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Simple Java I/O

Simple Java I/O. Part I General Principles. Prologue.

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Simple Java I/O

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  1. Simple Java I/O Part I General Principles

  2. Prologue “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, author of Thinking In Javahttp://www.artima.com/intv/aboutme2.html

  3. Streams • 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 end

  4. How to do I/O • import java.io.*; • Open the stream • Use the stream (read, write, or both) • Close the stream

  5. openuseclose Why 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 possibilities

  6. openuseclose Opening 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 stream

  7. openuseclose Example 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 fileReader

  8. openuseclose Using 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 out

  9. openuseclose Example 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( );

  10. openuseclose Manipulating 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);

  11. openuseclose Reading lines • String s;s = bufferedReader.readLine( ); • A BufferedReader will return null if there is nothing more to read

  12. openuseclose Closing • 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 this

  13. Simple Java I/O Part IILineReader and LineWriter

  14. Text files • Text (.txt) files are the simplest kind of files • Text files can be used by many different programs • Formatted text files (such as.docfiles) 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 text

  15. My LineReader class class LineReader { BufferedReader bufferedReader; LineReader(String fileName) {...} String readLine( ) {...} void close( ) {...} }

  16. Basics 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 Exceptions

  17. The 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); }

  18. readLine String readLine( ) { try { return bufferedReader.readLine( ); } catch(IOException e) { e.printStackTrace( ); } return null; }

  19. close void close() { try { bufferedReader.close( ); } catch(IOException e) { } }

  20. How 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 name

  21. The LineWriter class class LineWriter { PrintWriter printWriter; LineWriter(String fileName) {...} void writeLine(String line) {...} void close( ) {...} }

  22. The constructor for LineWriter LineWriter(String fileName) { try { printWriter = new PrintWriter( new FileOutputStream(fileName), true);} catch(Exception e) { System.err.println("LineWriter can't " + "use output file: " + fileName); } }

  23. Flushing the buffer • When you put information into a buffered output stream, it goes into a buffer • The buffer may or may not be written out right away • If your program crashes, you may not know how far it got before it crashed • Flushing the buffer forces the information to be written out

  24. PrintWriter • Buffers are automatically flushed when the program ends normally • Usually it is your responsibility to flush buffers if the program does not end normally • PrintWriter can do the flushing for you • public PrintWriter(OutputStream out, boolean autoFlush)

  25. writeLine void writeLine(String line) { printWriter.println(line); }

  26. close void close( ) { printWriter.flush( ); try { printWriter.close( ); } catch(Exception e) { } }

  27. Simple Java I/O Part III JFileChoosers

  28. About JFileChoosers • The JFileChooser class displays a window from which the user can select a file • The dialog window is modal--the application cannot continue until it is closed • Applets cannot use a JFileChooser, because applets cannot access files

  29. Typical JFileChooser window

  30. JFileChooser constructors • JFileChooser() • Creates a JFileChooser starting from the user’s directory • JFileChooser(File currentDirectory) • Constructs a JFileChooser using the given File as the path • JFileChooser(String currentDirectoryPath) • Constructs a JFileChooser using the given path

  31. Useful JFileChooser methods I • int showOpenDialog(Component enclosingJFrame); • Asks for a file to read; returns a flag (see below) • int showSaveDialog(Component enclosingJFrame); • Asks where to save a file; returns a flag (see below) • Returned flag value may be: • JFileChooser.APPROVE_OPTION • JFileChooser.CANCEL_OPTION • JFileChooser.ERROR_OPTION

  32. Useful JFileChooser methods II • File getSelectedFile() • showOpenDialog and showSaveDialog return a flag telling what happened, but don’t return the selected file • After we return from one of these methods, we have to ask the JFileChooser what file was selected • If we are saving a file, the File may not actually exist yet—that’s OK, we still have a Fileobject we can use

  33. Using a File • Assuming that we have successfully selected a File: • File file = chooser.getSelectedFile();if (file != null) { String fileName = file.getCanonicalPath(); FileReader fileReader = new FileReader(fileName); BufferedReader reader = new BufferedReader(fileReader); } • File file = chooser.getSelectedFile();if (file != null) { String fileName = file.getCanonicalPath(); FileOutputStream stream = new FileOutputStream(fileName);writer = new PrintWriter(stream, true);}

  34. Simple Java I/O Part IV Scanner and printf

  35. java.util.Scanner • Java finally has a fairly simple way to read input • First, you must create a Scanner object • To read from the keyboard (System.in), do: • Scanner scanner = new Scanner(System.in); • To read from a file, do: • File myFile = new File("myFileName.txt");Scanner scanner = new Scanner(myFile); • You have to be prepared to handle a FileNotFound exception • You can even “read” from a String: • Scanner scanner = new Scanner(myString); • This can be handy for parsing a string

  36. Using the Scanner • First, you should make sure there is something to scan • scanner.hasNext()  boolean • You wouldn’t use this when reading from the keyboard • You can read a line at a time • scanner.nextLine() String • Or, you can read one “token” at a time • A token is any sequence of nonwhitespace characters • scanner.next () String • You must be prepared to deal with exceptions • Eclipse will tell you what you need to do • nextLine and next return Strings, which you can convert to numbers or other types if you like • There are also methods to check for and return primitives directly

  37. You can read in and convert text to primitives: boolean b = sc.nextBoolean(); byte by = sc.nextByte(); short sh = sc.nextShort(); int i = sc.nextInt(); long l = sc.nextLong(); float f = sc.nextFloat(); double d = sc.nextDouble(); And test if you have something to read: hasNextBoolean() hasNextByte() hasNextShort() hasNextInt() hasNextLong() hasNextFloat() hasNextDouble() Scanning for primitives

  38. Formatted output • Java 5 has a printf method, similar to that of C • Each format code is % width code • Some format codes are s for strings, d for integers, f for floating point numbers • Example: • double pi = Math.PI;System.out.printf("%8s %-8s %6d %-6d %8f %-8.2f :) \n", "abc", "def", 123, 456, pi, pi);System.out.printf("%8s %-8s %6d %-6d", "abcdef", "ghijkl", 12345, 6789); • Output: abc def 123 456 3.141593 3.14 :) abcdef ghijkl 12345 6789

  39. Simple Java I/O Part V Serialization

  40. Serialization • You can also read and write objects to files • Object I/O goes by the awkward name of serialization • Serialization in other languages can be very difficult, because objects may contain references to other objects • Java makes serialization (almost) easy

  41. Conditions for serializability • If an object is to be serialized: • The class must be declared as public • The class must implement Serializable • The class must have a no-argument constructor • All fields of the class must be serializable: either primitive types or serializable objects

  42. Implementing Serializable • To “implement” an interface means to define all the methods declared by that interface, but... • The Serializable interface does not define any methods! • Question: What possible use is there for an interface that does not declare any methods? • Answer: Serializable is used as flag to tell Java it needs to do extra work with this class

  43. Writing objects to a file • ObjectOutputStream objectOut = new ObjectOutputStream( new BufferedOutputStream( new FileOutputStream(fileName))); • objectOut.writeObject(serializableObject); • objectOut.close( );

  44. Reading objects from a file • ObjectInputStream objectIn = new ObjectInputStream( new BufferedInputStream( new FileInputStream(fileName))); • myObject = (itsType)objectIn.readObject( ); • objectIn.close( );

  45. What have I left out? • Encrypted files, compressed files, files sent over internet connections, ... • Exceptions! All I/O involves Exceptions! • try {statements involving I/O}catch (IOException e) { e.printStackTrace ( );}

  46. The End “There is no reason anyone would want a computer in their home.” --Ken Olson, President/founder of Digital Equipment Corp., 1977 “I think there is a world market for maybe five computers.” --Thomas Watson Chairman of IBM,| 1943

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