STANDARD ERROR - Computer Science Department ...sprenkle/cs209/pre_class/11-streams.pdf• Example:...

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Objec&ves• StandardError• Streams

Ø ByteStreamsØ TextStreams

Oct 5, 2016 Sprenkle - CSCI209 1

STANDARDERROR


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StandardStreams• Preconnectedstreams

Ø StandardOut:stdoutØ StandardIn:stdinØ StandardError:stderr

• Forerrormessagesanddiagnos&cs•  InJava:System.err


Benefits of two output streams (out and err)?

Redirec&ngOutput• Recallearlierthissemester

Ø Redirectedstdout toscore.outØ stderr woulds&llgototerminal

• Toredirectstderrtofileaswell


> java OlympicScore > score.out

> java OlympicScore >& score.out

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STREAMS



Streams• Javahandlesinput/outputusingstreams,whicharesequencesofbytes

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

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Streams• Javahandlesinput/outputusingstreams,whicharesequencesofbytes

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


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

• Abstractbaseclassesforbinarydata:

• Abstractbaseclassesfortextdata:

InputStream OutputStream

Reader Writer

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ByteStreams

AbstractBaseClasses

Shaded: Read to/write from data sinks White: 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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STREAMS



FileInputandOutputStreams• FileInputStream:providesaninputstreamthatcanreadfromafileØ Constructortakesthenameofthefile:

Ø Or,usesaFileobject…

FileInputStream fin = new FileInputStream("chicken.data");

File inputFile = new File("chicken.data");FileInputStream fin = new FileInputStream(inputFile);

FileTest.java

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MorePowerfulStreamObjects• DataInputStream

Ø ReadsJavaprimi&vetypesthroughmethodssuchasreadDouble(), readChar(), readBoolean()

• DataOutputStreamØ WritesJavaprimi&vetypeswithwriteDouble(), writeChar(), …


ConnectedStreams

• FileInputStreamcanreadfromafilebuthasnomethodstoreadnumerictypes• DataInputStreamcanreadnumerictypesbuthasnomethodstoreadfromafile

• JavaallowsyoutocombinetwotypesofstreamsintoaconnectedstreamØ FileInputStreamàchocolateØ DataInputStreamàpeanutbuPer


Our goal: read numbers from a file

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ConnectedStreams

• Thinkofastreamasapipe• FileInputStream knowshowtoreadfromafile• DataInputStreamknowshowtoreadanInputStreamintousefultypes

• ConnectoutendofFileInputStreamtoinendofDataInputStream…

FileInputStream DataInputStreamdouble

charfile

Data Source stream Filtered Stream

Reads from a stream


Connec&ngStreams•  Ifwewanttoreadnumbersfromafile

Ø FileInputStreamreadsbytesfromfileØ DataInputStreamhandlesnumerictypereading

• ConnecttheDataInputStreamtotheFileInputStreamØ FileInputStreamgetsthebytesfromthefileandDataInputStreamreadsthemasassembledtypes

FileInputStream fin = new FileInputStream(“chicken.data”);

DataInputStream din = new DataInputStream(fin);

double num1 = din.readDouble();“wrap” fin in din

DataIODemo.java

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DataSourcevs.FilteredStreams

FilteredStreams•  Subclasses of FilterInputStream or FilterOutputStream

•  Always contains another stream

•  Adds functionality to other streamØ  Automatically buffered IO Ø  Automatic compressionØ  Automatic encryption Ø  Automatic conversion

between objects and bytes

DataSourceStreams

• Communicate with a data source Ø  file, byte array,

network socket, or URL



BufferedStreams• UseaBufferedInputStreamobjecttobufferyourinputstreamsØ ApipeinthechainthataddsbufferingØ SpeedsupaccessDataInputStream din = new DataInputStream ( new BufferedInputStream ( new FileInputStream("chicken.data")));

FileInputStreamdouble

charfile BufferedInputStream

What functionality does each stream add?

DataInputStream

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ConnectedStreams

• Whatarethetradeoffsforthisdesigndecision?

Combine different types of streamsto get functionality you want


ConnectedStreams

• Crea&ngaclassforeveryclasswouldresultinevenmoreclassesandalotofredundantcodeØ Considerwhatisrequiredifsomefunc&onalitymustbeupdated


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ConnectedStreams:Output

• SimilarforoutputØ Forbufferedoutputtothefileandtowritetypes• CreateaFileOutputStream• APachaBufferedOutputStream• APachaDataOutputStream• Performtypedwri&ngusingmethodsoftheDataOutputStreamobject


TEXTSTREAMS


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TextStreams• Previousstreams:operateonbinarydata,nottext

• JavausesUnicodetorepresentcharacters/stringsandsomeopera&ngsystemsdonotØ NeedsomethingthatconvertscharactersfromUnicodetowhateverencodingtheunderlyingopera&ngsystemuses

Ø Luckily,thisismostlyhiddenfromyou



CharacterStreams

AbstractBaseClasses

Shaded: Read to/write from data sinks White: Does some processing

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

in Unicode set

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TextStreams• DerivedfromReaderandWriterclasses

Ø ReaderandWritergenerallyrefertotextI/O• Example:MakeaninputreaderoftypeInputStreamReader thatreadsfromkeyboard

Ø in readscharactersfromkeyboardandconvertsthemintoUnicodeforJava


InputStreamReader in = new InputStreamReader(System.in);


TextStreamsandEncodings• APachanInputStreamReadertoaFileInputStream

Ø AssumesfilehasbeenencodedinthedefaultencodingofunderlyingOS

• YoucanspecifyadifferentencodinginconstructorofInputStreamReader…

InputStreamReader in = new InputStreamReader(new FileInputStream("employee.data"));

InputStreamReader in = new InputStreamReader(new FileInputStream("employee.data"), "UTF-8");

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ConvenienceClasses• Readingandwri&ngtotextfilesiscommon• FileReader

Ø ConvenienceclasscombinesaInputStreamReaderwithaFileInputStream

• Similarforoutputoftextfile

isequivalenttoFileWriter out = new FileWriter("output.txt");

OutputStreamWriter out = new OutputStreamWriter(new FileOutputStream("output.txt"));

PrintWriter• Useforwri&ngtextoutput

Ø Easiestwritertouse• SimilartoaDataOutputStream, PrintStream à has methods for printing various data types

• Methods: print,printfandprintlnØ SimilartoSystem.out(aPrintStream)todisplaystrings


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PrintWriter Example

PrintWriter out = new PrintWriter("output.txt");

String myName = "Homer Simpson";double mySalary = 35700;

out.print(myName);out.print(" makes ");out.print(salary);out.println(" per year.");

orout.println(myName + " makes " + salary +

" per year.");

File to write to

Review:FormaPedOutput• printforformat

Ø PrintStreamnewfunc&onalitysinceJava1.5


double f1=3.14159, f2=1.45, total=9.43;// simple formatting...System.out.printf("%6.5f and %5.2f", f1, f2);// getting fancy (%n = \n or \r\n)...System.out.printf("%-6s%5.2f%\n", "Tax:", total);

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PrintWritersandBuffering• PrintWritersarealwaysbuffered• Op&on:autoflushmode

Ø Causesanywritestobeexecuteddirectlyontargetdes&na&on•  Ineffect,defeatsthepurposeofbuffering

Ø Constructorwithsecondparametersettotrue

// create an autoflushing PrintWriterPrintWriter out = new PrintWriter("output.txt",

true);

ReadingTextfromaStream• ThereisnoPrintReaderclass• UseaBufferedReader

Ø ConstructorrequiresaReaderobject

• Readfile,line-by-lineusingreadLine()Ø ReadsinalineoftextandreturnsitasaStringØ Returnsnullwhennomoreinputisavailable


String line;while ((line = in.readLine()) != null) {

// process the line}

BufferedReader in = new BufferedReader(new FileReader("inputfile.txt"));

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ReadingTextfromaStream• YoucanalsoaPachaBufferedReadertoanInputStreamReader:

• Usedtobethebestwaytoreadfromtheconsole

BufferedReader consoleReader= new BufferedReader(new InputStreamReader(System.in));

BufferedReader webpageReader = new BufferedReader( new InputStreamReader(url.openStream());

Note how easy it is to read �from different sources

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