Algorithmics 3
Algorithmics 3
Error Management
October 18th, 2006
Algorithmics 3
Program
Program
Errors
About errors
Exceptions
Java choices
A first look at I/O
Bottom line
I/O
About I/O
Algorithmics 3
Program
Warning
Some of you can’t program yet.
Algorithmics 3
Program
Warning
Some of you can’t program yet.
It’s unfortunate, but it’s not too late.
Algorithmics 3
Program
Warning
Some of you can’t program yet.
It’s unfortunate, but it’s not too late.
So, let me give a little advice.
Algorithmics 3
Program
Programming is not
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
Writing random things found in the lecture.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Giving random answers.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Giving random answers.
I
Adding stuff until it works.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Giving random answers.
I
Adding stuff until it works.
Hoping that the answer to every single question appears in the
lecture.
I
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Giving random answers.
I
I
Adding stuff until it works.
Hoping that the answer to every single question appears in the
lecture.
I
Waiting for the end of the class.
Algorithmics 3
Program
Programming is not
I
Nodding approvingly whenever you see some source code.
I
Looking at a blank screen waiting for the miracle to come.
I
I
Writing random things found in the lecture.
Random things found on the Internet.
I
Giving random answers.
I
I
Adding stuff until it works.
Hoping that the answer to every single question appears in the
lecture.
I
Waiting for the end of the class.
Note that most of these remarks are valid for all sciences.
Algorithmics 3
Program
Programming is
Algorithmics 3
Program
Programming is
I
Reasoning.
Algorithmics 3
Program
Programming is
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
Algorithmics 3
Program
Programming is
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Algorithmics 3
Program
Programming is
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Algorithmics 3
Program
Programming is
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Algorithmics 3
Program
Programming is
I
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Finding simple solutions to simple problems.
Algorithmics 3
Program
Programming is
I
I
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Finding simple solutions to simple problems.
Understanding the solution well enough to be able to explain them
without the slightest ambiguity.
Algorithmics 3
Program
Programming is
I
I
I
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Finding simple solutions to simple problems.
Understanding the solution well enough to be able to explain them
without the slightest ambiguity.
Having a clear idea of what the tools will let you do and what they
won’t.
Algorithmics 3
Program
Programming is
I
I
I
I
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Finding simple solutions to simple problems.
Understanding the solution well enough to be able to explain them
without the slightest ambiguity.
Having a clear idea of what the tools will let you do and what they
won’t.
Understanding what you read (typically, in a documentation).
Algorithmics 3
Program
Programming is
I
I
I
I
I
I
I
I
Reasoning.
Explaining things clearly in a foreign language to someone deaf,
blind, stupid and who remembers every single of your contradictions.
(The computer, mind you, not the teacher).
Checking that the deaf, blind, stupid person understands what you
have explained.
Understanding problems well enough to be able to explain them
without the slightest ambiguity.
Finding simple solutions to simple problems.
Understanding the solution well enough to be able to explain them
without the slightest ambiguity.
Having a clear idea of what the tools will let you do and what they
won’t.
Understanding what you read (typically, in a documentation).
Once again, it’s not that different from what you’re asked in other
sciences and technologies.
Algorithmics 3
Program
So ?
If you can succeed in maths or physics, you can succeed in programming.
Algorithmics 3
Program
Exam
For the exam, you’ll be allowed to use just about everything, including
the lectures, google, Javasoft’s website. . .
Algorithmics 3
Program
Exam
For the exam, you’ll be allowed to use just about everything, including
the lectures, google, Javasoft’s website. . .
Well, ok, not everything: not cellphones, your e-mail or Messenger, for
instance.
Algorithmics 3
Program
Exam
For the exam, you’ll be allowed to use just about everything, including
the lectures, google, Javasoft’s website. . .
Well, ok, not everything: not cellphones, your e-mail or Messenger, for
instance.
None of this will help you if you can’t program.
Algorithmics 3
Program
Time for questions
Is there any question ?
Algorithmics 3
Program
Time for questions
Is there any question ?
If there’s no question, I’ll assume that you have understood everything up
to this point, including Algorithmics 1 and Algorithmics 2.
Algorithmics 3
Program
Time for questions
Is there any question ?
If there’s no question, I’ll assume that you have understood everything up
to this point, including Algorithmics 1 and Algorithmics 2.
In particular, in lab classes, I may decide that I have spent enough time
trying to help you at the whiteboard.
Algorithmics 3
Program
Time for questions
Is there any question ?
If there’s no question, I’ll assume that you have understood everything up
to this point, including Algorithmics 1 and Algorithmics 2.
In particular, in lab classes, I may decide that I have spent enough time
trying to help you at the whiteboard.
So ?
Algorithmics 3
Program
Today’s program
1. Errors.
2. Exceptions.
3. A few words about files.
Algorithmics 3
Program
Note
The first part of this session is back to algorithmics.
We’re going to introduce a new algorithmic construction, available in
most modern programming languages: exceptions.
Algorithmics 3
Program
Note
The first part of this session is back to algorithmics.
We’re going to introduce a new algorithmic construction, available in
most modern programming languages: exceptions.
The second part of this session is, as Swing, putting algorithmics and
object-orientation to work.
Algorithmics 3
Errors
About errors
Program
Errors
About errors
Exceptions
Java choices
A first look at I/O
Bottom line
I/O
About I/O
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
You should fix this.
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
You should fix this.
type error The computer guesses you’re doing something
counter-natural.
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
You should fix this.
type error The computer guesses you’re doing something
counter-natural.
You should fix this.
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
You should fix this.
type error The computer guesses you’re doing something
counter-natural.
You should fix this.
runtime error Something wrong happens.
Algorithmics 3
Errors
About errors
Errors
When can errors happen ?
syntax error The computer doesn’t understand your program.
You should fix this.
type error The computer guesses you’re doing something
counter-natural.
You should fix this.
runtime error Something wrong happens.
Your program should handle this.
Algorithmics 3
Errors
About errors
Runtime errors
Let’s take a look at division
function d i v i d e (p : integer , q : integer ) : double
begin
d i v i d e := p/q
end
Q What’s missing ?
Algorithmics 3
Errors
About errors
Runtime errors
Let’s take a look at division
function d i v i d e (p : integer , q : integer ) : double
begin
d i v i d e := p/q
end
Q What’s missing ?
A Well, error management ?
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Why ?
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Why ?
I
It’s not always possible to return a special value.
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Why ?
I
It’s not always possible to return a special value.
Why ?
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Why ?
I
It’s not always possible to return a special value.
Why ?
I
gotos are evil.
Algorithmics 3
Errors
About errors
Ideal error management
Q Ideally, what should happen ?
Q What are the difficulties ?
I
You need to be able to stop a computation or an operation when
something goes wrong.
I
Abruptly stopping the program is too brutal.
Why ?
I
It’s not always possible to return a special value.
Why ?
I
gotos are evil.
I
The library designer typically doesn’t know how errors/exceptional
circumstances should be handled.
Algorithmics 3
Errors
Exceptions
Program
Errors
About errors
Exceptions
Java choices
A first look at I/O
Bottom line
I/O
About I/O
Algorithmics 3
Errors
Exceptions
Introducing exceptions
Exceptions are the standard manner of dealing with exceptional
circumstances, that require stopping a computation.
Algorithmics 3
Errors
Exceptions
Introducing exceptions
Exceptions are the standard manner of dealing with exceptional
circumstances, that require stopping a computation.
Typical exceptional circumstances:
I disk error (the disk is full, the file doesn’t exist. . . )
I
the user has given you an inacceptable value
I
this part of the program hasn’t been written yet
...
I
Algorithmics 3
Errors
Exceptions
How exceptions work
1. Some algorithm A is declared as being able to trigger a set of
exceptional circumstances.
Algorithmics 3
Errors
Exceptions
How exceptions work
1. Some algorithm A is declared as being able to trigger a set of
exceptional circumstances.
2. Any algorithm using A must declare that it is going to try and
execute A – and how it is going to handle the exceptional
circumstance.
Algorithmics 3
Errors
Exceptions
How exceptions work
1. Some algorithm A is declared as being able to trigger a set of
exceptional circumstances.
2. Any algorithm using A must declare that it is going to try and
execute A – and how it is going to handle the exceptional
circumstance.
3. If, for some reason, A triggers some exceptional circumstance, the
execution of the program immediately jumps to the exception
handler and proceeds from there.
4. Otherwise, the algorithm proceeds normally.
Algorithmics 3
Errors
Exceptions
Syntax and semantics – callee
public
{
double
division ( double p ,
double
q)
throws
MyArithmeticExceptionException
i f ( q==0)
{
t h r o w new M y A r i t h m e t i c E x c e p t i o n E x c e p t i o n ( ) ;
// The f u n c t i o n e x i t s , no v a l u e i s r e t u r n e d .
}
return
p/q ;
}
I
Exceptions must be declared.
Exceptions may be thrown.
I
When the exception is thrown, the execution of the method ends.
I
Algorithmics 3
Errors
Exceptions
Syntax and semantics – caller
public
{
double
try
}
actionPerformed ( ActionEvent
e)
{
catch
double result
= d i v i s i o n (p , q ) ;
S t r i n g m e s s a g e = " The ␣ r e s u l t ␣ o f ␣ t h e ␣ d i v i s i o n ␣ "+p+" / "+q+" ␣ i s ␣ "+ r e s u l t ;
JOptionPane . showMessage ( n u l l , message ) ;
( MyArithmeticExceptionException exc ) {
JOptionPane . showMessage ( n u l l ,
" E r r o r : ␣ d i v i s i o n ␣ by ␣ z e r o " ,
J O p t i o n P a n e . ERROR_MESSAGE ) ;
}
}
I
Exceptions must be either caught or re-declared.
I
The caller must declare that it is going to try executing some block.
The caller must declare what it is going to do if it catches an
exception.
I
Algorithmics 3
Errors
Exceptions
Benefits of exceptions
Q Any idea why exceptions are good ?
Algorithmics 3
Errors
Exceptions
Benefits of exceptions
Q Any idea why exceptions are good ?
A Here are a few:
1. Normal code is separated from error-handling. Your code is not
cluttered with error-checking.
2. There is a clear notion of what a method knows how to handle and
what it doesn’t.
3. You can get detailed error reports.
4. You can handle different errors differently.
Algorithmics 3
Errors
Exceptions
Exception design
try
{
//
// Do e r r o r −p r o n e o p e r a t i o n s and e v e r y t h i n g t h a t
//
} catch ( S o m e E x c e p t i o n e1 ) {
//
// Ha ndle t h e e r r o r
//
} catch ( A n o t h e r E x c e p t i o n e2 ) {
// Ha ndle t h e e r r o r
} finally {
//
// P e r f o r m c l e a n −up
//
}
Algorithmics 3
Errors
Exceptions
Finally ?
Oh, yeah, finally.
Algorithmics 3
Errors
Exceptions
Finally ?
Oh, yeah, finally.
try
{
// . . . a t t e m p t do s o m e t h i n g
} finally {
// . . . t h i s w i l l a l w a y s be e x e c u t e d
}
Algorithmics 3
Errors
Exceptions
Finally ?
Oh, yeah, finally.
try
{
// . . . a t t e m p t do s o m e t h i n g
} finally {
// . . . t h i s w i l l a l w a y s be e x e c u t e d
}
Keyword finally lets us define a block that will always be executed, no
matter what happens in the try block or in any catch block.
Algorithmics 3
Errors
Exceptions
What’s the point ?
What’s the point of finally ?
Algorithmics 3
Errors
Exceptions
What’s the point ?
What’s the point of finally ?
// L e t ’ s
try {
}
}
play
a
short
animation
// Open t h e window
// Load i m a g e 1 −− t h i s c a n f a i l
// Load i m a g e 2 −− t h i s c a n f a i l
// . . .
// I f l o a d i n g was t o o l o n g , s t o p now
w h i l e ( /∗ l i t t l e t i m e h a s e l a p s e d ∗/ )
{
// D i s p l a y n e x t i m a g e −− t h i s
}
// . . .
finally {
//No m a t t e r what h a s h a p p e n e d
// C l o s e t h e window
can
fail
Algorithmics 3
Errors
Exceptions
More generally
try
{
//
// Do e r r o r −p r o n e o p e r a t i o n s and e v e r y t h i n g t h a t
//
} catch ( S o m e E x c e p t i o n e1 ) {
//
// Ha ndle t h e e r r o r
//
} catch ( A n o t h e r E x c e p t i o n e2 ) {
// Ha ndle t h e e r r o r
} finally {
//
// P e r f o r m c l e a n −up
//
}
Algorithmics 3
Errors
Exceptions
Throwing exception
Of course, you can also throw an exception by yourself.
Algorithmics 3
Errors
Exceptions
Throwing exception
Of course, you can also throw an exception by yourself.
throw new Exception("I have a problem");
throw new
URISyntaxExpression("http;//www.univ-orleans.fr", "Your
URI doesn’t mean anything.");
Algorithmics 3
Errors
Exceptions
Throwing exception
Of course, you can also throw an exception by yourself.
throw new Exception("I have a problem");
throw new
URISyntaxExpression("http;//www.univ-orleans.fr", "Your
URI doesn’t mean anything.");
. . . or re-throw it !
try {
// . . .
} catch ( U R I S y n t a x E x c e p t i o n e ) {
// . . .
throw e ;
}
Algorithmics 3
Errors
Java choices
Program
Errors
About errors
Exceptions
Java choices
A first look at I/O
Bottom line
I/O
About I/O
Algorithmics 3
Errors
Java choices
Technical questions
Q What kind of entity is an exception ?
Algorithmics 3
Errors
Java choices
Technical questions
Q What kind of entity is an exception ?
A It’s an object. It’s a subclass of class Exception.
Algorithmics 3
Errors
Java choices
Technical questions
Q What kind of entity is an exception ?
A It’s an object. It’s a subclass of class Exception.
You can (and probably should) declare new exceptions.
Algorithmics 3
Errors
Java choices
Technical questions
Q What kind of entity is an exception ?
A It’s an object. It’s a subclass of class Exception.
You can (and probably should) declare new exceptions.
Exception catching interacts with this subclass system.
Algorithmics 3
Errors
Java choices
Exception checking
What does this do ?
p u b l i c c l a s s TestClass
{
p u b l i c void test () throws
{
return ;
}
p u b l i c v o i d moreTest ( )
{
test ();
}
}
Exception
Algorithmics 3
Errors
Java choices
Exception checking
What does this do ?
p u b l i c c l a s s TestClass
{
p u b l i c void test () throws
{
return ;
}
p u b l i c v o i d moreTest ( )
{
test ();
}
}
Exception
1 error found:
File: /tmp/TestClass.java [line: 9]
Error: unreported exception java.lang.Exception;
must be caught or declared to be thrown
Algorithmics 3
Errors
Java choices
Exceptions are checked
The compiler checks that all exceptions are either caught or declared.
Algorithmics 3
Errors
Java choices
Exceptions are checked
The compiler checks that all exceptions are either caught or declared.
Well, not all exceptions. Not if they extend RuntimeException.
Algorithmics 3
Errors
Java choices
Exceptions are checked
The compiler checks that all exceptions are either caught or declared.
Well, not all exceptions. Not if they extend RuntimeException.
QWhat’s the point ? What’s the downside ?
Algorithmics 3
Errors
Java choices
Exceptions are checked
The compiler checks that all exceptions are either caught or declared.
Well, not all exceptions. Not if they extend RuntimeException.
QWhat’s the point ? What’s the downside ?
AThe point is that otherwise, you would need to catch or declare an
exception for each array access !
The downside is that it’s easy to abuse this. Just don’t.
Algorithmics 3
Errors
Java choices
Uncaught exceptions
Q What happens if you fail to catch an exception ?
Algorithmics 3
Errors
Java choices
Uncaught exceptions
Q What happens if you fail to catch an exception ?
A Several answers:
Algorithmics 3
Errors
Java choices
Uncaught exceptions
Q What happens if you fail to catch an exception ?
A Several answers:
I You can’t.
Algorithmics 3
Errors
Java choices
Uncaught exceptions
Q What happens if you fail to catch an exception ?
A Several answers:
I You can’t.
I
Ok, you can if it’s a RuntimeException.
Algorithmics 3
Errors
Java choices
Uncaught exceptions
Q What happens if you fail to catch an exception ?
A Several answers:
I You can’t.
I
Ok, you can if it’s a RuntimeException.
I
Let’s look at an example.
Algorithmics 3
Errors
Java choices
Exceptions as a debugging tool
You’re encouraged to use exceptions to find out when you’re reaching a
part of the program that shouldn’t be reachable.
Algorithmics 3
I/O
About I/O
Program
Errors
About errors
Exceptions
Java choices
A first look at I/O
Bottom line
I/O
About I/O
Algorithmics 3
I/O
About I/O
I/O ?
I/O stands for Input/Output: getting data in the program / getting data
out of the program.
Algorithmics 3
I/O
About I/O
I/O ?
I/O stands for Input/Output: getting data in the program / getting data
out of the program.
Input Reading a file. Receiving a message from the network.
Receiving instructions from the keyboard.
Output Writing a file. Sending a message to the network. Writing
something on the screen.
Algorithmics 3
I/O
About I/O
Do you I/O ?
Truth be told, you’ve been programming I/O for some time.
Algorithmics 3
I/O
About I/O
Do you I/O ?
Truth be told, you’ve been programming I/O for some time.
I’m talking of System.out.println().
Algorithmics 3
I/O
About I/O
Do you I/O ?
Truth be told, you’ve been programming I/O for some time.
I’m talking of System.out.println().
Let’s take a look at the documentation.
Algorithmics 3
I/O
About I/O
Streams
Input streams A construction able to provide a (potentially infinite) set
of informations to your program.
.
Output streams A construction able to accept a (potentially infinite) set
of informations coming from your program.
.
Algorithmics 3
I/O
About I/O
Streams
Input streams A construction able to provide a (potentially infinite) set
of informations to your program. In Java, they’re
subclasses of InputStream.
Output streams A construction able to accept a (potentially infinite) set
of informations coming from your program. In Java,
they’re subclasses of OutputStream.
Let’s take a look at the documentation.
Algorithmics 3
I/O
About I/O
Streams
Input streams A construction able to provide a (potentially infinite) set
of informations to your program. In Java, they’re
subclasses of InputStream.
Output streams A construction able to accept a (potentially infinite) set
of informations coming from your program. In Java,
they’re subclasses of OutputStream.
Let’s take a look at the documentation.
Note the exceptions.
Algorithmics 3
I/O
About I/O
Most common streams
Algorithmics 3
I/O
About I/O
Most common streams
From the keyboard System.in
Algorithmics 3
I/O
About I/O
Most common streams
From the keyboard System.in
To the screen System.out
Algorithmics 3
I/O
About I/O
Most common streams
From the keyboard System.in
To the screen System.out
From a file FileInputStream
Algorithmics 3
I/O
About I/O
Most common streams
From the keyboard System.in
To the screen System.out
From a file FileInputStream
To a file FileOutputStream
Algorithmics 3
I/O
About I/O
Enough for today
Next time: putting files to good use.