COBOL Tutorial

COBOL PROTECTED - OO Access Control

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Overview

PROTECTED is an object-oriented access modifier. It lets the defining class and its subclasses access a member (data or method), while blocking unrelated external code. It balances encapsulation and extensibility.

🏛️ Analogy

Think of a company building with floors: PUBLIC areas are the lobby (anyone enters), PROTECTED areas are staff-only floors (employees and trainees can enter), and PRIVATE areas are executive offices (only the specific team can enter). Subclasses are like trainees who get access to staff-only floors.

Key Concepts

Visibility: class + subclasses; not visible to unrelated code
Encapsulation: hide internals while enabling safe extension points
Overriding: allow child classes to adjust behavior
Stability: reduce accidental external dependencies

How to Use PROTECTED

Declare members as PROTECTED when they are internal extension points meant for subclasses. Expose stable PUBLIC methods for general use.

Subclass Access Example

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*> Conceptual OO COBOL-style example (dialect-specific)
 CLASS-ID. ACCOUNT.
 DATA DIVISION.
 WORKING-STORAGE SECTION.
 01  BALANCE         PIC 9(9)V99.
 01  MIN-BALANCE     PIC 9(5)V99 VALUE 100.00.
*> PROTECTED member
 01  PROTECTED-FEE   PIC 9(5)V99 VALUE 5.00.
 
 METHOD-ID. APPLY-FEE PROTECTED.
 PROCEDURE DIVISION.
     SUBTRACT PROTECTED-FEE FROM BALANCE
     EXIT METHOD.
 END METHOD APPLY-FEE.
 END CLASS ACCOUNT.
 
 CLASS-ID. SAVINGS-ACCOUNT INHERITS FROM ACCOUNT.
 METHOD-ID. MONTH-END-CLOSE PUBLIC.
 PROCEDURE DIVISION.
     *> Subclass can use protected hook
     INVOKE SELF "APPLY-FEE"
     IF BALANCE < MIN-BALANCE
         DISPLAY "Below minimum."
     END-IF
     EXIT METHOD.
 END METHOD MONTH-END-CLOSE.
 END CLASS SAVINGS-ACCOUNT.

Subclasses can invoke PROTECTED members. External programs cannot access them directly.

Non-Code Example

A bank exposes PUBLIC operations like deposit/withdraw.
It keeps PROTECTED hooks like apply-fee or reconcile used by derived account types.
External callers cannot trigger internal hooks; only specialized account types can.

Quick Reference

Action	Concept	Notes
Mark member	PROTECTED data/method	Visible to class + subclasses
Override	METHOD ... OVERRIDE	Child adjusts behavior
Access	From subclass	External access blocked

Best Practices and Pitfalls

Best Practices

Expose minimal PUBLIC surface; keep extension points PROTECTED
Document PROTECTED contracts for subclass authors
Keep invariants enforced by PRIVATE state, validated in PROTECTED hooks
Favor composition if many PROTECTED hooks indicate class doing too much

Common Mistakes

Mistake	Problem	Fix
Using PUBLIC for internal hooks	External coupling to internals	Use PROTECTED, document as internal
Too many PROTECTED members	Class surface becomes complex	Refactor or compose smaller classes
Assuming runtime security	Visibility is compile-time only	Use platform security for runtime protection

Test Your Knowledge

1. What does PROTECTED mean in OO COBOL?

Accessible by any program
Accessible only within the class and its subclasses
Accessible only within the same paragraph
Accessible only by the operating system

2. Which is true about PROTECTED methods?

They can be invoked by any object in any program
They can only be invoked from the defining class
They can be invoked from the defining class and subclasses
They cannot be overridden

3. Why use PROTECTED instead of PUBLIC for a method?

To improve CPU performance
To prevent external callers from depending on internal hooks
To reduce memory usage
To allow dynamic typing

4. Which access levels commonly appear with OO COBOL?

PRIVATE, PROTECTED, PUBLIC
SECURE, GUARDED, OPEN
HIDDEN, VISIBLE
LOCAL, EXTERNAL

5. What happens when external code tries to access a PROTECTED property?

It compiles and runs
It compiles but fails at runtime
It fails to compile (visibility error)
It silently succeeds

COBOL Tutorial

COBOL PROTECTED - OO Access Control

Overview

🏛️ Analogy

Key Concepts

How to Use PROTECTED

Subclass Access Example

Non-Code Example

Quick Reference

Best Practices and Pitfalls

Best Practices

Common Mistakes

Test Your Knowledge

Frequently Asked Questions

Related Concepts

Object-Oriented COBOL

OVERRIDE

OBJECT-REFERENCE

ENCAPSULATION

Related Pages

COBOL Tutorial

COBOL PROTECTED - OO Access Control

Overview

🏛️ Analogy

Key Concepts

How to Use PROTECTED

Subclass Access Example

Non-Code Example

Quick Reference

Best Practices and Pitfalls

Best Practices

Common Mistakes

Test Your Knowledge

Frequently Asked Questions

What is the difference between PRIVATE, PROTECTED, and PUBLIC?

Can PROTECTED members be overridden?

When should I mark a data item PROTECTED?

Is PROTECTED available in all COBOL compilers?

Does PROTECTED improve performance or security?

Related Concepts

Object-Oriented COBOL

OVERRIDE

OBJECT-REFERENCE

ENCAPSULATION

Related Pages