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OO Design and OO Design and Programming II Programming II

Chapter 17Chapter 17Exception Exception HandlingHandling

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What is an Exception?What is an Exception? An exception is an event that occurs during the execution of

a program that disrupts the normal flow of instructions. When an error occurs within a method, the method creates

an exception object and hands it off to the runtime system The exception object contains information about the error,

including its type and state of the program when the error occurred.

Creating an exception object and handing it to the runtime system is called throwing an exception.

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main

Method that has an exception handler

Method without an exception handler

Method where exception occurred

The call stack

Method call

Method call

Method call

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main

Method that has an exception handler

Method without an exception handler

Method where error occurred Looking for

appropriate handler

Looking for appropriate handler

Throws exception

not caught

Catches exception

When an error occurs, the runtime system searches the call stack for an appropriate error handler

“ducks”

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Catch and SpecifyCatch and Specify Catch: a method can catch an

exception by providing an exception handler for that type of exception.

Specify: If a method chooses not to catch an exception, the method must specify that it can throw that exception.

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Two kinds of ExceptionsTwo kinds of Exceptions1.1. Runtime exceptionsRuntime exceptions

Arithmetic exceptionsArithmetic exceptions Pointer exceptions (accessing an Pointer exceptions (accessing an

object’s members via null reference)object’s members via null reference) Indexing exceptions (accessing array Indexing exceptions (accessing array

element)element)2.2. Non-runtime exception (also called Non-runtime exception (also called

checked exceptionchecked exception)) I/OI/O

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The Throwable CLassThe Throwable CLass

Throwable

Object

Error Exception

IOException

RuntimeException

MyExceptionunchecked

unchecked

checked

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Checked or Unchecked Checked or Unchecked ExceptionsExceptions

Unchecked Exceptions

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Runtime exceptionsRuntime exceptions Arithmetic exceptionsArithmetic exceptions See example: BJ_Exceptions/Exc0.javaSee example: BJ_Exceptions/Exc0.java

compiled but runtime errorcompiled but runtime error BJ_Exceptions/Exc1.javaBJ_Exceptions/Exc1.java Compare Exc0 and Exc1 when the Compare Exc0 and Exc1 when the

exception occursexception occurs See the stack trace when exception See the stack trace when exception

occursoccurs

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Writing your own Writing your own HandlerHandler

Examples Exc0.java and Exc1.java use Examples Exc0.java and Exc1.java use the Java run-time environment the Java run-time environment exception handlersexception handlers

Write your own handlers with Write your own handlers with trytry catchcatch

Examples:Examples: BJ_Exceptions/Exc2.javaBJ_Exceptions/Exc2.java BJ_Exceptions/HandleErrorA.javaBJ_Exceptions/HandleErrorA.java BJ_Exceptions/HandleErrorB.javaBJ_Exceptions/HandleErrorB.java

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Not just one catchNot just one catch Multiple catchesMultiple catches are allowed in a try- are allowed in a try-

catch statementcatch statement Examples:Examples:

BJ_Exceptions/Multicatch.javaBJ_Exceptions/Multicatch.java Caution: Exception subclasses must Caution: Exception subclasses must

precede their exception superclasses. precede their exception superclasses. Otherwise the subclasses are not reachableOtherwise the subclasses are not reachable

Example: Example: BJ_Exceptions/SuperSubCatch.javaBJ_Exceptions/SuperSubCatch.java

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Nested try clausesNested try clauses A try clause can be nested in another try.A try clause can be nested in another try. Each time a try clause is entered, the context of Each time a try clause is entered, the context of

that exception is pushed on the stack.that exception is pushed on the stack. If an inner try clause does not have a catch If an inner try clause does not have a catch

handler for a particular exception, the stack is handler for a particular exception, the stack is unwound and the next try clause's catch handlers unwound and the next try clause's catch handlers are inspected for a match. This continues until are inspected for a match. This continues until one of the catch clauses succeeds, or until all of one of the catch clauses succeeds, or until all of the nested try statements are exhausted.the nested try statements are exhausted.

If no catch clause matches, then the Java run-time If no catch clause matches, then the Java run-time environment system will handle the exception.environment system will handle the exception.

Example: BJ_Exception/NestTry.javaExample: BJ_Exception/NestTry.java

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You can be a pitcher tooYou can be a pitcher too So far you have been catching exceptionsSo far you have been catching exceptions You can throw exceptions tooYou can throw exceptions too General form of General form of throw:throw:

throw ThrowableInstancethrow ThrowableInstance Primitive types, non-Throwable classes e.g. Primitive types, non-Throwable classes e.g.

String and Object cannot be used as String and Object cannot be used as exceptionsexceptions

Two ways to obtain a Throwable object:Two ways to obtain a Throwable object: using a parameter in a catch phraseusing a parameter in a catch phrase creating a Throwable object with the creating a Throwable object with the newnew

operatoroperator

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Method must declare a Method must declare a throwthrow

If method does not catch, don’t just throw If method does not catch, don’t just throw but you need to but you need to declaredeclare up front for up front for intention intention to throwto throw

General form of method declaration that General form of method declaration that includes a includes a throwsthrows clause clause

type method-name(parameter-list) type method-name(parameter-list) throws throws exception-listexception-list

{{// body of method// body of method

}}

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ExamplesExamples BJ_Throw/BJ_Throw/

IncorrectThrowsDemo.javaIncorrectThrowsDemo.java BJ_Throw/CorrectThrowsDemo.javaBJ_Throw/CorrectThrowsDemo.java BJ_Throw/ThrowDemo.javaBJ_Throw/ThrowDemo.java

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try-catch--finallytry-catch--finallytry { Java statements} catch (ExceptionType e1) {//exception

Java Statements //handler} catch (ExceptionType e2) {

Java Statements} finally { // optional

Java Statements // If present, // always

executes// close files

// cleanup}

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LabLab To determine the number of To determine the number of

characters, words, and lines in a filecharacters, words, and lines in a file Use FileReader to read a text fileUse FileReader to read a text file Use a simple finite state machine to Use a simple finite state machine to

count the number of words or linescount the number of words or lines

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FileReaderFileReaderTo construct a FileReader, use the To construct a FileReader, use the

following constructors:following constructors:public FileReader(String filename)public FileReader(String filename)public FileReader(File file)public FileReader(File file)Ref: Ref:

http://java.sun.com/j2se/1.5.0/docs/api/java/io/FileReadehttp://java.sun.com/j2se/1.5.0/docs/api/java/io/FileReader.htmlr.html

A A java.io.FileNotFoundExceptionjava.io.FileNotFoundException would would occur if you attempt to create a occur if you attempt to create a FileReaderFileReader with a nonexistent file. with a nonexistent file.

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How to read with How to read with FileReaderFileReader

// Create an input stream// Create an input stream input = new FileReader("temp.txt");input = new FileReader("temp.txt");

int code;int code; /* Repeatedly read a character and /* Repeatedly read a character and

display it on the console*/display it on the console*/ while ((code = input.read()) != -1)while ((code = input.read()) != -1) System.out.print((char)code);System.out.print((char)code);

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// To catch I/O exceptions try { // Create an input stream // Repeatedly read a character and display it on the console } catch (FileNotFoundException ex) { System.out.println("File temp.txt does not exist"); } catch (IOException ex) { ex.printStackTrace(); } finally { try { input.close(); // Close the stream } catch (IOException ex) { ex.printStackTrace(); } }

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FSM to count number of FSM to count number of lineslines

StateState Input Input char readchar read

ActionAction New New StateState

newLinenewLine ‘‘\n’\n’ lineCount +lineCount +++

newLinenewLine

newLinenewLine Not ‘\n’Not ‘\n’ nonenone !newLine!newLine

!newLine!newLine ‘‘\n’\n’ lineCount +lineCount +++

newLinenewLine

!newLine!newLine Not ‘\n’Not ‘\n’ nonenone !newLine!newLine

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FSM to count wordsFSM to count wordsStateState Input char Input char

readreadActionAction New StateNew State

newWordnewWord None of WS None of WS characterscharacters

wordCountwordCount++++

notNewWornotNewWordd

newWordnewWord WS WS charactercharacter

nonenone newWordnewWord

notNewWornotNewWordd

Any WS Any WS charactercharacter

nonenone newWordnewWord

notNewWornotNewWordd

Non-WS Non-WS charactercharacter

nonenone notNewWornotNewWordd

WS are white space characters: ‘’, ‘\t’, or ‘\n’

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checked checked exceptionsexceptionsSource Code Review and DemoSource Code Review and Demo

ListOfNumbers0.javaListOfNumbers0.javanew FileWriter("OutFile.txt") // Note new FileWriter("OutFile.txt") // Note

thisthisBut in But in

http://java.sun.com/j2se/1.5.0/docshttp://java.sun.com/j2se/1.5.0/docs/api//api/

FileWriter throws an exceptionFileWriter throws an exceptionpublic public FileWriterFileWriter((StringString fileName)  fileName)

throws throws IOExceptionIOException

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Exercise 5.1Exercise 5.1 Go to directory “Ex5.1 Checked Go to directory “Ex5.1 Checked

Exceptions”Exceptions” 1. Down load and compile 1. Down load and compile

ListOfNumers0.java as is.ListOfNumers0.java as is. 2. Study the changes in 2. Study the changes in

ListOfNumbers1.java and compile.ListOfNumbers1.java and compile. 3. Study the changes in 3. Study the changes in

ListOFNumbers2.java and compileListOFNumbers2.java and compile

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try-catch--finallytry-catch--finallytry { Java statements} catch (ExceptionType e1) {//exception

Java Statements //handler} catch (ExceptionType e2) {

Java Statements} finally { // optional

Java Statements // If present, // always

executes// close files

// cleanup}

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An ExampleAn Examplepublic void writeList() { PrintWriter out = null; try { System.out.println("Entering try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < size; i++) out.println("i + " = " + vector.elementAt(i)); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException:" +

e.getMessage()); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); } }}

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Source Code Source Code Review and Review and

DemoDemoDirectory “Ex5.2 Checked Directory “Ex5.2 Checked Exceptions”Exceptions”

ListOfNumbers.javaListOfNumbers.javaListOfNumbersTest.javaListOfNumbersTest.java

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Exercise 5.2Exercise 5.21.1. Down load and run Down load and run

ListOfNumersTest.java as is.ListOfNumersTest.java as is.2.2. Change outFile.txt to read-only and Change outFile.txt to read-only and

run it againrun it again3.3. Change outFile.txt back to r/wChange outFile.txt back to r/w4.4. Change “i<size” to (i<=size) in Change “i<size” to (i<=size) in

ListOfNumbers.javaListOfNumbers.java5.5. Compile and run againCompile and run again6.6. Change the program backChange the program back

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throwthrow We have looked at catching exceptions We have looked at catching exceptions

thrown by the Java Runtime systemthrown by the Java Runtime system Your program may also throw an Your program may also throw an

exceptionexception throw throw ThrowableInstance;ThrowableInstance; ThrowableInstance ThrowableInstance is an object of type is an object of type

ThrowableThrowable or a subclass of or a subclass of ThrowableThrowable

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Exceptions Thrown by a Method

public Object pop() throws EmptyStackException { Object obj;

if (size == 0) throw new EmptyStackException(); obj = objectAt(size - 1); setObjectAt(size - 1, null); size--; return obj;}

*The EmptyStackException class is defined in the java.util package.

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The Throwable CLassThe Throwable CLass

Throwable

Object

Error Exception

IOException

RuntimeException

MyExceptionunchecked

unchecked

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What can you throw?What can you throw? ErrorError deals with internal errors and deals with internal errors and

resource exhaustion resource exhaustion Not much you can do if it occursNot much you can do if it occurs You should not throw a object of this typeYou should not throw a object of this type

ExceptionException Hierarchy Hierarchy Focus on ExceptionFocus on Exception

Runtime exception is due to a programming Runtime exception is due to a programming errorerror

I/O exceptions may occur to a good programI/O exceptions may occur to a good program

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ExamplesExamples RuntimeExceptionsRuntimeExceptions

A bad castA bad cast An out-of-bounds array accessAn out-of-bounds array access A null pointer accessA null pointer access

Non RuntimeExceptions:Non RuntimeExceptions: Trying to read past the end of a fileTrying to read past the end of a file Trying to open a malformed URLTrying to open a malformed URL Trying to find a class object for a string Trying to find a class object for a string

that does not denote an existing classthat does not denote an existing class

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Where do you advertise Where do you advertise your method may throw?your method may throw?

In the header of the methodIn the header of the method For example: readLine of BufferedReaderFor example: readLine of BufferedReaderpublic public StringString readLinereadLine() throws () throws

IOExceptionIOExceptionhttp://java.sun.com/j2se/1.4.2/docs/api/java/io/BufferedReader.html#BufferedReader(java.io.Readehttp://java.sun.com/j2se/1.4.2/docs/api/java/io/BufferedReader.html#BufferedReader(java.io.Readerr

))

Returns a StringReturns a String May also throw an IOExceptionMay also throw an IOException If such exception occurs, runtime system will If such exception occurs, runtime system will

search for an exception handler.search for an exception handler.

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To Advertise a throw or To Advertise a throw or NotNot

You don’t have to advertise every possible throwable You don’t have to advertise every possible throwable objectobject

Then when and what you have to advertise?Then when and what you have to advertise? An exception is thrown in 4 situations:An exception is thrown in 4 situations:

1.1. You call a method that throws a checked exception, You call a method that throws a checked exception, for example, the readLine method of the for example, the readLine method of the BuffredReader classBuffredReader class

2.2. You detect an fault and throw a checked exception You detect an fault and throw a checked exception with the throw statementwith the throw statement

3.3. You make a programming error, such as array[-1] = You make a programming error, such as array[-1] = 0 and that gives rise to an unchecked exception such 0 and that gives rise to an unchecked exception such as ArrayIndexOutOfBoundsExceptionas ArrayIndexOutOfBoundsException

4.4. An internal error occurs in the virtual machine or An internal error occurs in the virtual machine or runtime libraryruntime library

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To Advertise a throw or To Advertise a throw or NotNot

For cases 1 and 2, you must tell the For cases 1 and 2, you must tell the programmers who will use your method that programmers who will use your method that there is the possibility of an exceptionthere is the possibility of an exception Why? Any method that throws an exception may Why? Any method that throws an exception may

terminate the threadterminate the thread For cases 3 and 4, you should not and need For cases 3 and 4, you should not and need

not advertise not advertise Why? Why? You should fix the unchecked runtime exception You should fix the unchecked runtime exception

instead of advertising its possibility. It is instead of advertising its possibility. It is completely under your controlcompletely under your control

You cannot fix internal Java errorsYou cannot fix internal Java errors

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Source Code Review and DemoSource Code Review and DemoBJ_ExceptionInPackage\BJ_ExceptionInPackage\

ExceptionDemo.javaExceptionDemo.java

Exercise .3: download and run Exercise .3: download and run ExceptionDemo.javaExceptionDemo.java

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Why Exceptions?Why Exceptions? Separating Separating error handlingerror handling code from code from regular regular

code.code. Propagating errors up the call stack Propagating errors up the call stack

towards the callerstowards the callers Grouping and differentiating error Grouping and differentiating error

messages and typesmessages and types

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Advantage 1 – Separating Advantage 1 – Separating Error-Handling code from Error-Handling code from

Regular CodeRegular CodereadFile { try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; } catch (fileOpenFailed) { doSomething; } catch (sizeDeterminationFailed) { doSomething; } catch (memoryAllocationFailed) { doSomething; } catch (readFailed) { doSomething; } catch (fileCloseFailed) { doSomething; }}

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Advantage 2 – Propagating Advantage 2 – Propagating Errors Errors

Up the Call StackUp the Call Stackmethod1 { // Only I am interested in errors try { call method2; } catch (Exception e) { doErrorProcessing; }}method2 throws Exception { // ‘throws’needed // to duck call method3;}method3 throws Exception { call readFile;}

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Advantage 3Advantage 3

Throwable

Object

Error Exception

IOException

RuntimeException

MyException

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Exercise 5.4Exercise 5.4 What are the exceptions you see in What are the exceptions you see in

Assignment #4?Assignment #4? No command line argumentsNo command line arguments Cannot open input filesCannot open input files Empty employee data fileEmpty employee data file Data format (syntax) errorsData format (syntax) errors Data semantic errors (sup does not Data semantic errors (sup does not

exist)exist) Table overflow Table overflow