exceptions handling the unexpected

ExceptionsHandling the unexpected

DCS – SWC 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

?

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 7

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

main askUser doAction makeWithdraw withdraw

Error detected – throw!

Who will catch the exception?

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 DrawExceptiondraw(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

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

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 exceptionout.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 Clausetry{}catch{}finally{}

try{try{}finally{}

}catch{}

GOODNOT SOGOOD

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 InsufficientFundsExceptionextends 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

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

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