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COBOL Tutorial

COBOL INTERFACE - Quick Reference

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Overview

The INTERFACE statement in object-oriented COBOL defines a contract that specifies what methods a class must implement. It provides a way to define a common set of methods that different classes can implement, enabling polymorphism and loose coupling.

Purpose and Usage

  • Define method contracts for classes to implement
  • Enable polymorphism through common interfaces
  • Provide loose coupling between components
  • Support modular design and testing

Syntax

The INTERFACE statement defines method signatures and contracts:

Basic Syntax

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INTERFACE-ID. interface-name.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
OBJECT-COMPUTER.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
METHOD-ID. method-name.
    DATA DIVISION.
    LINKAGE SECTION.
    PROCEDURE DIVISION.
END METHOD method-name.
END INTERFACE interface-name.

Interfaces define method signatures without implementation.

Example: Simple Interface

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INTERFACE-ID. ICalculator.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
OBJECT-COMPUTER.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
METHOD-ID. Add.
    DATA DIVISION.
    LINKAGE SECTION.
        01  a PIC 9(5).
        01  b PIC 9(5).
        01  result PIC 9(6).
    PROCEDURE DIVISION USING a, b RETURNING result.
END METHOD Add.
END INTERFACE ICalculator.

Example: Class Implementing Interface

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CLASS-ID. Calculator IMPLEMENTS ICalculator.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
OBJECT-COMPUTER.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
METHOD-ID. Add.
    DATA DIVISION.
    LINKAGE SECTION.
        01  a PIC 9(5).
        01  b PIC 9(5).
        01  result PIC 9(6).
    PROCEDURE DIVISION USING a, b RETURNING result.
        COMPUTE result = a + b
        EXIT METHOD.
END METHOD Add.
END CLASS Calculator.

Practical Examples

Here are some practical uses of INTERFACE in object-oriented COBOL:

Database Interface

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INTERFACE-ID. IDatabase.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
OBJECT-COMPUTER.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
METHOD-ID. Connect.
    DATA DIVISION.
    LINKAGE SECTION.
        01  connection-string PIC X(100).
        01  success PIC X.
    PROCEDURE DIVISION USING connection-string RETURNING success.
END METHOD Connect.
METHOD-ID. ExecuteQuery.
    DATA DIVISION.
    LINKAGE SECTION.
        01  query PIC X(500).
        01  result PIC X(1000).
    PROCEDURE DIVISION USING query RETURNING result.
END METHOD ExecuteQuery.
END INTERFACE IDatabase.

Defines a contract for database operations that different database classes can implement.

Logger Interface

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INTERFACE-ID. ILogger.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
OBJECT-COMPUTER.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
METHOD-ID. LogInfo.
    DATA DIVISION.
    LINKAGE SECTION.
        01  message PIC X(200).
    PROCEDURE DIVISION USING message.
END METHOD LogInfo.
METHOD-ID. LogError.
    DATA DIVISION.
    LINKAGE SECTION.
        01  message PIC X(200).
    PROCEDURE DIVISION USING message.
END METHOD LogError.
END INTERFACE ILogger.

Defines logging methods that different logger implementations can provide.

Best Practices

  • Define clear, focused interfaces with a single responsibility.
  • Use descriptive names for interfaces and methods.
  • Keep interfaces simple and avoid complex parameter structures.
  • Document the purpose and expected behavior of each method.
  • Use interfaces to enable testing through mock objects.

Common Pitfalls

  • Creating interfaces that are too large or complex.
  • Not implementing all required methods in classes.
  • Forgetting to use IMPLEMENTS keyword in class definitions.
  • Mixing interface and implementation concerns.

Test Your Knowledge

1. What is the primary purpose of the INTERFACE statement in COBOL?

  • To define a user interface
  • To define a contract for object methods
  • To create a file interface
  • To define program parameters

2. In which type of COBOL is INTERFACE primarily used?

  • Standard COBOL
  • Object-oriented COBOL
  • Procedural COBOL
  • Legacy COBOL

3. What does an INTERFACE define?

  • Data structures only
  • Method signatures and contracts
  • File definitions
  • Program logic

4. How do classes use INTERFACE?

  • By inheriting from it
  • By implementing it
  • By calling it
  • By extending it

5. Can an INTERFACE contain data fields?

  • Yes, always
  • No, never
  • Only constants
  • Only static fields

Frequently Asked Questions

Related Concepts

CLASS Statement

Defining classes in object-oriented COBOL.

METHOD-ID

Defining methods in COBOL classes.

IMPLEMENTS

Implementing interfaces in classes.

Object-Oriented COBOL

Overview of OO COBOL concepts.

Polymorphism

Understanding polymorphism in COBOL.

INITIALIZEDMETHOD-ID