COBOL Tutorial

COBOL OVERRIDE - Method Overriding

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What is OVERRIDE?

OVERRIDE is a keyword in object-oriented COBOL that allows a subclass to provide a new implementation of a method that is already defined in its parent class. Think of it as "customizing" inherited behavior - you're taking a method from your parent class and giving it your own special implementation while keeping the same interface.

🎭 Real-World Analogy

Imagine you're running a restaurant chain:

Parent Class: The main restaurant (has a standard "cook" method)
Subclass: A pizza restaurant (inherits from main restaurant)
OVERRIDE: The pizza restaurant provides its own "cook" method for making pizza
Polymorphism: When someone asks any restaurant to "cook", each one does it their way

OVERRIDE lets each restaurant type customize how they cook while keeping the same "cook" interface.

Key Concepts

Inheritance - A subclass inherits methods from its parent class
Method Overriding - Replacing an inherited method with a new implementation
Polymorphism - Using the same method name for different behaviors
Liskov Substitution - Subclasses can be used wherever parent classes are expected
Method Signature - The name, parameters, and return type must match exactly
Runtime Resolution - The actual method called depends on the object type at runtime

How to Use OVERRIDE

Using OVERRIDE involves creating a subclass that inherits from a parent class and then providing your own implementation of inherited methods. The OVERRIDE keyword tells COBOL that you're intentionally replacing the parent's method.

Basic Method Override

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* Basic method override example
IDENTIFICATION DIVISION.
PROGRAM-ID. OVERRIDE-EXAMPLE.
 
* Parent class definition
CLASS-ID. Animal.
 
METHOD-ID. makeSound.
    DISPLAY "Some animal sound"
    EXIT METHOD.
END METHOD makeSound.
 
END CLASS Animal.
 
* Subclass with override
CLASS-ID. Dog INHERITS FROM Animal.
 
METHOD-ID. makeSound OVERRIDE.
    DISPLAY "Woof! Woof!"
    EXIT METHOD.
END METHOD makeSound.
 
END CLASS Dog.
 
* Another subclass with different override
CLASS-ID. Cat INHERITS FROM Animal.
 
METHOD-ID. makeSound OVERRIDE.
    DISPLAY "Meow! Meow!"
    EXIT METHOD.
END METHOD makeSound.
 
END CLASS Cat.

This example shows how different subclasses can override the same method with different implementations.

Method Signature Requirements

Element	Requirement	Example
Method name	Must be identical	calculateArea
Parameter types	Must match in order and type	PIC 9(8)V99, PIC X(50)
Return type	Must be compatible	PIC 9(8)V99
OVERRIDE keyword	Must be included	METHOD-ID. name OVERRIDE

Practical Examples

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

Bank Account Hierarchy

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* Bank account inheritance with method overriding
IDENTIFICATION DIVISION.
PROGRAM-ID. BANK-ACCOUNTS.
 
* Base account class
CLASS-ID. BankAccount.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  ACCOUNT-NUMBER  PIC 9(10).
01  BALANCE         PIC 9(10)V99.
01  INTEREST-RATE   PIC 9(3)V99 VALUE 0.05.
 
METHOD-ID. calculateInterest.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(10)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    COMPUTE RETURN-VALUE = BALANCE * INTEREST-RATE
    EXIT METHOD.
END METHOD calculateInterest.
 
METHOD-ID. displayInfo.
    DISPLAY "Account: " ACCOUNT-NUMBER
    DISPLAY "Balance: " BALANCE
    DISPLAY "Interest Rate: " INTEREST-RATE
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS BankAccount.
 
* Savings account with override
CLASS-ID. SavingsAccount INHERITS FROM BankAccount.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  SAVINGS-RATE    PIC 9(3)V99 VALUE 0.08.
01  MINIMUM-BALANCE  PIC 9(10)V99 VALUE 100.00.
 
METHOD-ID. calculateInterest OVERRIDE.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(10)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Higher interest rate for savings
    COMPUTE RETURN-VALUE = BALANCE * SAVINGS-RATE
    EXIT METHOD.
END METHOD calculateInterest.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method first
    INVOKE SUPER "displayInfo"
    * Add savings-specific information
    DISPLAY "Account Type: Savings"
    DISPLAY "Minimum Balance: " MINIMUM-BALANCE
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS SavingsAccount.
 
* Checking account with override
CLASS-ID. CheckingAccount INHERITS FROM BankAccount.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CHECKING-RATE   PIC 9(3)V99 VALUE 0.02.
01  MONTHLY-FEE     PIC 9(5)V99 VALUE 10.00.
 
METHOD-ID. calculateInterest OVERRIDE.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(10)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Lower interest rate for checking
    COMPUTE RETURN-VALUE = BALANCE * CHECKING-RATE
    EXIT METHOD.
END METHOD calculateInterest.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method first
    INVOKE SUPER "displayInfo"
    * Add checking-specific information
    DISPLAY "Account Type: Checking"
    DISPLAY "Monthly Fee: " MONTHLY-FEE
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS CheckingAccount.

This example shows how different account types can override interest calculation and display methods.

Employee Management System

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* Employee inheritance with salary calculation override
IDENTIFICATION DIVISION.
PROGRAM-ID. EMPLOYEE-MANAGER.
 
* Base employee class
CLASS-ID. Employee.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  EMPLOYEE-ID     PIC 9(6).
01  NAME            PIC X(50).
01  BASE-SALARY     PIC 9(8)V99.
01  HOURS-WORKED    PIC 9(3).
 
METHOD-ID. calculateSalary.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(8)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Basic salary calculation
    COMPUTE RETURN-VALUE = BASE-SALARY
    EXIT METHOD.
END METHOD calculateSalary.
 
METHOD-ID. displayInfo.
    DISPLAY "Employee ID: " EMPLOYEE-ID
    DISPLAY "Name: " NAME
    DISPLAY "Base Salary: " BASE-SALARY
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS Employee.
 
* Manager subclass with override
CLASS-ID. Manager INHERITS FROM Employee.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  BONUS-RATE      PIC 9(3)V99 VALUE 0.15.
01  TEAM-SIZE       PIC 9(3).
 
METHOD-ID. calculateSalary OVERRIDE.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(8)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Manager gets base salary plus bonus
    COMPUTE RETURN-VALUE = BASE-SALARY + 
                          (BASE-SALARY * BONUS-RATE)
    EXIT METHOD.
END METHOD calculateSalary.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method first
    INVOKE SUPER "displayInfo"
    * Add manager-specific information
    DISPLAY "Position: Manager"
    DISPLAY "Team Size: " TEAM-SIZE
    DISPLAY "Bonus Rate: " BONUS-RATE
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS Manager.
 
* Part-time employee subclass
CLASS-ID. PartTimeEmployee INHERITS FROM Employee.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  HOURLY-RATE     PIC 9(5)V99.
 
METHOD-ID. calculateSalary OVERRIDE.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(8)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Part-time salary based on hours worked
    COMPUTE RETURN-VALUE = HOURS-WORKED * HOURLY-RATE
    EXIT METHOD.
END METHOD calculateSalary.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method first
    INVOKE SUPER "displayInfo"
    * Add part-time specific information
    DISPLAY "Position: Part-Time"
    DISPLAY "Hours Worked: " HOURS-WORKED
    DISPLAY "Hourly Rate: " HOURLY-RATE
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS PartTimeEmployee.

This example demonstrates salary calculation overrides for different employee types.

Advanced OVERRIDE Features

OVERRIDE supports advanced features for complex inheritance scenarios.

Calling Parent Method (SUPER)

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* Calling parent method from override
CLASS-ID. EnhancedAccount INHERITS FROM BankAccount.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method first to show basic info
    INVOKE SUPER "displayInfo"
    
    * Add enhanced information
    DISPLAY "Enhanced Features:"
    DISPLAY "- Online Banking"
    DISPLAY "- Mobile App"
    DISPLAY "- 24/7 Support"
    EXIT METHOD.
END METHOD displayInfo.
 
METHOD-ID. calculateInterest OVERRIDE.
DATA DIVISION.
LINKAGE SECTION.
01  RETURN-VALUE    PIC 9(10)V99.
 
PROCEDURE DIVISION RETURNING RETURN-VALUE.
    * Get base interest from parent
    INVOKE SUPER "calculateInterest" 
        RETURNING RETURN-VALUE
    
    * Add bonus for enhanced account
    COMPUTE RETURN-VALUE = RETURN-VALUE + 
                          (BALANCE * 0.01)
    EXIT METHOD.
END METHOD calculateInterest.
 
END CLASS EnhancedAccount.

You can call the parent method using INVOKE SUPER to extend rather than replace functionality.

Multiple Level Inheritance

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* Multiple inheritance levels with overrides
CLASS-ID. GrandParent.
 
METHOD-ID. displayInfo.
    DISPLAY "GrandParent Information"
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS GrandParent.
 
CLASS-ID. Parent INHERITS FROM GrandParent.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call grandparent method
    INVOKE SUPER "displayInfo"
    DISPLAY "Parent Information"
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS Parent.
 
CLASS-ID. Child INHERITS FROM Parent.
 
METHOD-ID. displayInfo OVERRIDE.
    * Call parent method (which calls grandparent)
    INVOKE SUPER "displayInfo"
    DISPLAY "Child Information"
    EXIT METHOD.
END METHOD displayInfo.
 
END CLASS Child.

Method overriding can occur at multiple levels in an inheritance hierarchy.

Best Practices and Tips

Following these best practices will help you use OVERRIDE effectively in your COBOL applications.

OVERRIDE Best Practices

Always use OVERRIDE keyword - Makes intentions clear and catches signature changes
Maintain Liskov Substitution Principle - Ensure overridden methods maintain the same contract
Document behavioral changes - Document how overridden behavior differs from parent
Consider using SUPER - Use INVOKE SUPER when appropriate to extend functionality
Test polymorphic behavior - Test with different object types to ensure correct method resolution
Keep signatures consistent - Ensure method signatures match exactly with parent
Use meaningful method names - Choose method names that clearly indicate their purpose
Handle exceptions properly - Ensure overridden methods handle exceptions appropriately

Common Mistakes to Avoid

Mistake	Problem	Solution
Not using OVERRIDE keyword	Compiler may not catch signature mismatches	Always include OVERRIDE in METHOD-ID clause
Changing method signature	Breaks inheritance contract	Keep exact same signature as parent method
Violating Liskov Principle	Subclasses can't be used as parent types	Maintain same behavioral contract
Not calling SUPER when needed	Lose parent functionality	Use INVOKE SUPER to extend parent behavior
Overriding too many methods	Complex inheritance hierarchies	Only override methods that need customization

OVERRIDE Quick Reference

Action	Syntax	Example
Basic override	METHOD-ID. name OVERRIDE	METHOD-ID. calculateArea OVERRIDE
Override with parameters	METHOD-ID. name OVERRIDE PROCEDURE DIVISION USING params	METHOD-ID. processData OVERRIDE PROCEDURE DIVISION USING INPUT-DATA
Call parent method	INVOKE SUPER "methodName"	INVOKE SUPER "processData"
Inheritance declaration	CLASS-ID. name INHERITS FROM parent	CLASS-ID. Rectangle INHERITS FROM Shape
Polymorphic usage	INVOKE obj "methodName"	INVOKE SHAPE-OBJECT "calculateArea"
Check object type	IF obj IS INSTANCE OF class	IF SHAPE-OBJECT IS INSTANCE OF Rectangle

Test Your Knowledge

1. What is the primary purpose of OVERRIDE in COBOL?

To create new methods
To override inherited methods in subclasses
To define data structures
To control program flow

2. Which COBOL concept does OVERRIDE support?

Data validation
Polymorphism
File I/O operations
Arithmetic calculations

3. Where is the OVERRIDE keyword placed in a method definition?

At the end of the method
In the METHOD-ID clause
In the PROCEDURE DIVISION
In the DATA DIVISION

4. What must match exactly when overriding a method?

Only the method name
Method name, parameters, and return type
Only the parameters
Only the return type

5. How do you call the parent method from an overridden method?

Using CALL PARENT
Using INVOKE SUPER
Using PERFORM PARENT
Using GO TO PARENT

COBOL Tutorial

COBOL OVERRIDE - Method Overriding

What is OVERRIDE?

🎭 Real-World Analogy

Key Concepts

How to Use OVERRIDE

Basic Method Override

Method Signature Requirements

Practical Examples

Bank Account Hierarchy

Employee Management System

Advanced OVERRIDE Features

Calling Parent Method (SUPER)

Multiple Level Inheritance

Best Practices and Tips

OVERRIDE Best Practices

Common Mistakes to Avoid

OVERRIDE Quick Reference

Test Your Knowledge

Frequently Asked Questions

Related Concepts

Inheritance in COBOL

Polymorphism

Method Overriding

Object-Oriented Design

Liskov Substitution Principle

Related Pages

COBOL Tutorial

COBOL OVERRIDE - Method Overriding

What is OVERRIDE?

🎭 Real-World Analogy

Key Concepts

How to Use OVERRIDE

Basic Method Override

Method Signature Requirements

Practical Examples

Bank Account Hierarchy

Employee Management System

Advanced OVERRIDE Features

Calling Parent Method (SUPER)

Multiple Level Inheritance

Best Practices and Tips

OVERRIDE Best Practices

Common Mistakes to Avoid

OVERRIDE Quick Reference

Test Your Knowledge

Frequently Asked Questions

What is the difference between OVERRIDE and creating a new method?

Can you override any method from a parent class?

What happens if you don't use the OVERRIDE keyword when overriding a method?

How does OVERRIDE support the Liskov Substitution Principle?

Can you override multiple methods in the same class?

Related Concepts

Inheritance in COBOL

Polymorphism

Method Overriding

Object-Oriented Design

Liskov Substitution Principle

Related Pages