Introduction to COBOL Object-Oriented Programming

Object-Oriented Programming (OOP) has transformed the way we think about software design and implementation. With its emphasis on encapsulation, inheritance, and polymorphism, OOP allows developers to create modular, reusable, and maintainable code. While COBOL may initially seem like a traditional procedural language, it has evolved to support object-oriented programming paradigms. In this article, we'll explore the core concepts of OOP and how they are realized within the COBOL programming language.

Core Concepts of Object-Oriented Programming

Before diving into COBOL-specific implementations, let's review the foundational concepts of OOP:

1. Encapsulation

Encapsulation is the principle of bundling data and the methods that operate on that data within a single unit, or object. This concept allows you to hide the internal state of an object from the outside world, exposing only the necessary functionalities. In COBOL, encapsulation is achieved through the creation of classes and objects, where you define data items (attributes) and the methods (procedures) to manipulate these data items.

2. Inheritance

Inheritance is a mechanism that allows a new class (called a subclass) to inherit attributes and behaviors (methods) from an existing class (called a superclass). This promotes code reusability and establishes a natural hierarchy between classes. In COBOL, inheritance is implemented through the INHERIT verb, allowing one class to utilize the properties and methods of another.

3. Polymorphism

Polymorphism allows a single interface to represent different underlying forms (data types). This can be achieved in two main ways: compile-time polymorphism (method overloading) and runtime polymorphism (method overriding). COBOL supports polymorphism through method overriding, letting subclasses provide specific implementations of methods as defined in their superclasses.

4. Abstraction

Abstraction is the concept of simplifying complex systems by modeling classes based on essential properties while ignoring unnecessary details. In COBOL, this is facilitated by defining classes that represent real-world entities with attributes and operations relevant to the application, allowing developers to work at a higher level of problem-solving without delving into the complexities of each object's implementation.

Implementing Object-Oriented Programming in COBOL

Defining Classes

In COBOL, you define a class using the CLASS keyword. A class consists of data members (attributes) and methods (procedures). Here’s a simple example of how to define a class in COBOL:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. OOPExample.
       ENVIRONMENT DIVISION.
       DATA DIVISION.

       WORKING-STORAGE SECTION.
       01  DogClass.
           05  DogName       PIC X(20).
           05  DogBreed      PIC X(20).

       CLASS-ID. Dog.
       DATA DIVISION.
       LINKAGE SECTION.
       01  Surname    PIC X(20).
       PROCEDURE DIVISION.
       METHOD-ID. SET-DOG-NAME.
           PROCEDURE DIVISION USING Surname.
           MOVE Surname TO DogName.
       END METHOD SET-DOG-NAME.
       END CLASS Dog.

In this code snippet, we define a class named Dog with a data member DogName and a method SET-DOG-NAME that sets the name of the dog.

Creating Objects

Once a class is defined, you can create objects (instances of the class) using the NEW keyword. Here’s how you can declare and work with an object of the class we previously defined:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. Main.
       WORKING-STORAGE SECTION.
       01  MyDog       TYPE Dog.
       01  InputName   PIC X(20).

       PROCEDURE DIVISION.
           DISPLAY "Enter dog's name: ".
           ACCEPT InputName.
           CALL 'Dog' "MyDog" SET-DOG-NAME USING InputName.
           DISPLAY 'Dog Name Set to: ' MyDog-DogName.
           STOP RUN.

In this example, we create an instance of the Dog class named MyDog. We accept a name for the dog from the user and then invoke the method to set the dog’s name.

Inheritance in COBOL

Inheritance allows us to create a new class based on an existing class, inheriting its attributes and methods. In COBOL, you can extend the Dog class to create a new class GuideDog as follows:

       CLASS-ID. GuideDog EXTENDS Dog.
       DATA DIVISION.
       LINKAGE SECTION.
       01  ServiceLevel  PIC X(20).
       PROCEDURE DIVISION.
       METHOD-ID. SET-SERVICE-LEVEL.
           PROCEDURE DIVISION USING ServiceLevel.
           MOVE ServiceLevel TO ServiceLevel.
       END METHOD SET-SERVICE-LEVEL.
       END CLASS GuideDog.

In this code, the GuideDog class inherits from the Dog class and adds an additional attribute, ServiceLevel, along with its method SET-SERVICE-LEVEL.

Polymorphism through Method Overriding

When a subclass defines a method with the same name as a method in its superclass, the subclass method overrides the superclass method. Here’s an example of polymorphism in action:

       CLASS-ID. GuideDog EXTENDS Dog.
       METHOD-ID. SET-DOG-NAME.
           MOVE "Guide " TO DogName.
           DISPLAY "Dog's Name Set to: " DogName.
       END METHOD SET-DOG-NAME.

In this example, calling SET-DOG-NAME on a GuideDog object will display "Guide " followed by whatever name you set, effectively overriding the original SET-DOG-NAME method from the Dog class.

Working with Abstraction in COBOL

To demonstrate abstraction in COBOL, consider that while we can create different types of dogs, the class should not expose all of its internals or operations. By only exposing methods relevant to the dog behaviors and not its internal data handling, we maintain an abstraction level.

Here's how you ensure users of the class don't manipulate its data directly:

       CLASS-ID. Dog.
       DATA DIVISION.
       LINKAGE SECTION.
       01  DogName     PIC X(20) PRIVATE.
       01  DogBreed    PIC X(20) PRIVATE.

       METHOD-ID. GET-DOG-INFO RETURNING STRING.
           STRING DogName DELIMITED BY SPACE "is a" DogBreed DELIMITED BY SPACE
           INTO DogInfo.
           GOBACK WITH DogInfo.
       END METHOD GET-DOG-INFO.
       END CLASS Dog.

In this construct, the attributes are made private, limiting access, and the only way to access dog information is through the GET-DOG-INFO method.

Conclusion

COBOL has embraced object-oriented programming, allowing developers to take advantage of modern programming techniques while working within a familiar language framework. By incorporating encapsulation, inheritance, polymorphism, and abstraction into your COBOL applications, you can create powerful, maintainable, and reusable codebases that meet the demands of today's software development landscape. As you explore OOP in COBOL, you’ll discover how these concepts can breathe new life into classic applications while ensuring robust functionality.