Clean Error Handling

Error handling important because things can go wrong. Therefore as a developer, we have to make sure our code acts properly in such situations. This talk covers things that will help you handle errors in clean and elegant way in your code.
Provided sample code fragments use Java.

Transcript

1. Clean Error Handling Natig Babayev
2. Error handling is important
3. Why?
4. Things can go wrong.
5. We have to make sure our code acts properly.
6. Clean error handling?
8. Use Exceptions Rather Than Return Codes
9. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
10. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
11. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
12. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
13. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
14. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
15. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
16. public void authorize(String pin) { pinCode.verify(pin); int verificationResult = pinCode.lastError(); if (verificationResult == PinCode.ERROR_NONE) { paymentVerifier.acceptLastPayment(); int result = paymentVerifier.lastError(); if (result == PaymentVerifier.ERROR_NONE) { view.show("Payment has been processed"); } else if (result == PaymentVerifier.ERROR_EXPIRED) { view.show("Payment request has been expired."); } else { view.show("Unknown error"); } } else if (verificationResult == PinCode.ERROR_NOT_SET) { view.show("You have to set PIN first."); } else if (verificationResult == PinCode.ERROR_INCORRECT_PIN) { view.show("PIN is not correct. Try again."); } else { view.show("Too many wrong attempts"); } }
17. public void authorize(String pin) { try { pinCode.verify(pin); // throws exception paymentVerifier.acceptLastPayment(); // throws exception } catch (Throwable throwable) { handleErrors(throwable); } }
18. public void authorize(String pin) { try { pinCode.verify(pin); // throws exception paymentVerifier.acceptLastPayment(); // throws exception } catch (Throwable throwable) { handleErrors(throwable); } } Easier to read and understand what method is supposed to do
19. public void authorize(String pin) { try { pinCode.verify(pin); // throws exception paymentVerifier.acceptLastPayment(); // throws exception } catch (Throwable throwable) { handleErrors(throwable); } } Easier to read and understand what method is supposed to do Concerns are separated
20. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First
21. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions
22. What are unchecked exceptions?
23. Throwable (checked) Exception (checked) Error (unchecked) RuntimeException (unchecked) Other subclasses (checked)
24. “Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.” Open/Closed Principle
25. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions Provide Context with Exceptions
26. ✅ Failed operation & the type of failure Informative error messages ❌ Sensitive data ✅ Values of parameters and fields
27. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions Provide Context with Exceptions Define Exception Classes in Terms of a Caller’s Needs
28. try { paymentsAPI.getPayments(); } catch (HttpException e) { view.show("Request failed: " + e.code()); } catch (SocketTimeoutException e) { view.show("Request failed: 504"); } catch (ConnectException e) { view.show("Request failed: 503"); } catch (InternalError e) { view.show("Request failed: 500"); } catch (Exception e) { view.show("Request failed: 418"); }
29. public class PaymentsApiWrapper { private final PaymentsAPI paymentsAPI; public PaymentsApiWrapper(PaymentsAPI paymentsAPI) { this.paymentsAPI = paymentsAPI; } public List<Payment> getPayments() { try { return paymentsAPI.getPayments(); } catch (HttpException e) { throw new PaymentsApiException("getPayments request failed: " + e.code()); } catch (SocketTimeoutException e) { throw new PaymentsApiException("getPayments request failed: 504"); } catch (ConnectException e) { throw new PaymentsApiException("getPayments request failed: 503"); } catch (InternalError e) { throw new PaymentsApiException("getPayments request failed: 500"); } catch (Exception e) { throw new PaymentsApiException("getPayments request failed: 418"); } } }
30. try { paymentsApiWrapper.getPayments(); } catch (PaymentsApiException e) { view.show(e.getMessage()); }
31. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions Provide Context with Exceptions Define Exception Classes in Terms of a Caller’s Needs Define the Normal Flow
32. try { MealExpenses expenses = expenseReportDAO.getMeals(employee.getID()); m_total += expenses.getTotal(); } catch (MealExpensesNotFound e) { m_total += getMealPerDiem(); }
33. try { MealExpenses expenses = expenseReportDAO.getMeals(employee.getID()); m_total += expenses.getTotal(); } catch (MealExpensesNotFound e) { m_total += getMealPerDiem(); }
34. public class PerDiemMealExpenses implements MealExpenses { public int getTotal() { // return the per diem default } }
35. Special Case Pattern
36. Create a class or configure an object so that it handles a special case for you.
37. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions Provide Context with Exceptions Define Exception Classes in Terms of a Caller’s Needs Define the Normal Flow Don't Return Null
38. @Nullable public List<Payment> getPendingPayments() { if(paymentAPI.retrievePendingPayments().size == 0) { return null; } else { ... } }
39. private void retrievePendingPayments() { List<Payment> payments = payments.getPendingPayments(); if (payments != null && payments.size() > 0) { view.displayPendingPayments(payments); } }
40. @NotNull public List<Payment> getPendingPayments() { if(paymentAPI.retrievePendingPayments().size == 0) { return Collections.emptyList(); } else { ... } }
41. @NotNull public List<Payment> getPendingPayments() { if(paymentAPI.retrievePendingPayments().size == 0) { return Collections.emptyList(); } else { ... } }
42. @NotNull public List<Payment> getPendingPayments() { if(paymentAPI.retrievePendingPayments().size == 0) { return Collections.emptyList(); } else { ... } }
43. private void retrievePendingPayments() { List<Payment> payments = payments.getPendingPayments(); if (payments.size() > 0) { view.displayPendingPayments(payments); } }
44. Use Exceptions Rather Than Return Codes Write Your Try-Catch-Finally Statement First Use Unchecked Exceptions Provide Context with Exceptions Define Exception Classes in Terms of a Caller’s Needs Define the Normal Flow Don't Return Null Don't Pass Null
45. public class PinCode { ... public void verify(String pin) { ... } .. }
46. public class PinCode { ... public void verify(@NotNull String pin) { if (pin == null) throw new NullPointerException("pin == null"); ... } .. }
47. public class PinCode { ... public void verify(@NotNull String pin) { if (pin == null) throw new NullPointerException("pin == null"); ... } .. }
48. public class PinCode { ... public void verify(@NotNull String pin) { if (pin == null) throw new NullPointerException("pin == null"); ... } .. }
49. Thank you!