What is class design in COBOL?

Class design in COBOL involves creating object-oriented structures using CLASS-ID definitions, defining methods, properties, and inheritance relationships to build reusable and maintainable code components.

How do you define a class in COBOL?

Classes in COBOL are defined using CLASS-ID statements in the IDENTIFICATION DIVISION, followed by METHOD-ID definitions for class methods and PROPERTY definitions for class attributes.

What is inheritance in COBOL?

Inheritance in COBOL allows classes to inherit properties and methods from parent classes using INHERITS clause, enabling code reuse and hierarchical class relationships.

How do you implement encapsulation in COBOL?

Encapsulation in COBOL is implemented using PRIVATE and PUBLIC access modifiers, controlling access to class properties and methods from outside the class.

What is polymorphism in COBOL?

Polymorphism in COBOL allows objects of different classes to be treated uniformly through method overriding and dynamic method dispatch, enabling flexible object interactions.

How do you create objects from classes in COBOL?

Objects are created from classes using INVOKE statements with NEW keyword, which instantiates the class and returns an object reference for method calls.

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COBOL Class Design

Class design in COBOL represents the foundation of object-oriented programming, allowing developers to create reusable, maintainable, and well-structured code. Modern COBOL supports full object-oriented capabilities including classes, inheritance, encapsulation, and polymorphism.

Understanding Class Design Concepts

A class in COBOL is a blueprint that defines the structure and behavior of objects. It encapsulates data (properties) and functions (methods) into a single unit, providing a template for creating multiple instances of similar objects. This approach promotes code reusability and makes programs easier to maintain.

Basic Class Structure

1. Class Definition

Every class in COBOL begins with a CLASS-ID statement in the IDENTIFICATION DIVISION. This statement defines the class name and can include inheritance specifications. The class definition serves as the foundation for all object-oriented functionality.

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       IDENTIFICATION DIVISION.
       CLASS-ID. CUSTOMER-CLASS.
       *> This defines a class named CUSTOMER-CLASS
       *> The class will contain customer-related data and operations
       
       ENVIRONMENT DIVISION.
       CONFIGURATION SECTION.
       *> Environment settings for the class
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       *> Class-level data definitions go here
       
       PROCEDURE DIVISION.
       *> Class methods will be defined here

The CLASS-ID statement creates a new class definition. The name CUSTOMER-CLASS becomes the identifier for this class throughout the program. This is similar to defining a data type, but with the added capability of containing both data and methods that operate on that data.

2. Property Definitions

Properties in COBOL classes define the data attributes that each object instance will contain. These properties can have different access levels (PUBLIC, PRIVATE, PROTECTED) to control how they can be accessed from outside the class.

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       DATA DIVISION.
       WORKING-STORAGE SECTION.
       
       *> Public properties - accessible from outside the class
       01 CUSTOMER-ID           PIC 9(8) VALUE ZERO.
       01 CUSTOMER-NAME        PIC X(50) VALUE SPACES.
       01 CUSTOMER-EMAIL       PIC X(100) VALUE SPACES.
       
       *> Private properties - only accessible within the class
       01 PRIVATE-CUSTOMER-BALANCE PIC 9(10)V99 VALUE ZERO.
       01 PRIVATE-CREDIT-LIMIT     PIC 9(8)V99 VALUE ZERO.
       
       *> Protected properties - accessible by subclasses
       01 PROTECTED-ACCOUNT-STATUS PIC X(1) VALUE 'A'.
           88 ACTIVE-ACCOUNT VALUE 'A'.
           88 INACTIVE-ACCOUNT VALUE 'I'.
           88 SUSPENDED-ACCOUNT VALUE 'S'.

Properties are defined in the WORKING-STORAGE SECTION of the class. Public properties can be accessed directly from outside the class, while private properties are hidden and can only be accessed through class methods. Protected properties are accessible to the class itself and any subclasses that inherit from it.

Method Definitions

1. Basic Method Structure

Methods in COBOL classes are defined using METHOD-ID statements. Each method represents a specific behavior or operation that objects of this class can perform. Methods can accept parameters and return values, making them flexible tools for object interaction.

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       PROCEDURE DIVISION.
       
       METHOD-ID. SET-CUSTOMER-DATA.
       *> This method sets customer information
       *> Parameters: customer-id, customer-name, customer-email
       DATA DIVISION.
       LINKAGE SECTION.
       01 CUSTOMER-ID-PARAM    PIC 9(8).
       01 CUSTOMER-NAME-PARAM   PIC X(50).
       01 CUSTOMER-EMAIL-PARAM  PIC X(100).
       
       PROCEDURE DIVISION USING CUSTOMER-ID-PARAM
                                CUSTOMER-NAME-PARAM
                                CUSTOMER-EMAIL-PARAM.
       *> Store the passed parameters into class properties
           MOVE CUSTOMER-ID-PARAM TO CUSTOMER-ID
           MOVE CUSTOMER-NAME-PARAM TO CUSTOMER-NAME
           MOVE CUSTOMER-EMAIL-PARAM TO CUSTOMER-EMAIL
           
       *> Validate the data before storing
           IF CUSTOMER-ID-PARAM > 0
               DISPLAY 'Customer data set successfully'
           ELSE
               DISPLAY 'Invalid customer ID provided'
           END-IF
           
       END METHOD SET-CUSTOMER-DATA.

The METHOD-ID statement defines a new method within the class. The USING clause specifies the parameters that the method accepts. Inside the method, we can access both the parameters and the class properties. This method demonstrates data validation and assignment, which are common patterns in class design.

2. Method with Return Value

Methods can return values to the calling code. This is useful for retrieving calculated results or accessing private data through controlled interfaces. The RETURNING clause specifies what value the method will return.

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       METHOD-ID. GET-CUSTOMER-BALANCE.
       *> This method returns the customer's current balance
       *> Returns: customer balance as a numeric value
       DATA DIVISION.
       LINKAGE SECTION.
       01 RETURN-BALANCE        PIC 9(10)V99.
       
       PROCEDURE DIVISION RETURNING RETURN-BALANCE.
       *> Return the private balance property
           MOVE PRIVATE-CUSTOMER-BALANCE TO RETURN-BALANCE
           
       *> Log the balance access for audit purposes
           DISPLAY 'Balance accessed for customer: ' CUSTOMER-ID
           
       END METHOD GET-CUSTOMER-BALANCE.
       
       METHOD-ID. CALCULATE-TOTAL-CHARGES.
       *> This method calculates total charges based on transactions
       DATA DIVISION.
       LINKAGE SECTION.
       01 TRANSACTION-COUNT     PIC 9(4).
       01 AVERAGE-TRANSACTION   PIC 9(6)V99.
       01 RETURN-TOTAL          PIC 9(10)V99.
       
       PROCEDURE DIVISION USING TRANSACTION-COUNT
                                AVERAGE-TRANSACTION
                       RETURNING RETURN-TOTAL.
       *> Calculate total charges
           COMPUTE RETURN-TOTAL = TRANSACTION-COUNT * AVERAGE-TRANSACTION
           
       *> Apply any discounts or fees
           IF TRANSACTION-COUNT > 100
               COMPUTE RETURN-TOTAL = RETURN-TOTAL * 0.95
               DISPLAY 'Volume discount applied'
           END-IF
           
       END METHOD CALCULATE-TOTAL-CHARGES.

The RETURNING clause allows methods to return calculated values. The first method demonstrates accessing private data through a controlled interface, while the second method shows how to perform calculations and return results. This pattern maintains encapsulation while providing necessary functionality.

Inheritance in Class Design

1. Creating a Subclass

Inheritance allows new classes to be created based on existing classes, inheriting their properties and methods. This promotes code reuse and creates hierarchical relationships between classes. The INHERITS clause establishes the inheritance relationship.

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       IDENTIFICATION DIVISION.
       CLASS-ID. PREMIUM-CUSTOMER-CLASS
           INHERITS FROM CUSTOMER-CLASS.
       *> This class inherits all properties and methods from CUSTOMER-CLASS
       *> It adds premium-specific functionality
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       *> Additional properties specific to premium customers
       01 PREMIUM-MEMBERSHIP-TYPE PIC X(10) VALUE SPACES.
       01 PREMIUM-DISCOUNT-RATE  PIC 9(3)V99 VALUE ZERO.
       01 CONCIERGE-SERVICE-FLAG PIC X(1) VALUE 'N'.
           88 CONCIERGE-AVAILABLE VALUE 'Y'.
           88 CONCIERGE-NOT-AVAILABLE VALUE 'N'.
       
       PROCEDURE DIVISION.
       
       METHOD-ID. SET-PREMIUM-MEMBERSHIP.
       *> This method sets premium membership details
       DATA DIVISION.
       LINKAGE SECTION.
       01 MEMBERSHIP-TYPE-PARAM PIC X(10).
       01 DISCOUNT-RATE-PARAM   PIC 9(3)V99.
       
       PROCEDURE DIVISION USING MEMBERSHIP-TYPE-PARAM
                                DISCOUNT-RATE-PARAM.
       *> Set premium-specific properties
           MOVE MEMBERSHIP-TYPE-PARAM TO PREMIUM-MEMBERSHIP-TYPE
           MOVE DISCOUNT-RATE-PARAM TO PREMIUM-DISCOUNT-RATE
           
       *> Enable concierge service for certain membership types
           IF MEMBERSHIP-TYPE-PARAM = 'PLATINUM'
               SET CONCIERGE-AVAILABLE TO TRUE
               DISPLAY 'Concierge service enabled'
           ELSE
               SET CONCIERGE-NOT-AVAILABLE TO TRUE
           END-IF
           
       END METHOD SET-PREMIUM-MEMBERSHIP.

The INHERITS FROM clause creates a subclass that automatically includes all properties and methods from the parent class. The subclass can add new properties and methods, and can also override inherited methods to provide specialized behavior. This demonstrates the "is-a" relationship in object-oriented design.

2. Method Overriding

Method overriding allows subclasses to provide their own implementation of inherited methods. This enables polymorphic behavior where the same method call can produce different results depending on the actual object type.

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       METHOD-ID. CALCULATE-TOTAL-CHARGES.
       *> This method overrides the inherited method to apply premium discounts
       DATA DIVISION.
       LINKAGE SECTION.
       01 TRANSACTION-COUNT     PIC 9(4).
       01 AVERAGE-TRANSACTION   PIC 9(6)V99.
       01 RETURN-TOTAL          PIC 9(10)V99.
       
       PROCEDURE DIVISION USING TRANSACTION-COUNT
                                AVERAGE-TRANSACTION
                       RETURNING RETURN-TOTAL.
       *> First calculate the base total using parent class logic
           INVOKE SUPER "CALCULATE-TOTAL-CHARGES" USING TRANSACTION-COUNT
                                                       AVERAGE-TRANSACTION
                                               RETURNING RETURN-TOTAL
       
       *> Apply premium customer discount
           COMPUTE RETURN-TOTAL = RETURN-TOTAL * (1 - PREMIUM-DISCOUNT-RATE / 100)
           
       *> Additional premium benefits
           IF CONCIERGE-AVAILABLE
               DISPLAY 'Premium concierge service included'
           END-IF
           
       END METHOD CALCULATE-TOTAL-CHARGES.

Method overriding allows the subclass to provide specialized behavior while still leveraging the parent class functionality. The INVOKE SUPER statement calls the parent class version of the method, then the subclass adds its own logic. This pattern maintains the inheritance hierarchy while providing customized behavior.

Encapsulation Techniques

1. Access Control

Encapsulation in COBOL classes is achieved through careful control of property and method access. Private properties are hidden from external access, while public methods provide controlled interfaces for data manipulation. This protects data integrity and provides a clean API for class usage.

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       METHOD-ID. UPDATE-BALANCE.
       *> This method provides controlled access to the private balance property
       DATA DIVISION.
       LINKAGE SECTION.
       01 TRANSACTION-AMOUNT    PIC S9(8)V99.
       01 TRANSACTION-TYPE      PIC X(1).
           88 DEPOSIT VALUE 'D'.
           88 WITHDRAWAL VALUE 'W'.
       01 RETURN-SUCCESS        PIC X(1).
           88 SUCCESS VALUE 'Y'.
           88 FAILURE VALUE 'N'.
       
       PROCEDURE DIVISION USING TRANSACTION-AMOUNT
                                TRANSACTION-TYPE
                       RETURNING RETURN-SUCCESS.
       *> Validate the transaction before processing
           IF TRANSACTION-AMOUNT <= 0
               SET FAILURE TO TRUE
               DISPLAY 'Invalid transaction amount'
               EXIT METHOD
           END-IF
           
       *> Process deposit
           IF DEPOSIT
               ADD TRANSACTION-AMOUNT TO PRIVATE-CUSTOMER-BALANCE
               SET SUCCESS TO TRUE
               DISPLAY 'Deposit processed successfully'
               
       *> Process withdrawal with balance check
           ELSE IF WITHDRAWAL
               IF TRANSACTION-AMOUNT <= PRIVATE-CUSTOMER-BALANCE
                   SUBTRACT TRANSACTION-AMOUNT FROM PRIVATE-CUSTOMER-BALANCE
                   SET SUCCESS TO TRUE
                   DISPLAY 'Withdrawal processed successfully'
               ELSE
                   SET FAILURE TO TRUE
                   DISPLAY 'Insufficient funds for withdrawal'
               END-IF
           ELSE
               SET FAILURE TO TRUE
               DISPLAY 'Invalid transaction type'
           END-IF
           
       END METHOD UPDATE-BALANCE.

This method demonstrates proper encapsulation by providing controlled access to the private balance property. The method validates all inputs, performs business logic checks (like sufficient funds), and only modifies the private data if all conditions are met. This protects the data from invalid operations and provides clear feedback about the operation's success or failure.

2. Data Validation Methods

Validation methods ensure data integrity by checking input values before they are stored in class properties. These methods encapsulate business rules and validation logic, making the class more robust and easier to maintain.

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       METHOD-ID. VALIDATE-CUSTOMER-DATA.
       *> This method validates all customer data before storage
       DATA DIVISION.
       LINKAGE SECTION.
       01 VALIDATION-RESULT     PIC X(1).
           88 DATA-VALID VALUE 'Y'.
           88 DATA-INVALID VALUE 'N'.
       01 ERROR-MESSAGE         PIC X(100).
       
       PROCEDURE DIVISION RETURNING VALIDATION-RESULT
                                ERROR-MESSAGE.
       *> Initialize validation result
           SET DATA-VALID TO TRUE
           MOVE SPACES TO ERROR-MESSAGE
           
       *> Validate customer ID
           IF CUSTOMER-ID <= 0
               SET DATA-INVALID TO TRUE
               MOVE 'Customer ID must be greater than zero' TO ERROR-MESSAGE
               EXIT METHOD
           END-IF
           
       *> Validate customer name
           IF CUSTOMER-NAME = SPACES
               SET DATA-INVALID TO TRUE
               MOVE 'Customer name cannot be empty' TO ERROR-MESSAGE
               EXIT METHOD
           END-IF
           
       *> Validate email format (basic check)
           IF CUSTOMER-EMAIL NOT CONTAINS '@'
               SET DATA-INVALID TO TRUE
               MOVE 'Email must contain @ symbol' TO ERROR-MESSAGE
               EXIT METHOD
           END-IF
           
       *> Validate account status
           IF NOT ACTIVE-ACCOUNT AND NOT INACTIVE-ACCOUNT AND NOT SUSPENDED-ACCOUNT
               SET DATA-INVALID TO TRUE
               MOVE 'Invalid account status' TO ERROR-MESSAGE
               EXIT METHOD
           END-IF
           
       END METHOD VALIDATE-CUSTOMER-DATA.

This validation method demonstrates comprehensive data checking. It examines each property for validity according to business rules, provides specific error messages for different types of validation failures, and returns both a success indicator and descriptive error information. This approach makes debugging easier and provides clear feedback to calling code.

Polymorphism Implementation

1. Interface-Based Polymorphism

Polymorphism in COBOL allows objects of different classes to be treated uniformly through common interfaces. This is achieved by defining methods with the same signature across different classes, enabling flexible and extensible code design.

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       METHOD-ID. PROCESS-PAYMENT.
       *> This method processes payments differently based on customer type
       DATA DIVISION.
       LINKAGE SECTION.
       01 PAYMENT-AMOUNT        PIC 9(8)V99.
       01 PAYMENT-METHOD        PIC X(10).
       01 PROCESSING-RESULT    PIC X(1).
           88 PAYMENT-SUCCESS VALUE 'Y'.
           88 PAYMENT-FAILED VALUE 'N'.
       
       PROCEDURE DIVISION USING PAYMENT-AMOUNT
                                PAYMENT-METHOD
                       RETURNING PROCESSING-RESULT.
       *> Different processing based on customer type
           EVALUATE PAYMENT-METHOD
               WHEN 'CREDIT-CARD'
                   PERFORM PROCESS-CREDIT-CARD-PAYMENT
               WHEN 'BANK-TRANSFER'
                   PERFORM PROCESS-BANK-TRANSFER
               WHEN 'CASH'
                   PERFORM PROCESS-CASH-PAYMENT
               WHEN OTHER
                   SET PAYMENT-FAILED TO TRUE
                   DISPLAY 'Unsupported payment method'
           END-EVALUATE
           
       END METHOD PROCESS-PAYMENT.
       
       METHOD-ID. PROCESS-CREDIT-CARD-PAYMENT.
       *> Process credit card payments with validation
       PROCEDURE DIVISION.
       *> Check credit limit
           IF PAYMENT-AMOUNT <= PRIVATE-CREDIT-LIMIT
               SET PAYMENT-SUCCESS TO TRUE
               DISPLAY 'Credit card payment processed'
           ELSE
               SET PAYMENT-FAILED TO TRUE
               DISPLAY 'Payment exceeds credit limit'
           END-IF
       END METHOD PROCESS-CREDIT-CARD-PAYMENT.

This method demonstrates polymorphic behavior by handling different payment methods through a unified interface. The same method call can process various payment types, with each type having its own specific implementation. This makes the code flexible and easy to extend with new payment methods.

Best Practices for Class Design

Single Responsibility: Each class should have one clear purpose and responsibility
Encapsulation: Hide internal implementation details and provide controlled access
Inheritance: Use inheritance to model "is-a" relationships and promote code reuse
Polymorphism: Design methods to work with multiple object types through common interfaces
Validation: Always validate input data before storing in class properties
Documentation: Document class purpose, methods, and properties clearly

Common Class Design Patterns

Factory Pattern: Create objects through factory methods
Singleton Pattern: Ensure only one instance of a class exists
Observer Pattern: Notify multiple objects of state changes
Strategy Pattern: Encapsulate algorithms in separate classes
Template Method: Define algorithm structure in base class
Builder Pattern: Construct complex objects step by step

COBOL Class Design

Understanding Class Design Concepts

Basic Class Structure

1. Class Definition

2. Property Definitions

Method Definitions

1. Basic Method Structure

2. Method with Return Value

Inheritance in Class Design

1. Creating a Subclass

2. Method Overriding

Encapsulation Techniques

1. Access Control

2. Data Validation Methods

Polymorphism Implementation

1. Interface-Based Polymorphism

Best Practices for Class Design

Common Class Design Patterns

Object-Oriented Programming

Classes and Objects

Inheritance

Polymorphism

Encapsulation

Method Programming