1. Exceptions Handling the unexpected

2. Motivation So far, most of our code has been somewhat näive
We have assumed that nothing goes wrong…
User enters correct input
We never address outside the boundaries of an array
…and so on
DCS – SWC

3. Motivation Of course, the real world works differently
public class BankAccount {
public void withdraw(double amount)
if (amount > balance)
// Now what? }
...
DCS – SWC

4. Motivation
Possible actions depend on our interpre-tation of this situation
Legal – just do business logic
Illegal, and we know what to do – perhaps just do nothing
Illegal, and we do not know what to do!
Error detection and error handling are often separated in code!
DCS – SWC

5. Motivation
Problem: Can become very complex to ”drag” error handling code around in code for business logic
Error-handling may be application-specific
GUI-application: Pop-up window
Other: Write to a log file
Business logic code should not choose strategy for error handling!
DCS – SWC

6. Motivation
Next problem: Error handling code is often very ”far away” from error detction
main
askUser
doAction
makeWithdraw
withdraw ?
Error detected
Error handling
DCS – SWC

7. Motivation Management of errors can be broken down into several tasks:
Detection – realising an error situation has occurred
Signaling – making the surrounding code aware that an error has been detected
Capturing – taking responsibility for handling the error
Handling – performing the error handling actions
DCS – SWC

8. Exceptions
The mechanism for crossing the gap of method calls is exceptions
An exception in itself is ”just another class”
We can create exception objects just as we can create other objects
An exception object contains information about the type of error which occurred
Java contains several built-in exception classes, forming an inheritance hierarchy
DCS – SWC

9. Throwing and catching Exception can be thrown and catched
What does that mean!?
A very different flow than usual method calls
An exception is thrown up through the chain of method calls
DCS – SWC

10. Throwing and catching Who will catch the exception? main askUser
doAction
makeWithdraw
withdraw
Who will catch the exception?
Error detected – throw!
DCS – SWC

11. Throwing and catching A throw can look like this in Java:
public void withdraw(double amount) {
if (amount > balance)
IllegalArgumentException ex =
new IllegalArgumentException(”...”);
throw ex; }
balance = balance – amount;
NOTE!
DCS – SWC

12. Throwing and catching
An exception is now thrown; this changes the flow of code immediately!
Remaining code in the method throwing the exception is not executed
Somebody must catch the exception
In order to catch the exception, we must write an exception handler
DCS – SWC

13. Throwing and catching General exception handler structure: try {
// Code which may throw an exception
... }
catch (ExceptionType ex)
// Proper handling of exception
DCS – SWC

14. Throwing and catching try { myAccount.withdraw(1000);
myAccount.getTransactions();
... }
catch (IllegalArgumentException ex)
System.out.println(ex.getMessage());
ex.printStackTrace();
DCS – SWC

15. Throwing and catching Things to note:
Error detection (throw) and error handling (try/catch) is usually not in the same method
The catch statement only catches exceptions of the specified type
Information about the error is found implicitly – by the type of the exception – and explicitly from e.g the text stored in the object
DCS – SWC

16. Throwing and catching Throw early, catch late!
If you cannot fix a pro-blem correctly, throw an exception
Only catch an exception if you really know how to fix the problem
DCS – SWC

17. Throwing and catching try { myAccount.withdraw(1000);
myAccount.getTransactions();
... }
catch (Exception ex)
// do nothing...
Tempting, but bad…!
DCS – SWC

18. Checked and Unchecked
How do I know what exceptions some piece of code can throw…?
Difficult to code a method correctly without this knowledge
Two types of exceptions exist
Checked exception
Unchecked exception
DCS – SWC

19. Checked and Unchecked Checked exception
Used for problems beyond the control of the programmer
Corrupted file, network problems, etc..
Compiler insists that you explicitly decide what to do about it
Option 1: Re-throw the exception
Option 2: Handle the exception, using a catch clause matching the exception
DCS – SWC

20. Checked and Unchecked // Suppose draw() can throw DrawException
draw(Figure f);
// Compiler will not like this!
public void drawOne(Figure f) {
getScreen().draw(f); }
DCS – SWC

21. Checked and Unchecked // Option 1: Re-throw the exception
// (i.e. do nothing…)
public void drawOne(Figure f)
throws DrawException {
getScreen().draw(f); }
DCS – SWC

22. Checked and Unchecked // Option 2: Handle the exception
public void drawOne(Figure f) {
try
getScreen().draw(f); }
catch (DrawException de)
// Code for handling the problem
DCS – SWC

23. Checked and Unchecked draw drawOne draw drawOne
Option 1: drawOne does nothing, so it must annonce that it will (re)throw the exception
draw
drawOne
Option 2: drawOne handles the exception, so it is ”consumed” by drawOne
DCS – SWC

24. Checked and Unchecked Unchecked exception
Used for problems which the programmer should be able to prevent
Null reference, out of bounds reference,…
Why do we have these…? Accidents do happen…!
Unchecked exceptions are not announced
DCS – SWC

25. The finally Clause
Sometimes we need to execute some specific code after an exception occurs
Typically ”clean-up” code – close a file connection, a database connection, etc.
Where do we put this code…?
In exception handlers? Difficult, who actually catches the exception…
In a finally clause!
DCS – SWC

26. The finally Clause PrintWriter out = new PrintWriter(filename); ...
try {
writeData(out); }
finally
// This code will always be executed,
// even if the above code throws an exception
out.close();
DCS – SWC

27. The finally Clause
The code in the finally clause is guaran-teed to be executed, in one of these ways:
If no exceptions are thrown: After completing the last statement in the try block
If an exception is thrown:
Execute code in finally clause
Exit to exception handler
DCS – SWC

28. The finally Clause NOT SO GOOD GOOD try { } catch finally try { }
DCS – SWC

29. Making your own exceptions
Throw exceptions that are as specific as possible – also in terms of type
Many built-in exceptions to choose from
Can be appropriate to create your own exceptions
Just extend existing class
DCS – SWC

30. Making your own exceptions
public class InsufficientFundsException
extends RunTimeException {
public InsufficientFundsException() {}
public InsufficientFundsException(String message)
super(message); }
DCS – SWC

31. Exceptions vs. Flow control
Exceptions change the linear flow of code, just like if, while, etc.
However, they are only intended for error detection and handling
Do not use exceptions as a substitute for ordinary flow control
DCS – SWC

32. Exceptions vs. Flow control
GOOD
for (int i = 0; i < noOfElements; i++)
myArray[i] = i;
try {
for (int i = 0; /* No Test?? */ ; i++) }
catch (ArrayIndexOutOfBoundsException e) {}
BAD
DCS – SWC

33. Exceptions summary Throw early, catch late
Only catch, if you can handle the problem correctly
You must deal with checked exceptions
Use try, catch and finally appropriately
Make your own exception classes, if you really need them
Exceptions are not for flow control
DCS – SWC