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Penn CIT 591 - Simple Java

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Simple Java I/OPrologueStreamsHow to do I/OWhy Java I/O is hardOpening a streamExample of opening a streamUsing a streamExample of using a streamManipulating the input dataReading linesClosingSlide 13Text filesMy LineReader classBasics of the LineReader constructorThe full LineReader constructorreadLinecloseHow did I figure that out?The LineWriter classThe constructor for LineWriterFlushing the bufferPrintWriterwriteLineSlide 26Slide 27About JFileChoosersTypical JFileChooser windowJFileChooser constructorsUseful JFileChooser methods IUseful JFileChooser methods IIUsing a FileSlide 34SerializationConditions for serializabilityImplementing SerializableWriting objects to a fileReading objects from a fileWhat have I left out?The EndJan 14, 2019Simple Java I/OPart IGeneral Principles2Prologue“They say you can hold seven plus or minus two pieces of information in your mind. I can’t remember how to open files in Java. I’ve written chapters on it. I’ve done it a bunch of times, but it’s too many steps. And when I actually analyze it, I realize these are just silly design decisions that they made. Even if they insisted on using the Decorator pattern in java.io, they should have had a convenience constructor for opening files simply. Because we open files all the time, but nobody can remember how. It is too much information to hold in your mind.”—Bruce Eckel, http://www.artima.com/intv/aboutme2.html3StreamsAll modern I/O is stream-basedA stream is a connection to a source of data or to a destination for data (sometimes both)An input stream may be associated with the keyboardAn input stream or an output stream may be associated with a file Different streams have different characteristics:A file has a definite length, and therefore an endKeyboard input has no specific end4How to do I/O import java.io.*;Open the streamUse the stream (read, write, or both)Close the stream5Why Java I/O is hardJava I/O is very powerful, with an overwhelming number of optionsAny given kind of I/O is not particularly difficultThe trick is to find your way through the maze of possibilitiesopenuseclose6Opening a streamThere is data external to your program that you want to get, or you want to put data somewhere outside your programWhen you open a stream, you are making a connection to that external placeOnce the connection is made, you forget about the external place and just use the streamopenuseclose7Example of opening a streamA FileReader is a used to connect to a file that will be used for input: FileReader fileReader = new FileReader(fileName);The fileName specifies where the (external) file is to be foundYou never use fileName again; instead, you use fileReaderopenuseclose8Using a streamSome streams can be used only for input, others only for output, still others for bothUsing a stream means doing input from it or output to itBut it’s not usually that simple--you need to manipulate the data in some way as it comes in or goes outopenuseclose9Example of using a stream int charAsInt;charAsInt = fileReader.read( );The fileReader.read() method reads one character and returns it as an integer, or -1 if there are no more characters to readThe meaning of the integer depends on the file encoding (ASCII, Unicode, other)You can cast from int to char: char ch = (char)fileReader.read( );openuseclose10Manipulating the input dataReading characters as integers isn’t usually what you want to doA BufferedReader will convert integers to characters; it can also read whole linesThe constructor for BufferedReader takes a FileReader parameter: BufferedReader bufferedReader = new BufferedReader(fileReader);openuseclose11Reading lines String s;s = bufferedReader.readLine( );A BufferedReader will return null if there is nothing more to readopenuseclose12ClosingA stream is an expensive resourceThere is a limit on the number of streams that you can have open at one timeYou should not have more than one stream open on the same fileYou must close a stream before you can open it againAlways close your streams!Java will normally close your streams for you when your program ends, but it isn’t good style to depend on thisopenusecloseJan 14, 2019Simple Java I/OPart IILineReader and LineWriter14Text filesText (.txt) files are the simplest kind of filesText files can be used by many different programsFormatted text files (such as .doc files) also contain binary formatting informationOnly programs that “know the secret code” can make sense of formatted text filesCompilers, in general, work only with text15My LineReader classclass LineReader { BufferedReader bufferedReader; LineReader(String fileName) {...} String readLine( ) {...} void close( ) {...}}16Basics of the LineReader constructorCreate a FileReader for the named file: FileReader fileReader = new FileReader(fileName);Use it as input to a BufferedReader: BufferedReader bufferedReader = new BufferedReader(fileReader);Use the BufferedReader; but first, we need to catch possible Exceptions17The full LineReader constructor LineReader(String fileName) { FileReader fileReader = null; try { fileReader = new FileReader(fileName); } catch (FileNotFoundException e) { System.err.println ("LineReader can't find input file: " + fileName); e.printStackTrace( ); } bufferedReader = new BufferedReader(fileReader); }18readLineString readLine( ) { try { return bufferedReader.readLine( ); } catch(IOException e) { e.printStackTrace( ); } return null;}19closevoid close() { try { bufferedReader.close( ); } catch(IOException e) { }}20How did I figure that out?I wanted to read lines from a fileI thought there might be a suitable readSomething method, so I went to the API IndexNote: Capital letters are all alphabetized before lowercase in the IndexI found a readLine method in several classes; the most promising was the BufferedReader classThe constructor for BufferedReader takes a Reader as an argumentReader is an abstract class, but it has several implementations, including InputStreamReaderFileReader is a subclass of InputStreamReaderThere is a constructor for FileReader that takes as its argument a (String) file name21The LineWriter classclass LineWriter { PrintWriter printWriter; LineWriter(String fileName) {...} void writeLine(String line) {...} void close( )


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Penn CIT 591 - Simple Java

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