10-exceptions files streams 2 Jar (Java Archive) Files • Archives of Java ﬁles • Package code...

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Objec-ves• Jarfiles

• Excep-onsØ WrapupØ WhyExcep-ons?

• Files• Streams

Oct3,2016 Sprenkle-CSCI209 1

JARFILES


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Jar(JavaArchive)Files• ArchivesofJavafiles• Packagecodeintoaneatbundletodistribute

Ø Easier,fastertodownloadØ Easierforotherstouse

• jar command:create,view,andextractJarfilesØ Workssimilarlytotarjar cf myapplication.jar *.class

• Runitusingjavajava -jar myapplication.jar


Jar/TarCommands• Commonop-ons:

• Commonuse:Ø jar cfz archive.jar.gz arch_directoryØ jar xfz archive.jar.gz


Option/Operations Meaning

f The name of the archive file c Create an archive file x Extract the archive file v Verbose z Zip (compress) t Table of contents (list contents)

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Jarfile:Metadata• JarfileincludesaspecialmetadatafilewiththepathMETA-INF/MANIFEST.MFØ SayhowJarfileisusedØ jar createsadefaultmetadatafile,ifnotspecified


Jarfile:Metadata• ExamplemetadatafilethatallowsyoutoexecutetheJARwithjava

• Tocreatethejarfile:jar cmf myManifest myapplication.jar *.class

• Runitusingjavajava -jar myapplication.jar


Specifying the metadata file

Manifest-Version: 1.0Main-Class: MyApplication

Note the newline

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Crea-ngJarFilesinEclipse• ExportàJavaàJarfile

Ø Op-onstocreateaMANIFEST.MFfileØ Op-onstoincludesourcefilesoronlyclassfiles

• ShouldsubmitassignmentsthiswayØ Mustincludesourcefiles

• Lookforcheckbox


EXCEPTIONS


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Discussion:WhyCheckedandUncheckedExcep-ons?

• Whydowehaveexcep-onsthatthecompilerdoesn’tforcetheprogrammertocheck?Ø Thinkaboutexamplesofuncheckedexcep-ons(ArrayOutOfBoundsException, NullPointerException, ClassCastException)andwhenthoseexcep-onscanoccur


MethodsandExcep-onsExample• BufferedReader hasmethodreadLine()

Ø Readsalinefromastream,suchasafileornetworkconnec-on

• Methodheader:

• Interpre-ngtheheader:readLine willØ returnaString(ifeverythingwentright)Ø throwanIOException(ifsomethingwentwrong)


public String readLine() throws IOException

PartofAdver-sing

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Adver-singCheckedExcep-ons• Adver-sing:inJavadoc,documentunderwhatcondi-onseachexcep-onisthrownØ @throws tag

• Examplesofwhenyourmethodshouldadver-sethecheckedexcep-onsthatitmaythrowØ Yourmethodcallsamethodthatthrowsacheckedexcep-on

Ø Yourmethoddetectsanerrorinitsprocessinganddecidestothrowanexcep-on


JavadocGuidelinesabout@throws• Alwaysreportifthrowcheckedexcep-ons• Reportanyuncheckedexcep-onsthatthecallermightreasonablywanttocatchØ Excep-on:NullPointerExceptionØ Allowscallertohandle(ornot)Ø Documentexcep-onsthatareindependentoftheunderlyingimplementa-on

• Errorsshouldnotbedocumentedastheyareunpredictable


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WhattodowithaCaughtExcep-on?• Dumpthestackadertheexcep-onoccurs

Ø Whatelsecanwedo?

• Generally,twoop-ons:1.  Catchtheexcep-onandrecoverfromit2.  Passexcep-onuptowhoevercalledit


ToThroworCatch?• Problem:lower-levelexcep-onpropagateduptohigher-levelcode

• Example:userentersaccountinforma-onandgetsexcep-onmessage“fieldexceedsallowedlengthindatabase”Ø LostcontextØ Lower-leveldetailpollu-nghigher-levelAPI


Solution: higher-levels should catch lower-level exceptions �and throw them in terms of higher-level abstraction

GUI

DB

…

Excep-onhere

Handled here

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Excep-onTransla-on

• Specialcase:Excep-onChainingØ Whenhigher-levelexcep-onneedsinfofromlower-levelexcep-on


try {// Call lower-level abstraction

}catch (LowerLevelException ex) {

// log exception …throw new HigherLevelException(…);

}

try {// Call lower-level abstraction

}catch (LowerLevelException cause) {

// log exception …throw new HigherLevelException(cause);

}

Most standard Exceptions have this

constructor

Summary:MethodsThrowingExcep-ons• APIdocumenta-ontellsyouifamethodcanthrowanexcep-onØ Ifso,youmusthandleit

• Ifyourmethodcouldpossiblythrowanexcep-on(bygenera-ngitorbycallinganothermethodthatcould),adver-seit!Ø Ifyoucan’thandleeveryerror,that’sOK…letwhoeveriscallingyouworryaboutit

Ø However,theycanonlyhandletheerrorifyouadver-setheexcep-onsyoucan’tdealwith


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ProgrammingwithExcep-ons• Excep-onhandlingisslow

• Useonebigtry blockinsteadofnes-ngtry-catch blocksØ SpeedsupExcep-onHandlingØ Otherwise,codegetstoomessy

• Don'tignoreexcep-ons(e.g.,catch blockdoesnothing)Ø Bemertopassthemalongtohighercalls


try {}catch () { }try {}catch () { }

try {try {}catch () { }

}catch () { }

try {……

}catch () { }

Crea-ngOurOwnExcep-onClass• Trytoreuseanexis-ngexcep-on

Ø Matchinnameaswellasseman-cs

• IfyoucannotfindapredefinedJavaExceptionclassthatdescribesyourcondi-on,implementanewExceptionclass!


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Crea-ngOurOwnExcep-onClass

public class FileFormatException extends IOException {public FileFormatException() {

}

public FileFormatException(String message) {super(message);

}

// other 2 standard constructors…}

• Cannowthrowexcep-onsoftypeFileFormatException

What happens in this constructor implicitly?

Is this a checked or unchecked exception?

GuidelinesforCrea-ngYourOwnExcep-onClasses

• Includeaccessormethodstogetmoreinforma-onaboutthecauseoftheexcep-onØ “failure-captureinforma-on”

• Checkedoruncheckedexcep-on?Ø Checked:forcesAPIusertohandleBUTmoredifficulttouseAPI• Hastohandleallcheckedexcep-ons

Ø Usecheckedexcep-onifexcep-onalcondi-oncannotbepreventedbyproperuseofAPIandAPIusercantakeausefulac-onaderward


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Discussion:BenefitsofExcep-ons• Beentalkingaboutdetails…

• WhydoesJavahaveexcep-onsaspartofthelanguage?

• WhydoesJavaaddsomefeaturesthatPythondoesn’thave?


BenefitsofExcep-ons• Forceerrorchecking/handling

Ø Otherwise,won’tcompileØ Doesnotguarantee“good”excep-onhandling

• EasedebuggingØ Stacktrace

• Separateserror-handlingcodefrom“regular”codeØ ErrorcodeisincatchblocksatendØ Descrip-vemessageswithexcep-ons

• PropagatemethodsupcallstackØ Letwhoever“cares”abouterrorhandleit

• Groupanddifferen-ateerrortypes

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FILES


java.io.FileClass• Representsafileordirectory• Providesfunc-onalitysuchas

Ø StorageofthefileonthediskØ Determineifapar-cularfileexistsØ WhenfilewaslastmodifiedØ RenamefileØ Remove/deletefileØ …


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MakingaFileObject• Simplestconstructortakesfullfilename(includingpath)Ø Ifdon’tsupplypath,Javaassumescurrentdirectory(.)

Ø CreatesaFile objectrepresen-ngafilenamed“chicken.data”inthecurrentdirectory

Ø Doesnotcreateafilewiththisnameondisk


File f1 = new File("chicken.data");


Files,Directories,andUsefulMethods• AFile objectcanrepresentafileoradirectoryØ Directoriesarespecialfilesinmostmodernopera-ngsystems

• UseisDirectory()and/orisFile()fortypeoffileFileobjectrepresents

• Use exists()methodØ Determinesifafileexistsonthedisk

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MoreFileConstructors• Stringforthepath,Stringforfilename

• Filefordirectory,Stringforfilename

File f2 = new File("/csdept/local/courses/cs209/handouts","chicken.data");

File dir = new File("/csdept/local/courses/cs209/handouts");

File f4 = new File(dir, "chicken.data");


“Break”anyofJava’sPrinciples?

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java.io.File Class• 25+methods

Ø ManipulatefilesanddirectoriesØ Crea-ngandremovingdirectoriesØ Making,renaming,anddele-ngfilesØ Informa-onaboutfile(size,lastmodified)Ø Crea-ngtemporaryfilesØ …

• SeeonlineAPIdocumenta-on

FileTest.java

SCANNERjava.u-l.


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java.util.Scanner• New(er)classforhandlinginput

Ø SinceJava1.5• Manyconstructors

Ø Readfromfile,inputstream,string…

• ManymethodsØ nextXXXX (int,long,line)Ø Skippingpamerns,matchingpamerns,etc.

Scanner sc = new Scanner(System.in);

Scanners• Breaksitsinputintotokensusingadelimiterpamern,whichmatcheswhitespace

• Convertsresul-ngtokensintovaluesofdifferenttypesusingnextXXX()

• Canchangetokendelimiterfromdefaultofwhitespace

• AssumesnumbersareinputasdecimalØ Canspecifyadifferentradix


What is “delimiter pattern”?What is “whitespace”?

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UsingScanners• UsenextXXX() toreadfromit…

long tempLong;

// create the scanner for the consoleScanner sc = new Scanner(System.in);

// read in an integer and a Stringint i = sc.nextInt();String restOfLine = sc.nextLine();

// read in a bunch of long integerswhile (sc.hasNextLong()) {

tempLong = sc.nextLong(); }

UsingScanner


public static void main(String[] args) {

// open the Scanner on the console input, System.inScanner scan = new Scanner(System.in);

scan.useDelimiter("\n"); // breaks up by lines, useful for // console I/O

System.out.print("Please enter the width of a rectangle: ");int width = scan.nextInt();

System.out.print("Please enter the height of a rectangle: ");int length = scan.nextInt();

System.out.println("The area of your square is " + length * width +

".");} ConsoleUsingScannerDemo.java

Simplified version of online example

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Output


This program calculates the area of a rectangle.

Please enter the width of a rectangle (as an integer): the number is 1Incorrect input.Please enter the width of a rectangle (as an integer): 1 2Incorrect input.Please enter the width of a rectangle (as an integer): 2Please enter the height of a rectangle (as an integer): 3The area of your rectangle is 6.

Read in as one token

Scanners&Excep-ons• ScannersdonotthrowIOExceptions!

Ø Forasimpleconsoleprogram,main()doesnothavetodealwithorthrowIOExceptions

Ø RequiredwithBufferedReader/InputStreamReader combina-on

• ThrowsInputMismatchExceptionwhentokendoesn’tmatchpamernforexpectedtypeØ e.g.,nextLong()calledwithnexttoken“AAA”Ø RuntimeException(nocatchingrequired)


How do you prevent such errors?

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Consoleclass• GetaConsoleobjectusingSystem.console()

• Has some useful methods for requesting passwords

•  Issue: does not work through an IDE


ConsoleUsingConsoleDemo.java

STREAMS


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Streams• Javahandlesinput/outputusingstreams,whicharesequencesofbytes

input stream: an object from which we can read a sequence of bytesabstract class: java.io.InputStream


Streams• Javahandlesinput/outputusingstreams,whicharesequencesofbytes

output stream: an object to which we can write a sequence of bytesabstract class: java.io.OutputStream

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JavaStreams• MANY(80+)typesofJavastreams• Injava.io package• Whystreamabstrac-on?

Ø Informa-onstoredindifferentsourcesisaccessedinessen-allythesameway• Examplesources:file,onawebserveracrossthenetwork,string

Ø Allowssamemethodstoreadorwritedata,regardlessofitssource• CreateanInputStreamorOutputStreamoftheappropriatetype



java.io ClassesOverview• Twotypesofstreamclasses,basedondatatype:Byte,Text

• Abstractbaseclassesforbinarydata:

• Abstractbaseclassesfortextdata:

InputStream OutputStream

Reader Writer

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ByteStreams

AbstractBaseClasses

Shaded: Read to/write from data sinksWhite: Does some processing

•  For binary data•  In java.io package


CharacterStreams

AbstractBaseClasses

Shaded: Read to/write from data sinksWhite: Does some processing

• For Text• In java.io package• Handle any character in

Unicode set

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ConsoleI/O• Output:

Ø System.outisaPrintStream object• Input

Ø System.in isanInputStream objectØ Throwsexcep-onsifformatofinputdataisnotcorrect• Handleintry/catch

Opening&ClosingStreams• Streamsareautoma/callyopenedwhenconstructed

• Closeastreambycallingitsclose() methodØ CloseastreamassoonasobjectisdonewithitØ Freeupsystemresources


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Reading&Wri-ngBytes• Abstractparentclass:InputStream

Ø abstract int read()•  readsonebytefromthestreamandreturnsit

• Concreteinputstreamclassesoverrideread()toprovideappropriatefunc-onalityØ e.g.,FileInputStream’s read()readsonebytefromafile

• Similarly,OutputStreamclasshasabstractwrite() towriteabytetothestream



Reading&Wri-ngBytes

• read()andwrite()areblockingopera-onsØ Ifabytecannotbereadfromthestream,themethodwaits(doesnotreturn)un-labyteisread

• available():getthenumberofbytesthatareavailableforreading

• Exampleuse:

int bytesAvailable = System.in.available();if (bytesAvailable > 0)

System.in.read(byteBuffer);

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