Exception Handling (Cambridge (CIE) A Level Computer Science): Revision Note

Exception handling

What is exception handling?

• An exception is an unexpected event that disrupts normal program execution

• Exception handling is how a program detects and responds to these errors, allowing it to recover or shut down cleanly

Why use exception handling?

• Prevents programs from crashing unexpectedly

• Allows for user-friendly error messages

• Makes programs more robust and reliable

• Handles problems caused by:

  • Division by zero

  • File not found or end-of-file errors

  • Hardware or device failures (e.g. printers, lost connections)

  • Invalid input from users

Causes of exceptions

Pseudocode

TRY
    // Code that might cause an error
    OPEN "data.txt" FOR READ
    READ record FROM "data.txt"
    SET result = 10 / userInput
EXCEPT
    // Code to handle the error
    OUTPUT "An error occurred during file processing or division."
    CLOSE "data.txt"
ENDTRY

Key terms

Python

try:
    file = open("data.txt", "r")
    line = file.readline()
    user_input = int(input("Enter a number: "))
    result = 10 / user_input
    print("Result:", result)
    file.close()

except FileNotFoundError:
    print("Error: File not found.")

except ZeroDivisionError:
    print("Error: Cannot divide by zero.")

except Exception as e:
    print("An unexpected error occurred:", e)

finally:
    try:
        file.close()
    except:
        pass  # file might not have opened

Java

import java.io.*;
import java.util.Scanner;

public class ExceptionHandlingExample {
    public static void main(String[] args) {
        BufferedReader reader = null;

        try {
            reader = new BufferedReader(new FileReader("data.txt"));
            String line = reader.readLine();

            Scanner scanner = new Scanner(System.in);
            System.out.print("Enter a number: ");
            int userInput = scanner.nextInt();

            int result = 10 / userInput;
            System.out.println("Result: " + result);

        } catch (FileNotFoundException e) {
            System.out.println("Error: File not found.");
        } catch (ArithmeticException e) {
            System.out.println("Error: Cannot divide by zero.");
        } catch (IOException e) {
            System.out.println("Error reading from file.");
        } catch (Exception e) {
            System.out.println("An unexpected error occurred: " + e.getMessage());
        } finally {
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                // Ignore closing error
            }
        }
    }
}