Tài liệu Object Oriented Programming With Cobol pptx

The main function of the factory object is to create new instance objects, but it can also contain data and methods that are shared by all instance objects.. For example, the factory obj

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Table of Contents

About this Book 11

Audience 11

Notation 11

Command Lines 12

Side Headings 13

Part 1: Overview 1 Introduction 17

Object-oriented COBOL syntax 17

Developing OO Programs 18

What to Read Next 19

What to Read Next

We advise you to start by reading the chapter OO Programming

Concepts which provides an introduction to object-oriented

programming concepts and terminology

When you are ready to find out more about coding in object-oriented COBOL, move onto Parts 2 and Part 3 Part 2 contains more detailed information about object-oriented programming, specifically in COBOL, while Part 3 contains some tutorials If you want to go straight to the tutorials, we recommend that you try only the tutorials described in the

chapters Objects and Messages Tutorial and Simple Class Tutorial, before returning to Part 2, starting with the chapter Using Objects in

Programs All the chapters in Parts 2 and 3 use the ISO 2002 OO COBOL

syntax and terminology

The chapters in Parts 4, 5 and 6 all relate to features and facilities of Micro Focus COBOL and use the Micro Focus OO COBOL syntax and terminology Part 4 describes Micro Focus syntax that provides alternatives and extensions to the ISO 2002 syntax Part 5 is devoted to

the Micro Focus class libraries; start with the chapter Introduction to the

Class Libraries Part 6 contains tutorials that use Micro Focus alternatives

and extensions to the ISO 2002 syntax

Object-oriented Analysis and Design

The documentation supplied with your COBOL system will help you to start programming in OO COBOL However, if you are new to OO, learning about the principles of Object-Oriented Design and Analysis (OOD and OOA) will enable you to make the best use of this new technology

The reading list below suggests some books dealing with oriented methodologies and technologies

object-Booch, Grady Object-Oriented Design Benjamin/Cummings, 1994

ISBN: 0-8053-0091-0

Jacobson , Ivor Object-Oriented Software Engineering

Addison-Wesley, 1992 ISBN: 0-201-54435-0

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Rumbaugh James Object-Oriented Modeling and Design

Prentice Hall, 1991 ISBN: 0-13-629841-9

Shlaer, Sally and Mellor, Steve Object-Oriented Systems Analysis:

Modeling the World in Data Prentice Hall, 1988 ISBN: 0-13-629023-1

and Object Lifecycles: Modeling the World in States Prentice Hall, 1992

ISBN: 0-13-629940-7

Taylor, David Object Oriented Information Systems: Planning and

Implementation John Wiley, ISBN: 0-471-54364-0.

Wirfs-Brock, Rebecca Designing Object-oriented Software

Prentice Hall, 1990 ISBN: 0-13-629825-7

There are also several training organizations that run independent courses on OOD and OOA

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This chapter describes the key concepts supported by an oriented programming language, and how they are implemented by Micro Focus COBOL Micro Focus COBOL supports the ISO 2002 standard for Object-Oriented COBOL (OO COBOL) and supplies some additional OO facilities

• Encapsulation

• Inheritance

• Polymorphism The following sections describe these elements and characteristics in turn, and how they are implemented in OO COBOL

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An object is a combination of data and the procedures to operate on that data The data is known as the object’s attributes, and the procedures are known as its methods Every object in an object-oriented application has a unique object identifier, allocated to it at creation and fixed for its lifetime

Objects hold data, but they do not replace files and databases The data

in an object exists only during the lifetime of the object Objects are created and destroyed by object-oriented applications

Many of the objects in an OO application represent objects in the real world For example, a banking system would include objects to represent customers, accounts and ledgers The attributes of an account would include the balance, and its methods would include Debit, Credit,

GetBalance Figure 2-1 shows two ways of representing such an object

Figure 2-1 An Object

The user of an object can only find out about or change its attributes by making requests to the object These requests are known as messages, and each message invokes a method supported by the object The object interface is a description of all the messages to which the object responds For example, to find out the balance of an account, you would send an account object the message GetBalance

The actual representation of the data is known only to the object As long as the object interface remains the same, a programmer can

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An account object is said to be an instance of the account class, or an instance object, or simply an instance.

A class is a programming language construct, while an object is an entity that exists in memory at run time

A class not only defines an instance object; it also defines a factory object The factory object defines the class’s own data and behavior

The class does not have the same behavior as the instances it creates A class is like a printer’s plate, printing identical forms A plate enables you to print a form, but is not a form itself

The main function of the factory object is to create new instance objects, but it can also contain data and methods that are shared by all instance objects For example, the factory object of an account class could include a data item for keeping a count of the number of account objects created

In OO COBOL, a class is a COBOL source element, which consists of a set

of nested source elements, including source elements for the factory object and the instance object The factory source element contains all the attributes and methods specific to the factory object The object source element contains the attributes and methods specific to all

instance objects Figure 2-2 shows a class and instances being created

from it

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Figure 2-2 A Class and Instances Created From It

Note: Micro Focus COBOL provides alternative terminology and

features:

• The equivalent of a factory object is the class object

• The top-level source element for the class can contain data that is common to all instance objects and the class object

Methods

Methods are the pieces of code that implement the behavior of an object In OO COBOL, each method is a separate source element nested within the factory or object source element An object method can access its own data, the instance data and the factory data declared in

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method prototypes see the section Interfaces

Interfaces

Interfaces are collections of method prototypes The set of method prototypes defines a common behavior that a variety of objects might share For example, you might have an interface Rentable, that defines methods appropriate for objects that people can rent, such as cars and video tapes The Car class is a subclass of the Vehicle class, while the VideoTape class is a subclass of the VideoRecording class, but both Car and VideoTape implement the Rentable interface The interface methods might include pickUp and dropOff These are defined as prototypes in the interface Each class that implements the interface must provide full method definitions for the method prototypes in the interface

Interfaces give you additional flexibility in designing your OO applications; they are one of the elements in an OO language that

provide polymorphism (see the section Polymorphism).

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Messages may also optionally contain input and output parameters Where there is an output parameter, the sender will be expecting a reply to its message

The object reference enables the run-time system to find the object for which the message is intended The target of a message is known as the receiver The COBOL run-time system uses dynamic binding to

determine the receiver of a message This means that the receiver of a message is determined at run time rather than at compile time

The method selector is the text of the message; it tells the receiver which method it should invoke OO COBOL uses the INVOKE verb to

send messages Figure 2-3 shows the components of a message both

diagramatically and in OO COBOL syntax

Figure 2-3 A Message

Encapsulation

Encapsulation in OO langauges is the inclusion within an object of everything it needs to function: its data and the implementation of its methods Other objects can use it as long as they adhere to the object’s interface; they do not need to know anything about the way the object holds and manipulates its data Encapsulation helps to preserve the integrity of data

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Inheritance 27

Inheritance

Inheritance enables OO languages to mirror the real world more closely

by establishing hierarchies of classes Most objects in the real world belong to specific categories; for example, cars and motor bikes are vehicles, managers and workers are employees Using inheritance in OO applications reduces coding effort, because you only need to include methods in a subclass if the required behavior is different from that provided in the superclass When you create a method in a subclass of the same name as a method in the class it inherits from, the method in the subclass overrides the method in the class; the method in the subclass is known as a reimplementation of the method in the class

For example, our banking application might handle the following types

of account: checking accounts, credit card accounts and savings accounts There are some features that all these accounts have in common; a balance attribute, methods credit, debit and getBalance But there are also differences; a savings account pays interest, while a checking account may allow overdrafts

An Account class implements all the methods and attributes common to all its subclasses The subclasses implement the attributes and methods which they uniquely require

A deposit account implements its own version of Debit that does not

allow overdrafts Figure 2-4 shows a possible inheritance hierarchy for

bank accounts

Figure 2-4 Bank Account Inheritance

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Inheritance makes subsequent updates easier, because you can add new subclasses as required For example, the bank might introduce two new types of savings account: an instant access account, and a high interest account where you have to give notice of a withdrawal These could be subclassed from the Savings Account class, with additions to provide the new behavior required Because they still respond to the message interface used by all account objects, the changes do not ripple through the rest of the system

Micro Focus COBOL comes with a number of class libraries All the classes in the Base Class Library are ultimately descended from class Base Base provides methods which are required by all classes, which include methods for creating and destroying objects The classes you write are also likely to be subclasses of Base You can create your own root class and subclass from that, but you may need to duplicate some

of the services provided by Base

In OO COBOL, a factory of a subclass has access to all the factory methods of its superclasses An instance of a subclass has access to all the object methods of its superclasses

OO COBOL provides multiple inheritance; this means that in addition to

a subclass inheriting all the methods of the classes above it in the hierarchy, it can also inherit directly from more than one class This is

illustrated in Figure 2-5

Figure 2-5 Multiple Inheritance

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Polymorphism is provided by two mechanisms in OO COBOL:

• Inheritance Where a number of subclasses inherit from a class, the methods in the class can be method prototypes, that is methods that don’t contain any code Complete methods of the same name appear in each subclass, and these can be different

• Interfaces Interfaces are collections of methods applicable to a variety of objects Again, the methods in the interface are prototypes If a class implements an interface, it must provide complete methods of the same name, and these methods can differ between classes that implement the same interface

For example, consider a graphics drawing system that has objects representing squares and objects representing circles Circle objects and square objects both have the method draw, either because both are descended from a class that has the method draw, or because both implement an interface Drawable that has the method draw The code for drawing squares and circles are different, but the display controller can send the message draw to any graphical object, without caring what type it is The receiver of the message will execute its own draw

method, producing the correct result (see Figure 2-6)

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Figure 2-6 Polymorphism

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Part 2: Object-oriented COBOL Programming

This part contains the following chapters:

• Chapter 3, “Using Objects in Programs”

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This chapter describes how you can use objects in programs It uses examples of code to illustrate points For full syntax definitions, see

your Language Reference.

Programs that use objects all have certain features in common:

• They have a Repository paragraph

This lists all the classes and interfaces the program is going to use If the program itself is an OO COBOL class, the Repository paragraph also lists its superclass

Micro Focus COBOL provides alternative syntax; see the section

Summary of Syntax Alternatives in the chapter Micro Focus OO COBOL Alternative Syntax

• They declare one or more data items of type OBJECT REFERENCE

An object reference data item holds an object handle, that is, a pointer to the location of the object in memory The run-time system assigns a unique object handle to every single object active

in your application An object handle enables you to send messages

to the object

• They send messages

When you send a message to an object, you invoke a method inside the object Some methods receive or return parameters; when you invoke the method you include the parameters as part of the message

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Generally, a program that uses objects sends a message to one or more factory objects to create the instance objects it requires For each instance object created, you must declare an object reference to hold the object handle, although you may overwrite the object handle in an object reference with a new one if you have finished with it When a program has finished with an object, it should destroy it

checking to take place (see the section Conformance) Information

about classes is added to the external repository when a class is compiled

The CLASS clause also:

• Creates a data item for each class named At run time, this data item holds an object handle to the factory object

• Enables the run-time system to find and load the file with the class executable code

A class is guaranteed to be loaded before the factory object receives its first message Usually this occurs when you send the first message

to the factory object, but before the factory object receives it.When you declare a class, it effectively makes the class’s factory object available The class names in the Repository paragraph are

automatically declared as object reference data items This enables you

to send messages to the factory object

The code below shows class registration for classes CharacterArray and Employee:

repository

class CharacterArray as "chararry"

class Employee

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If you code the AS clause, the Compiler uses the value of the literal as the external name of the class program; otherwise it uses the class name changed to upper case If you are writing a class, you do not have to include a CLASS clause for the class you are writing; if you do include one, the Compiler ignores it and uses the class name or literal in the Class-ID paragraph in the Identification Division

Micro Focus COBOL provides alternative syntax; see the section

Summary of Syntax Alternatives in the chapter Micro Focus OO COBOL Alternative Syntax

Using Object References

An object reference can hold a handle to a factory or instance object

Depending on the syntax you use, object references are untyped or typed An untyped object reference data item can hold an object reference for any factory or instance object Untyped object references are also known as universal object references A typed object reference can only hold an object reference of the type specified by the syntax

It is good programming practice to use typed object references rather than untyped; this is because they allow the Compiler to perform its conformance checking, which tends to lessen the number of run-time errors that occur For more information on conformance checking see

the section Conformance.

Declaring Object References

You need to declare data items of type OBJECT REFERENCE to hold handles to any objects you will be using

For example:

01 anObject usage object reference

01 secdObject usage object reference factory of BankAccount

01 thirdObject usage object reference active-class

01 fourthObject usage object reference Rentable

The first line defines an untyped or universal object reference

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The second, third and fourth lines define typed object references:

• The secondObject data item can hold a handle only to the factory object of the class BankAccount or a subclass of that class

• The thirdObject data item can hold a handle only to an object of the active class, that is, the class in which this data description appears (either in the factory source element or the object source element),

or any subclass of that class Object references of type active-class cannot appear in procedural COBOL programs

• The fourthObject data item can hold a handle only to an object that implements the Rentable interface Interfaces are collections of method prototypes that define behavior; for more information about interfaces and how objects implement them see the chapter

Interfaces.

You do not need to declare object references for the factory objects of the classes you are going to use in your program These are declared automatically when you list your classes in the Repository paragraph

Manipulating Object References

The only direct operations you can use on object references are:

• Send a message to the object represented by the handle in the object reference For example:

invoke anObject "message"

• Copy an object reference to another For example:

set anObject1 to anObject2

• Test whether two object references refer to the same object For example:

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No other operations are valid, because the object reference contains only a handle to an object To change or interrogate an object’s internal state, you have to send messages to it

If you clear all references to a particular object, you will be unable to send that object any more messages In particular, you will not be able

to destroy that object and deallocate its storage (see the section

Destroying Objects ).

You can pass object references as parameters when you send a message

or make a COBOL CALL, in effect enabling you to pass objects between different parts of an application

01 obj1 usage object reference class shape

01 obj2 usage object reference class circle

01 obj3 usage object reference class triangle

01 obj4 usage object reference class rectangle

set obj1 to obj2set obj1 to obj3set obj1 to obj4This syntax enables polymorphism; in this example you use it in order to execute the right methods for the particular shape you are processing, for example the method for calculating its area

However, the following code is not valid:

set obj2 to obj1This is because obj1 might not be pointing to a circle object However, you might know that there are certain points in your program where obj1 does point to a circle object Object views enable you to exploit this knowledge In this example, you would recode your SET statement as follows:

set obj2 to obj1 as circle

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This statement will be accepted at compile time The presence of the object view "as circle" signals to the run-time system that it must check that obj1 is actually pointing to an object of class circle, before

executing the statement If the check fails, you get a run-time error.The object view syntax also provides:

• A FACTORY clause, which asks the run-time system to check that the object reference is pointing to the factory object of the class

• An ONLY phrase, which asks the run-time system to check that the object reference is pointing to an object of that class only and not any subclass

• An option to specify an interface name as an alternative to a class name If you specify an interface name, the run-time system checks that the object reference is pointing to an object that implements that interface

• A UNIVERSAL clause, which causes the object reference to be treated as universal

Method Inheritance in the chapter Classes for more information).

There are three ways of sending a message:

• The INVOKE statement

• In-line method invocation

• Object properties

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Sending Messages 39

Using the INVOKE Statement

You can send a message by using the INVOKE statement For example: invoke myClass "new" returning myObject

This sends message "new" to myClass; in this example myClass is an object reference referring to the class declared in the example in the previous section The target of an INVOKE statement is always an object reference USING and RETURNING parameters are optional, and follow the same rules as a COBOL CALL statement

You do not have to use a literal for the message name You can also put

a message name into a PIC x(n) data item For example:

move "new " to aMessage

invoke myClass aMessage returning myObjectHowever, there is a disadvantage to using a data item: the object reference that precedes the message name data item, myClass in this example, must be a universal object reference, and hence the Compiler cannot do its usual conformance checking For more information on

conformance checking see the section Conformance.

The data item used to store the message must be large enough to allow

at least one space at the end of the message name The run-time system needs the space to be able to find the end of the message name

The results are unpredictable if you send a message and use the same variable for sending a parameter and returning a result For example,

do not code:

invoke myObject "add" using aValue returning aValue

Inline Method Invocation

You can invoke a method inline, as long as it has a returning parameter,

by coding the object reference followed by the method and separated

by two colons in place of an operand in a statement, for example:

move anAccount::"getBalance" to current-balance

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This is equivalent to the following INVOKE statement:

invoke anAccount "getBalance" returning current-balance

Invocation Using Object Properties

You can invoke get property and set property methods using object properties Get and set property methods are methods that are either generated automatically by the Compiler in response to object property syntax in data definitions, or coded by you using special syntax in the Method-ID paragraph To invoke a get or set property method, you use

a statement such as:

move balance of anAccount to current-balanceThis statement invokes the get property method balance of the account object referred to by the object reference anAccount

For more details see the section Get and Set Property Methods in the chapter Methods

• An interface can also be a collection of method prototypes, specifying some behavior

Part of an object’s interface (in the first sense) can be a set of methods that it provides because it implements an interface (in the second sense) Interfaces in the second sense are explained in more detail in the

chapter Interfaces.

It is essential in OO programming that, when your program is executed, each object conforms to the interface that you defined for it To help to

Tiêu đề	Overview of Object Oriented Programming with COBOL
Trường học	Micro Focus International Limited
Chuyên ngành	Object Oriented Programming with COBOL
Thể loại	Tài liệu
Năm xuất bản	2004

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