ExceptionsHandling the unexpected

RHS – SWC 2

The Real World

• 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

RHS – SWC 3

The Real World

• Of course, the real world works differently

public class BankAccount

{

public void withdraw(double amount)

{

if (amount > balance)

// Now what?

}

...

}

RHS – SWC 4

The Real World

• 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!

RHS – SWC 5

The Real World

• 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!

RHS – SWC 6

The Real World

• Next problem: Error handling code is often very ”far away” from error detction

main askUser doAction makeWithdraw withdraw

Error detected

Error handling

?

RHS – SWC 7

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

RHS – SWC 8

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

RHS – SWC 9

Throwing and catching

main askUser doAction makeWithdraw withdraw

Error detected – throw!

Who will catch the exception?

RHS – SWC 10

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!

RHS – SWC 11

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

RHS – SWC 12

Throwing and catching

• General exception handler structure:

try

{

// Code which may throw an exception

...

}

catch (ExceptionType ex)

{

// Proper handling of exception

...

}

RHS – SWC 13

Throwing and catching

try

{

myAccount.withdraw(1000);

myAccount.getTransactions();

...

}

catch (IllegalArgumentException ex)

{

System.out.println(ex.getMessage());

ex.printStackTrace();

}

RHS – SWC 14

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

RHS – SWC 15

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

RHS – SWC 16

Throwing and catching

try

{

myAccount.withdraw(1000);

myAccount.getTransactions();

...

}

catch (Exception ex)

{

// do nothing...

}

Tempting, but bad…!

RHS – SWC 17

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!

RHS – SWC 18

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 exceptionout.close();

}

RHS – SWC 19

The finally Clause

• The code in the finally clause is guaran-teed to be executed, in one of these ways:– After completing the last statement in the try

block– After completing the last statement in a catch clause, if an exception is caught

– When an exception was throw in the try block and not caught

RHS – SWC 20

The finally Clausetry{}catch{}finally{}

try{try{}finally{}

}catch{}

GOODNOT SOGOOD

RHS – SWC 21

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

RHS – SWC 22

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

RHS – SWC 23

Checked and Unchecked

// Suppose draw() can throw DrawExceptiondraw(Figure f);

// Compiler will not like this!public void drawOne(Figure f){getScreen().draw(f);

}

RHS – SWC 24

Checked and Unchecked

// Option 1: Re-throw the exception// (i.e. do nothing…)

public void drawOne(Figure f) throws DrawException{getScreen().draw(f);

}

RHS – SWC 25

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}

}

RHS – SWC 26

Checked and Unchecked

drawdrawOneOption 1: drawOne does nothing, so it must annonce that it will (re)throw the exception

drawdrawOneOption 2: drawOne handles the exception, so it is ”consumed” by drawOne

RHS – SWC 27

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

RHS – SWC 28

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

RHS – SWC 29

Making your own exceptions

public class InsufficientFundsExceptionextends RunTimeException

{public InsufficientFundsException() {}

public InsufficientFundsException(String message){ super(message);}

}

RHS – SWC 30

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

RHS – SWC 31

Exceptions vs. Flow control

for (int i = 0; i < noOfElements; i++)

myArray[i] = i;

try {

for (int i = 0; /* No Test?? */ ; i++) myArray[i] = i;

}catch (ArrayIndexOutOfBoundsException e)

{}

GOOD

BAD

RHS – SWC 32

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