	CS 535 Object-Oriented Programming Fall Semester, 2003 Exceptions
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Contents of Doc 9, Exceptions

References Object-Oriented Design with Smalltalk — a Pure Object Language and its Environment, Ducasse, University of Bern, Lecture notes 2000/2001, http://www.iam.unibe.ch/~ducasse/WebPages/Smalltalk/ST00_01.pdf

VisualWorks Application Developer Guide, doc/vwadg.pdf in the VisualWorks installation. Chapter 10 Exception and Error Handling

Reading VisualWorks Application Developer Guide, doc/vwadg.pdf in the VisualWorks installation. Chapter 10 Exception and Error Handling, pages 215-230


Doc 9, Exceptions Slide # 2

Exceptions

Basic Issues

How are exceptions raised (started)
How to handle exceptions
What can one do when handling exceptions
How is the correct handler found for an exception


Doc 9, Exceptions Slide # 3

Basic Handling of Exceptions

Format

   [ProtectedBlock]
      on: ExceptionList
      do: [:exception | HandlerBlock]

Example

[numerator := 5.
denominator := 0.0.
numerator / denominator]
   on: ZeroDivide
   do: 
      [:exception | 
      Transcript 
         show: exception description; 
         cr]

Unlike Java, in Smalltalk zero divide by both integer and floats cause a zero divide exception to be raised


Doc 9, Exceptions Slide # 4

Exceptions & Return Values
on:do: is a message, so returns a value

If an exception is raised the return value is:

The return value of the handler

If an exception is not raised the return value is:

The return value of the protected block

Example - No Exception Raised
This code assigns 10 to result

| result |
result := [10/1]
   on: ZeroDivide
   do: [:exception | Float zero ].
^result

Example - Exception Raised
This code assigns 0.0 to result

| result |
result := [10/0]
   on: ZeroDivide
   do: [:exception | Float zero ].
^result


Doc 9, Exceptions Slide # 5

Catching Multiple Exceptions
Use a comma or ExceptionSets

[1/0]
   on: Warning , ZeroDivide
   do: [:exception | code here]

| exceptions |
exceptions := ExceptionSet with: Warning with: ZeroDivide.
[1/0]
   on: exceptions 
   do: [:exception | code here]


Doc 9, Exceptions Slide # 6

Clean Up or Unwind Protection

mySafeMethod
   | grades |
   grades := 'cs535Grades' asFilename.
   gradeIO := grades readWriteStream.
   Bar yourUnsafeMethod.
   "Change some grades here - code not shown"
   gradeIO close.

If yourUnsafeMethod raises an exception gradeIO is not closed

If you do not know what exception might be raised it is hard to handle it

Use ensure: or ifCurtailed:


Doc 9, Exceptions Slide # 7

ensure:

Format

   [block] ensure: [clean up block]

Ensure that the clean up block will be done

If block ends due to an exception

Execute handler for exception
Execute clean up block

You code should not depend on the order of execution of the handler and clean up block

Example

[[10/0] ensure: [Transcript show: 'In ensure'; cr]]
   on: ZeroDivide
   do: [:exception | Transcript show: 'In handler';cr ]

Output in Transcript In handler
In ensure


Doc 9, Exceptions Slide # 8

ifCurtailed:

Format

   [block] ifCurtailed: [clean up block]

Clean up block is done only if [block] ends abnormally


Doc 9, Exceptions Slide # 9

Raising Exceptions

Implicitly Raised Exceptions
Exceptions can be raised by VM

12 / 0

Explicitly Raised Exceptions
Send one of following messages to an exception class

raiseSignal: aStringDescriptionOfProblem
raiseSignal

Examples
Warning raiseSignal: 'This string is the signal description' false

Error raiseSignal


Doc 9, Exceptions Slide # 10

Explicitly Raised Exceptions
Some Error Handling Messages in Object

self error: 'Error message'

Raises exception with given message

self halt
self halt: 'Message"

Raises Halt exception.
Allows user to invoke debugger or resume

self notify: 'Notify message"

Like halt except user does not see stack history

self shouldNotImplement

Used in subclasses in inherited methods that do not belong in the subclass

self subclassResponsibility

Used in methods to declare them abstract
Indicated subclasses must implement this method


Doc 9, Exceptions Slide # 11

self error: 'A message'
How this Works
Object defines a method error: that raises an exception

Object>>error: aStringOrMessage
   | lastNonSpace aString|
   aString :=  aStringOrMessage asString.
   lastNonSpace := aString findLast: [:ch | ch ~= Character space].
   ^self errorSignal raiseErrorString: 
         (aString copyFrom: 1 to: lastNonSpace)

Object>>errorSignal
   "Answer the Signal used for miscellaneous errors
   (self error:)."
   ^self class errorSignal
Object class>>errorSignal
   "Answer the Signal used for miscellaneous errors (self error:)."
   ^Error

Default exception raised is Error

To change the exception raised by error: override the class method errorSignal

Foo class>>errorSignal
   ^KeyNotFoundError
Doc 9, Exceptions Slide # 12

Template Method
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses

Subclasses redefine certain steps of an algorithm without changing the algorithm’s structure

Important in class libraries

Inverted control structure

Parent class calls subclass methods

Examples
printString and printOn:


Doc 9, Exceptions Slide # 13

Finding the Exception Handler

When an exception is raised

The enclosing handlers are searched

Start with the code that raised the exception
Search the "closest" enclosing handler first
Continue searching the enclosing handers

The first handler that deals with the exception is used

If no handlers handle the exception the exception's default action is done

[[1/0]
   on: ZeroDivide
   do: [:exception | Transcript show: 'First']]
      on: ZeroDivide
      do: [:exception | Transcript show: 'Second']

Result in Transcript First


Doc 9, Exceptions Slide # 14

Exceptions are Classes
Exception class is the parent of all exceptions

Subclasses define specialized exceptions

Important Subclasses

Error

A condition that prevents the normal continuation of processing

Notification

Something interesting has occurred
If it is not handled, it will pass by without effect

Warning

An unusual event the user needs to know about
Asks the user if the program should continue

MessageNotUnderstood

A method was sent to an object that does not implement it

Important exception methods

description

Returns a string describing the actual exception

defaultAction

Executed when an exception occurs


Doc 9, Exceptions Slide # 15

Inheritance and Exception

All subexceptions are caught by an exception in on:do:

ZeroDivide is a subclass of Error

The ZeroDivide exception will be caught in the following

[1/0]
   on: Error
   do: 
      [:exception | 
      Transcript 
         show: exception description; 
         cr]


Doc 9, Exceptions Slide # 16

Warning Default Action
Warning default action

Show dialog asking user if program should continue

Example 1
The following code:

Displays a dialog asking if user wishes to continue
Returns boolean result depending on users answer

Warning raiseSignal: 'An error occured, continue?'.

Dialog Displayed


Doc 9, Exceptions Slide # 17

Example 2
The following code:

Prints 'Handler' in the Transcript

[Warning raiseSignal: 'Hi Mom'.
Transcript show: 'End']
   on: Warning
   do: [:exception | Transcript show: 'Handler']

Result in Transcript

Handler


Doc 9, Exceptions Slide # 18

Notification Default Action
Notification default action

Do nothing & continue as normal

Example 1
The following code:

Prints 'End' in the Transcript

Notification raiseSignal: 'Hi Mom'.
Transcript show: 'End'

Example 2
The following code:

Prints 'Handler' in the Transcript

[Notification raiseSignal: 'Hi Mom'.
Transcript show: 'End']
on: Exception
do: [:exception | Transcript show: 'Handler']

This example shows why you may not want to wrap your code in a general catch all handler


Doc 9, Exceptions Slide # 19

What is the Default Action for Exception X?
Look at the defaultAction method in the exception's class


Doc 9, Exceptions Slide # 20

Resumable Exceptions

Some exceptions are resumable

| result |
[result := 10/0.
Transcript show: result printString]
on: ZeroDivide
do:
[:exception |
exception resume: Float zero ]

Output in Transcript 0.0


Doc 9, Exceptions Slide # 21

Exception Messages that exit the Handler

resume or resume:

Continue processing the protected block, immediately following the message that triggered the exception.

return or return:

Ends processing the protected block that triggered the exception

retry

Reevaluates the protected block

retryUsing:

Evaluates a new block in place of the protected block

resignalAs:

Resignal the exception as another exception

pass

Exit the current handler and pass to the next outer handler, control does not return to the passer

outer

as with pass, except will regain control if the outer handler resumes

resume: and return: return their argument as the return value, instead of the value of the final statement of the handler block


Doc 9, Exceptions Slide # 22

Example - resume:
10/0 raises an exception
The handler requests resumption with value 1
The expression 10/0 returns 1
The sum becomes 1 + 5

| result |
[result := 10/0 + 5.
Transcript show: result printString]
   on: ZeroDivide 
   do: 
      [:exception |
      exception resume: 1 ]

Output in Transcript 6


Doc 9, Exceptions Slide # 23

Example - resume
10/0 raises an exception
The handler requests resumption, no value
The expression 10/0 returns nil

| result |
[result := 10/0.
Transcript show: result printString]
   on: ZeroDivide 
   do: 
      [:exception |
      exception resume ]

Output in Transcript nil


Doc 9, Exceptions Slide # 24

Example - retry
x/y raises an exception
The handler sets y := 1.
The block is reexecuted

| x y result |
x := 10.
y := 0.
[result := x / y.
Transcript show: result printString]
   on: ZeroDivide 
   do: 
      [:exception |
      y := 1.
      exception retry ]

Output in Transcript 10


Doc 9, Exceptions Slide # 25

Example - return
The following are equivalent

| result |
[result := 10/0.
Transcript show: result printString]
   on: ZeroDivide 
   do: [:exception | exception return]

| result |
[result := 10/0.
Transcript show: result printString]
   on: ZeroDivide 
   do: [:exception | nil]

Result
No output in Transcript


Doc 9, Exceptions Slide # 26

Example - return:

The following are equivalent


| result |
result := [10/0]
   on: Error
   do: [:exception | Float zero ].
^result

| result |
result := [10/0]
   on: Error
   do: [:exception | exception return: Float zero ].
^result


Doc 9, Exceptions Slide # 27

Translating Exceptions

At times you may need to rethrow an exception, but as a different exception

[low-level I/O]
   on: OperatingSystemException
   do: 
      [ex|
      ex errorCode = -213
         ifTrue: [ex resignalAs: EndOfFile new]
         ifFalse: [ex resignalAs:
            (Error new messageText: 'OS Error']]


Doc 9, Exceptions Slide # 28

Creating Your Own Exceptions

Subclass the correct existing Exception

Usually Error or Notification

If you want the exception to be resumable

Make method isResumable return true

If you want non-standard default behavior

Override the method defaultAction

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