the object’s lifetimeends

Can be explicitly called

Name is the class name, preceded by atilde (~)




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Modules

C/C++ Header Files

C and C++ provide a primitive means of

module abstraction. Header files are separate group of files

wherein certain subprogram definitions areobtained.

• conio.h – console input and output library

• stdio.h – standard input/output library

Users may create their own header files

Usually named after the use they areintended for such as:

vga.h – VGA graphics driver header

mouse.h – mouse driver routine header


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Modules

Packages in Ada

Grouping of subprograms

Group together declarations of related variables, constants, and types

Create encapsulated or abstract data typesfor which the name and the applicableoperations are declared

Valuable for programming design and

implementation of large systems software




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Modules

Packages in Java

Java packages are mechanisms for

organizing Java classes into namespaces.• These can be stored in compressed files called

JAR files

In a Java source file, the package that thisfile's class or classes belong to is specifiedusing the package keyword, which is

usually the first keyword in source file.

To use the classes of a package inside a Java source file, import the classes from thepackage using the import keyword.

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