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COBOL Tutorial

COBOL ON EXCEPTION - Quick Reference

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Overview

The ON EXCEPTION clause in COBOL provides error handling and recovery mechanisms. It allows programs to respond to errors and exceptions that occur during execution, enabling graceful error handling instead of program termination.

Purpose and Usage

  • Error handling and recovery
  • Graceful failure instead of program termination
  • Robust programming practices
  • User-friendly error messages

Syntax

The ON EXCEPTION clause follows specific syntax patterns:

Basic Syntax

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* Basic ON EXCEPTION syntax
statement ON EXCEPTION
    * Exception handling code
    PERFORM error-handling
NOT ON EXCEPTION
    * Success handling code
    PERFORM success-processing
END-EXCEPTION.

ON EXCEPTION handles errors, NOT ON EXCEPTION handles success.

File Operation Example

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* File operation with exception handling
OPEN INPUT data-file
    ON EXCEPTION
        DISPLAY "Error opening file: " data-file
        PERFORM file-error-handling
    NOT ON EXCEPTION
        DISPLAY "File opened successfully"
        PERFORM process-file
END-EXCEPTION.

Method Invocation Example

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* Method invocation with exception handling
INVOKE object "method-name" USING parameter
    ON EXCEPTION
        DISPLAY "Method call failed"
        PERFORM method-error-handling
    NOT ON EXCEPTION
        DISPLAY "Method executed successfully"
        PERFORM continue-processing
END-EXCEPTION.

Practical Examples

Here are some practical uses of ON EXCEPTION in COBOL:

File Processing with Error Handling

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* Reading file with exception handling
READ data-file
    ON EXCEPTION
        IF file-status = "10"
            DISPLAY "End of file reached"
            PERFORM end-of-file-processing
        ELSE
            DISPLAY "Read error: " file-status
            PERFORM read-error-handling
        END-IF
    NOT ON EXCEPTION
        PERFORM process-record
END-EXCEPTION.
 
* Writing file with exception handling
WRITE output-record
    ON EXCEPTION
        DISPLAY "Write error: " file-status
        PERFORM write-error-handling
    NOT ON EXCEPTION
        ADD 1 TO records-written
END-EXCEPTION.

Comprehensive file I/O error handling.

Object Method Error Handling

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* Database connection with exception handling
INVOKE database "Connect" USING connection-string
    ON EXCEPTION
        DISPLAY "Database connection failed"
        PERFORM database-error-handling
        MOVE "N" TO connection-success
    NOT ON EXCEPTION
        DISPLAY "Database connected successfully"
        MOVE "Y" TO connection-success
        PERFORM database-operations
END-EXCEPTION.
 
* Data validation with exception handling
INVOKE validator "ValidateData" USING input-data
    ON EXCEPTION
        DISPLAY "Data validation failed"
        PERFORM validation-error-handling
    NOT ON EXCEPTION
        DISPLAY "Data validation successful"
        PERFORM process-valid-data
END-EXCEPTION.

Object method error handling for robust operations.

Best Practices

  • Always use ON EXCEPTION for critical operations that might fail.
  • Provide meaningful error messages to help with debugging.
  • Implement appropriate error recovery mechanisms.
  • Log errors for troubleshooting and monitoring.
  • Consider the user experience when handling errors.
  • Test exception handling paths thoroughly.

Common Pitfalls

  • Not handling all possible error conditions.
  • Providing generic error messages that don\'t help with debugging.
  • Forgetting to implement proper error recovery.
  • Not testing exception handling code paths.
  • Ignoring error conditions that could affect program stability.

Test Your Knowledge

1. What is the primary purpose of the ON EXCEPTION clause in COBOL?

  • To handle errors and exceptions
  • To terminate the program
  • To skip statements
  • To create loops

2. Where is ON EXCEPTION typically used?

  • In file operations
  • In method calls
  • In both file operations and method calls
  • Only in arithmetic operations

3. What happens when an exception occurs and ON EXCEPTION is specified?

  • The program terminates
  • Control transfers to the exception handling code
  • The statement is retried
  • Nothing happens

4. What is the relationship between ON EXCEPTION and NOT ON EXCEPTION?

  • They are the same
  • ON EXCEPTION handles errors, NOT ON EXCEPTION handles success
  • NOT ON EXCEPTION is obsolete
  • They cannot be used together

5. Is ON EXCEPTION required for all operations?

  • Yes, always
  • No, it is optional but recommended
  • Only for file operations
  • Only for method calls

Frequently Asked Questions

Related Concepts

NOT ON EXCEPTION

Handling successful operations in COBOL.

Error Handling

Comprehensive error handling strategies.

File Operations

File I/O with error handling.

INVOKE Statement

Method invocation with exception handling.

Robust Programming

Best practices for reliable COBOL programs.

OBJECTNOT ON EXCEPTION