COBOL Tutorial

COBOL OBJECT-REFERENCE - Object-Oriented Programming

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What is OBJECT-REFERENCE?

OBJECT-REFERENCE is a data type in object-oriented COBOL that holds a reference to an object (an instance of a class). Think of it as a "remote control" that lets you work with objects - you don't directly manipulate the object, but you use the reference to tell the object what to do.

🏗️ Real-World Analogy

Imagine you're building a house:

Class: The blueprint for a house (defines what a house should have)
Object: An actual house built from that blueprint
OBJECT-REFERENCE: The address of that house (lets you find and work with it)
Methods: Things you can do with the house (turn on lights, open doors)

Just like you need an address to find a house, you need an OBJECT-REFERENCE to work with an object.

Key Concepts

Object-Oriented Programming - A programming paradigm that uses objects to organize code
Class - A template that defines what an object should look like and what it can do
Instance - A specific object created from a class (like a specific house built from a blueprint)
Reference - A way to access and work with an object without directly manipulating its memory
Method Invocation - Telling an object to perform an action using its methods
Polymorphism - Using different types of objects through the same interface

How to Use OBJECT-REFERENCE

Using OBJECT-REFERENCE involves declaring variables, creating objects, and invoking methods. Let's look at the basic steps.

Basic OBJECT-REFERENCE Usage

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* Basic OBJECT-REFERENCE example
IDENTIFICATION DIVISION.
PROGRAM-ID. OBJECT-EXAMPLE.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
* Declare an object reference variable
01  CUSTOMER-OBJECT    OBJECT-REFERENCE.
01  ACCOUNT-OBJECT     OBJECT-REFERENCE.
01  RESULT-VALUE       PIC X(50).
 
PROCEDURE DIVISION.
    * Create a new customer object
    INVOKE Customer "NEW" RETURNING CUSTOMER-OBJECT
    
    * Set customer properties
    INVOKE CUSTOMER-OBJECT "setName" 
        USING BY CONTENT "John Doe"
    
    INVOKE CUSTOMER-OBJECT "setAccountNumber" 
        USING BY CONTENT "12345"
    
    * Get customer information
    INVOKE CUSTOMER-OBJECT "getName" 
        RETURNING RESULT-VALUE
    
    DISPLAY "Customer Name: " RESULT-VALUE
    
    * Create an account object
    INVOKE Account "NEW" RETURNING ACCOUNT-OBJECT
    
    * Link customer to account
    INVOKE CUSTOMER-OBJECT "setAccount" 
        USING BY REFERENCE ACCOUNT-OBJECT
    
    STOP RUN.

This example shows the basic pattern: declare, create, use, and invoke methods.

Object Reference Declaration

Declaration	Purpose	Example
01 var-name OBJECT-REFERENCE	Single object reference	01 CUSTOMER-OBJ OBJECT-REFERENCE
01 var-name OBJECT-REFERENCE OCCURS n TIMES	Array of object references	01 CUSTOMER-LIST OBJECT-REFERENCE OCCURS 10 TIMES
01 var-name OBJECT-REFERENCE CLASS class-name	Typed object reference	01 CUSTOMER-OBJ OBJECT-REFERENCE CLASS Customer
01 var-name OBJECT-REFERENCE INITIAL NULLS	Initialized to null	01 CUSTOMER-OBJ OBJECT-REFERENCE INITIAL NULLS

Practical Examples

Let's look at some real-world examples of how OBJECT-REFERENCE is used in COBOL applications.

Customer Management System

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* Customer management with object references
IDENTIFICATION DIVISION.
PROGRAM-ID. CUSTOMER-MANAGER.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CUSTOMER-OBJECT    OBJECT-REFERENCE.
01  ACCOUNT-OBJECT     OBJECT-REFERENCE.
01  CUSTOMER-NAME      PIC X(50).
01  ACCOUNT-NUMBER     PIC 9(10).
01  BALANCE            PIC 9(10)V99.
01  ACTION-CODE        PIC X(1).
 
PROCEDURE DIVISION.
    * Create customer object
    INVOKE Customer "NEW" RETURNING CUSTOMER-OBJECT
    
    * Set up customer information
    MOVE "John Smith" TO CUSTOMER-NAME
    MOVE "1234567890" TO ACCOUNT-NUMBER
    MOVE 1000.00 TO BALANCE
    
    INVOKE CUSTOMER-OBJECT "setName" 
        USING BY CONTENT CUSTOMER-NAME
    INVOKE CUSTOMER-OBJECT "setAccountNumber" 
        USING BY CONTENT ACCOUNT-NUMBER
    
    * Create account object
    INVOKE Account "NEW" RETURNING ACCOUNT-OBJECT
    INVOKE ACCOUNT-OBJECT "setBalance" 
        USING BY CONTENT BALANCE
    
    * Link customer to account
    INVOKE CUSTOMER-OBJECT "setAccount" 
        USING BY REFERENCE ACCOUNT-OBJECT
    
    * Perform customer operations
    PERFORM UNTIL ACTION-CODE = "Q"
        DISPLAY "Customer Operations:"
        DISPLAY "1 - View Customer Info"
        DISPLAY "2 - Deposit Money"
        DISPLAY "3 - Withdraw Money"
        DISPLAY "Q - Quit"
        ACCEPT ACTION-CODE
        
        EVALUATE ACTION-CODE
            WHEN "1" PERFORM DISPLAY-CUSTOMER-INFO
            WHEN "2" PERFORM DEPOSIT-MONEY
            WHEN "3" PERFORM WITHDRAW-MONEY
        END-EVALUATE
    END-PERFORM
    
    STOP RUN.
 
DISPLAY-CUSTOMER-INFO.
    INVOKE CUSTOMER-OBJECT "displayInfo".
    
DEPOSIT-MONEY.
    DISPLAY "Enter amount to deposit: "
    ACCEPT BALANCE
    INVOKE ACCOUNT-OBJECT "deposit" 
        USING BY CONTENT BALANCE.
    
WITHDRAW-MONEY.
    DISPLAY "Enter amount to withdraw: "
    ACCEPT BALANCE
    INVOKE ACCOUNT-OBJECT "withdraw" 
        USING BY CONTENT BALANCE.

This example shows a complete customer management system using object references.

Polymorphic Shape Calculator

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* Polymorphic shape calculator using object references
IDENTIFICATION DIVISION.
PROGRAM-ID. SHAPE-CALCULATOR.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  SHAPE-OBJECT       OBJECT-REFERENCE.
01  SHAPE-TYPE         PIC X(10).
01  LENGTH             PIC 9(3)V99.
01  WIDTH              PIC 9(3)V99.
01  RADIUS             PIC 9(3)V99.
01  AREA               PIC 9(6)V99.
 
PROCEDURE DIVISION.
    DISPLAY "Shape Calculator"
    DISPLAY "Enter shape type (RECTANGLE/CIRCLE): "
    ACCEPT SHAPE-TYPE
    
    * Create appropriate shape object based on type
    EVALUATE SHAPE-TYPE
        WHEN "RECTANGLE"
            DISPLAY "Enter length: "
            ACCEPT LENGTH
            DISPLAY "Enter width: "
            ACCEPT WIDTH
            
            INVOKE Rectangle "NEW" 
                USING BY CONTENT LENGTH WIDTH
                RETURNING SHAPE-OBJECT
                
        WHEN "CIRCLE"
            DISPLAY "Enter radius: "
            ACCEPT RADIUS
            
            INVOKE Circle "NEW" 
                USING BY CONTENT RADIUS
                RETURNING SHAPE-OBJECT
                
        WHEN OTHER
            DISPLAY "Unknown shape type"
            STOP RUN
    END-EVALUATE
    
    * Calculate area using polymorphic method call
    INVOKE SHAPE-OBJECT "calculateArea" 
        RETURNING AREA
    
    DISPLAY "Area: " AREA
    
    * Display shape information
    INVOKE SHAPE-OBJECT "displayInfo"
    
    STOP RUN.

This example demonstrates polymorphism - the same method call works for different object types.

Advanced OBJECT-REFERENCE Features

OBJECT-REFERENCE supports advanced features for complex object-oriented programming scenarios.

Object Collections and Arrays

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* Working with collections of objects
IDENTIFICATION DIVISION.
PROGRAM-ID. OBJECT-COLLECTION.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CUSTOMER-LIST     OBJECT-REFERENCE OCCURS 10 TIMES.
01  CUSTOMER-COUNT    PIC 9(2) VALUE 0.
01  I                 PIC 9(2).
01  CUSTOMER-NAME     PIC X(50).
01  CUSTOMER-OBJECT   OBJECT-REFERENCE.
 
PROCEDURE DIVISION.
    * Add customers to the collection
    PERFORM ADD-CUSTOMER UNTIL CUSTOMER-COUNT >= 10
    
    * Display all customers
    PERFORM VARYING I FROM 1 BY 1 
        UNTIL I > CUSTOMER-COUNT
        INVOKE CUSTOMER-LIST(I) "getName" 
            RETURNING CUSTOMER-NAME
        DISPLAY "Customer " I ": " CUSTOMER-NAME
    END-PERFORM
    
    STOP RUN.
 
ADD-CUSTOMER.
    ADD 1 TO CUSTOMER-COUNT
    MOVE CUSTOMER-COUNT TO I
    
    DISPLAY "Enter customer name: "
    ACCEPT CUSTOMER-NAME
    
    * Create new customer object
    INVOKE Customer "NEW" RETURNING CUSTOMER-OBJECT
    INVOKE CUSTOMER-OBJECT "setName" 
        USING BY CONTENT CUSTOMER-NAME
    
    * Store in collection
    MOVE CUSTOMER-OBJECT TO CUSTOMER-LIST(I).

This example shows how to work with collections of objects using arrays of OBJECT-REFERENCE.

Null Reference Handling

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* Safe object reference handling
IDENTIFICATION DIVISION.
PROGRAM-ID. SAFE-OBJECT-HANDLING.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CUSTOMER-OBJECT    OBJECT-REFERENCE INITIAL NULLS.
01  CUSTOMER-NAME      PIC X(50).
01  OBJECT-EXISTS      PIC X VALUE "N".
 
PROCEDURE DIVISION.
    * Check if object exists before using it
    IF CUSTOMER-OBJECT = NULLS
        DISPLAY "No customer object exists"
        PERFORM CREATE-CUSTOMER
    END-IF
    
    * Safe method invocation
    IF CUSTOMER-OBJECT NOT = NULLS
        INVOKE CUSTOMER-OBJECT "getName" 
            RETURNING CUSTOMER-NAME
        DISPLAY "Customer: " CUSTOMER-NAME
    ELSE
        DISPLAY "Error: Customer object is null"
    END-IF
    
    STOP RUN.
 
CREATE-CUSTOMER.
    INVOKE Customer "NEW" RETURNING CUSTOMER-OBJECT
    INVOKE CUSTOMER-OBJECT "setName" 
        USING BY CONTENT "Default Customer".

This example shows how to safely handle null object references to prevent runtime errors.

Best Practices and Tips

Following these best practices will help you use OBJECT-REFERENCE effectively in your COBOL applications.

OBJECT-REFERENCE Best Practices

Always check for null references - Check if OBJECT-REFERENCE is NULLS before using it
Use meaningful variable names - Name your object references clearly (e.g., CUSTOMER-OBJECT, ACCOUNT-OBJECT)
Initialize object references - Use INITIAL NULLS to ensure proper initialization
Handle exceptions - Use ON EXCEPTION clauses in INVOKE statements
Document object relationships - Document how objects relate to each other
Use polymorphism appropriately - Leverage polymorphism for flexible, maintainable code
Clean up resources - Set object references to NULLS when done with them

Common Mistakes to Avoid

Mistake	Problem	Solution
Not checking for null references	Runtime errors when invoking methods	Always check IF OBJECT-REF = NULLS before use
Using wrong parameter types	Method invocation failures	Match parameter types exactly with method signature
Forgetting to create objects	Null reference errors	Always use INVOKE class "NEW" to create objects
Not handling exceptions	Program crashes on errors	Use ON EXCEPTION clauses in INVOKE statements
Memory leaks	Excessive memory usage	Set object references to NULLS when done

OBJECT-REFERENCE Quick Reference

Action	Syntax	Example
Declare object reference	01 var-name OBJECT-REFERENCE	01 CUSTOMER-OBJ OBJECT-REFERENCE
Create object instance	INVOKE class "NEW" RETURNING obj-ref	INVOKE Customer "NEW" RETURNING CUSTOMER-OBJ
Invoke method	INVOKE obj-ref "method" USING params	INVOKE CUSTOMER-OBJ "setName" USING "John"
Check for null	IF obj-ref = NULLS	IF CUSTOMER-OBJ = NULLS
Set to null	MOVE NULLS TO obj-ref	MOVE NULLS TO CUSTOMER-OBJ
Array of objects	01 array OBJECT-REFERENCE OCCURS n	01 CUSTOMER-LIST OBJECT-REFERENCE OCCURS 10

Test Your Knowledge

1. What is an OBJECT-REFERENCE in COBOL?

A pointer to a data file
A reference to a class instance in object-oriented COBOL
A reference to a subroutine
A memory address

2. How do you declare an OBJECT-REFERENCE variable?

01 MY-OBJECT PIC X(10)
01 MY-OBJECT OBJECT-REFERENCE
01 MY-OBJECT POINTER
01 MY-OBJECT REFERENCE

3. How do you create an object instance using OBJECT-REFERENCE?

MOVE "CLASS-NAME" TO OBJECT-REF
INVOKE CLASS-NAME "NEW" RETURNING OBJECT-REF
CALL CLASS-NAME USING OBJECT-REF
SET OBJECT-REF TO CLASS-NAME

4. What is polymorphism in the context of OBJECT-REFERENCE?

Multiple objects with the same name
The ability to use different object types through the same interface
Objects that can change their type
Objects that can reference themselves

5. How do you check if an OBJECT-REFERENCE is null?

IF OBJECT-REF = NULL
IF OBJECT-REF = SPACES
IF OBJECT-REF = ZERO
IF OBJECT-REF = NULLS

COBOL Tutorial

COBOL OBJECT-REFERENCE - Object-Oriented Programming

What is OBJECT-REFERENCE?

🏗️ Real-World Analogy

Key Concepts

How to Use OBJECT-REFERENCE

Basic OBJECT-REFERENCE Usage

Object Reference Declaration

Practical Examples

Customer Management System

Polymorphic Shape Calculator

Advanced OBJECT-REFERENCE Features

Object Collections and Arrays

Null Reference Handling

Best Practices and Tips

OBJECT-REFERENCE Best Practices

Common Mistakes to Avoid

OBJECT-REFERENCE Quick Reference

Test Your Knowledge

Frequently Asked Questions

Related Concepts

Object-Oriented Programming in COBOL

Classes and Objects

Method Invocation

Polymorphism

Inheritance

Related Pages

COBOL Tutorial

COBOL OBJECT-REFERENCE - Object-Oriented Programming

What is OBJECT-REFERENCE?

🏗️ Real-World Analogy

Key Concepts

How to Use OBJECT-REFERENCE

Basic OBJECT-REFERENCE Usage

Object Reference Declaration

Practical Examples

Customer Management System

Polymorphic Shape Calculator

Advanced OBJECT-REFERENCE Features

Object Collections and Arrays

Null Reference Handling

Best Practices and Tips

OBJECT-REFERENCE Best Practices

Common Mistakes to Avoid

OBJECT-REFERENCE Quick Reference

Test Your Knowledge

Frequently Asked Questions

What is the difference between OBJECT-REFERENCE and POINTER?

Can I use OBJECT-REFERENCE in traditional procedural COBOL?

How do I handle errors when working with OBJECT-REFERENCE?

Can I store OBJECT-REFERENCE in files or pass them between programs?

What happens if I try to invoke a method on a null OBJECT-REFERENCE?

Related Concepts

Object-Oriented Programming in COBOL

Classes and Objects

Method Invocation

Polymorphism

Inheritance

Related Pages