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COBOL Tutorial

COBOL OBJECT - Quick Reference

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Overview

An OBJECT in object-oriented COBOL is an instance of a class that contains data (attributes) and can execute methods. Objects are created at runtime and represent specific instances of the class template.

Purpose and Usage

  • Runtime instances of classes
  • Data encapsulation and behavior
  • Method execution on object instances
  • Object-oriented programming support

Syntax

Object creation and usage in COBOL involves several components:

Object Reference Declaration

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* Declaring object references
01  my-object OBJECT REFERENCE.
01  employee-obj OBJECT REFERENCE Employee.
01  calculator-obj OBJECT REFERENCE Calculator.

OBJECT REFERENCE declares variables that can hold object instances.

Object Creation

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* Creating objects using factory methods
INVOKE Employee "NEW" RETURNING employee-obj.
 
* Alternative creation syntax
SET employee-obj TO NEW Employee.
 
* Creating with parameters
INVOKE Employee "NEW" USING "John Doe", 1001
    RETURNING employee-obj.

Method Invocation

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* Invoking methods on objects
INVOKE employee-obj "GetName" RETURNING employee-name.
INVOKE employee-obj "SetSalary" USING new-salary.
INVOKE calculator-obj "Add" USING a, b RETURNING result.

Practical Examples

Here are some practical uses of objects in COBOL:

Employee Object Usage

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* Working with employee objects
01  employee1 OBJECT REFERENCE Employee.
01  employee2 OBJECT REFERENCE Employee.
01  emp-name PIC X(30).
01  emp-salary PIC 9(7)V99.
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    * Create employee objects
    INVOKE Employee "NEW" USING "John Doe", 50000
        RETURNING employee1.
    INVOKE Employee "NEW" USING "Jane Smith", 60000
        RETURNING employee2.
    
    * Access object methods
    INVOKE employee1 "GetName" RETURNING emp-name.
    INVOKE employee1 "GetSalary" RETURNING emp-salary.
    
    DISPLAY "Employee: " emp-name.
    DISPLAY "Salary: " emp-salary.
    
    * Modify object state
    INVOKE employee1 "SetSalary" USING 55000.
    
    STOP RUN.

Creating and manipulating employee objects.

Calculator Object Usage

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* Working with calculator objects
01  calc OBJECT REFERENCE Calculator.
01  result PIC 9(6)V99.
01  a PIC 9(3) VALUE 10.
01  b PIC 9(3) VALUE 20.
 
PROCEDURE DIVISION.
CALCULATE.
    * Create calculator object
    INVOKE Calculator "NEW" RETURNING calc.
    
    * Perform calculations
    INVOKE calc "Add" USING a, b RETURNING result.
    DISPLAY "Sum: " result.
    
    INVOKE calc "Multiply" USING a, b RETURNING result.
    DISPLAY "Product: " result.
    
    INVOKE calc "Divide" USING b, a RETURNING result.
    DISPLAY "Quotient: " result.
    
    STOP RUN.

Using calculator objects for mathematical operations.

Best Practices

  • Use meaningful names for object references.
  • Always check if object references are valid before use.
  • Properly manage object lifecycle (creation and destruction).
  • Use encapsulation to protect object data.
  • Document object interfaces and expected behavior.

Common Pitfalls

  • Using uninitialized object references.
  • Not checking for null object references before method calls.
  • Memory leaks from not properly disposing of objects.
  • Incorrect method signatures when invoking object methods.

Test Your Knowledge

1. What is an OBJECT in object-oriented COBOL?

  • A data structure
  • An instance of a class
  • A file
  • A program

2. How do you create an object in COBOL?

  • Using the CREATE statement
  • Using the NEW statement
  • Using the FACTORY method
  • Using the INSTANTIATE statement

3. What does the OBJECT-REFERENCE data type represent?

  • A pointer to data
  • A reference to an object instance
  • A file handle
  • A memory address

4. How do you invoke methods on an object?

  • Using PERFORM
  • Using INVOKE
  • Using CALL
  • Using EXECUTE

5. What is the relationship between CLASS and OBJECT?

  • They are the same
  • CLASS is the template, OBJECT is the instance
  • OBJECT is the template, CLASS is the instance
  • They are unrelated

Frequently Asked Questions

Related Concepts

CLASS Statement

Defining classes in object-oriented COBOL.

METHOD-ID

Defining methods in COBOL classes.

INVOKE Statement

Invoking methods on objects.

Object-Oriented COBOL

Overview of OO COBOL concepts.

Encapsulation

Understanding encapsulation in COBOL.

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