COBOL Method Programming

Method programming in COBOL refers to the object-oriented (OO) features added in COBOL 2002 and supported by some compilers: classes, object references, and methods. You call methods with the INVOKE statement, passing parameters with USING and receiving return values with RETURNING. This page introduces method invocation, object references, and when OO COBOL is used.

Explain Like I'm Five: Methods and INVOKE

A method is an action that belongs to an object or a class. In procedural COBOL you use CALL or PERFORM to run code. In OO COBOL you say INVOKE and name the object and the method, like "tell this account object to do DEBIT." The object holds the data; the method does something with it. INVOKE is how you "tell" the object to run that method. Creating a new object is usually done by invoking the class's "NEW" method, which gives you back a new object reference.

INVOKE Statement

INVOKE calls a method on an object reference or on a class. Basic form: INVOKE object-reference method-name. You can pass arguments with USING and receive a result with RETURNING. Optional ON EXCEPTION and NOT ON EXCEPTION handle success or failure. The method that runs is determined at runtime by the object's class (polymorphism).

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       *> Assume MY-OBJECT is an OBJECT REFERENCE, and "DO-SOMETHING" is a method
       INVOKE MY-OBJECT "DO-SOMETHING"
           USING WS-INPUT
           RETURNING WS-RESULT
           ON EXCEPTION
               DISPLAY "Method failed"
           NOT ON EXCEPTION
               DISPLAY "Result: " WS-RESULT
           END-INVOKE
       
       *> Create a new object (factory method)
       INVOKE MyClass "NEW"
           RETURNING MY-OBJECT
           ON EXCEPTION
               DISPLAY "Creation failed"
           END-INVOKE

Object References

An object reference is a data item that refers to an object. You declare it with OBJECT REFERENCE [class-name]. It does not hold the object's data; it points to the object. You get an object by invoking "NEW" on a class and RETURNING into the object reference, or by receiving it from another method or parameter. You then use that reference in INVOKE to call instance methods.

Instance vs Class (Static) Methods

An instance method is invoked on an object reference (INVOKE my-obj method-name). It operates on that instance. A factory or static method is invoked on the class (INVOKE ClassName "NEW" RETURNING my-obj). "NEW" is the standard way to create a new instance. Other static methods might be called on the class name when they do not need instance data.

When to Use Method Programming

• When your compiler and project use OO COBOL and you need to call or implement classes and methods.
• When interfacing with OO frameworks or libraries that expose objects and methods.
• When you want polymorphism: the same INVOKE can run different implementations depending on the object's class.

Step-by-Step: Calling a Method

• Declare an object reference (OBJECT REFERENCE class-name) if you need to hold an object.
• Obtain the object (e.g. INVOKE ClassName "NEW" RETURNING object-ref, or receive it via USING).
• Call the method: INVOKE object-ref method-name [USING ...] [RETURNING ...] [ON EXCEPTION ...] END-INVOKE.
• Use the RETURNING value or handle ON EXCEPTION as needed.

Related Concepts