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File input and output

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File input and output Interactive vs. file I/O All of the programs we have seen or written thus far have assumed interaction with a user, who types in data at a ... – PowerPoint PPT presentation

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Title: File input and output


1
File input and output
2
Interactive vs. file I/O
  • All of the programs we have seen or written thus
    far have assumed interaction with a user, who
    types in data at a keyboard and sees output on a
    screen
  • We can also receive data from, or send data to,
    the secondary storage devices on the computer
  • This type of input and output is called file I/O

3
File I/O
  • The action of saving, or writing, data to a file
    is called file output.
  • The action of reading data from a file is called
    file input.
  • Both of these actions involve interacting with
    the operating system and its file structure

4
The file concept
  • We can envision a file as a special kind of
    container that can hold a collection of data or
    instructions
  • A file can also contain a collection of other
    files such a file is called a directory or
    folder
  • An important point to remember is that Java
    considers a folder to be just another file

5
File system organization
  • Under Windows and most other modern operating
    systems, information is stored on disk in a
    hierarchically ordered structure consisting of
    directories, subdirectories, and files
  • Disks are designated using letters for example,
    C is typically the computers hard drive (or its
    primary partition), while A is traditionally a
    floppy disk
  • The Windows GUI portrays directories as folders,
    which can contain other folders or files

6
Windows file system example
The illustration below depicts a typical file
system structure under Windows
7
Opening a file for input or output
  • Before we can read data from a file, we must
    create a File object and associate it to the file
    from which we wish to read.
  • When a valid association is established, we say a
    file is opened.
  • A file must be opened before we can do any input
    and output to the file.
  • We do this by calling a File constructor

8
Opening a file
  • The simplest syntax for opening a file is
  • File aFile new File (filename)
  • The String argument designates the name of the
    file (as seen by the operating system)
  • This syntax assumes that the argument is the name
    of a file (not a folder), and that the file is
    located in the same folder as the programs code
    file

9
Opening a file
  • Slightly more complicated syntax is necessary if
    the file to be opened is in a different folder
  • File aFile new File (path, file)
  • The first argument specifies the disk and file
    system hierarchy that contains the file
  • For example, if the file is in directory
    G\shared\cate\cs1, you would write
  • File aFile new File (g/shared/cate/cs1,
    file)
  • Note the use of the forward slash you can use
    either this format or
  • File aFile new File (g\\shared\\cate\\cs1,
    file)

10
Opening a file
  • A File object may also be associated to a
    directory.
  • As far as Java is concerned, a directory is just
    a special kind of file
  • The code on the next slide illustrates the use of
    a File object to open a directory, then list the
    names of all the files in the directory

11
Example
File directory new File (C/JavaPrograms/Ch12)
String filename directory.list() //
Returns an array of strings naming the files and
// directories in the directory denoted by this
pathname for (int i0 iltfilename.length
i) System.out.println(filenamei)
12
File methods
  • The previous slide illustrated the use of the
    list() method, which returns an array of Strings
    containing the names of files within a directory
  • Several other useful methods of the File class
    are listed in the Java API we will examine a few
    of these

13
File methods
  • We can check if a File object is associated
    correctly to an existing file by calling its
    exists method
  • if (inFile.exists())
  • // inFile is associated correctly to an
    existing file
  • else
  • // inFile is not associated to any existing
    file
  • It is particularly useful to check input files to
    see if they actually exist before we try to read
    from them

14
File methods
  • To determine if a File object is associated to a
    file or directory, we call its boolean method
    isFile.
  • A similar method, isDirectory, returns true if
    the File object is associated with a folder
    instead of an ordinary file.

15
Choosing a file from a list
  • A javax.swing.JFileChooser object allows the user
    to select a file from a list of available files.
  • The code fragment below illustrates how this is
    done
  • JFileChooser chooser new JFileChooser()
  • ...
  • chooser.showOpenDialog(null)

16
A JFileChooser object displayed with the
showOpenDialog method
17
JFileChooser Object methods
  • The getSelectedFile method retrieves the desired
    file.
  • The getName and getAbsolutePath methods retrieve
    the name and full path of a selected file
    (similar methods exist for File objects).
  • The showSaveDialog method displays a JFileChooser
    with a Save button.

18
A JFileChooser object displayed with the
showSaveDialog method
19
JFileChooser Example
  • The next couple of slides display a program that
    uses JFileChooser objects to display a list of
    files, then allow the user to choose which file
    should be open for output
  • Note the use of File and JFileChooser methods in
    the code

20
  • import java.io.
  • import javax.swing.
  • public class TestJFileChooser
  • public static void main (String args)
  • JFileChooser chooser
  • File file, directory
  • int status
  • chooser new JFileChooser( )
  • status chooser.showOpenDialog(null)
  • if (status JFileChooser.APPROVE_OPTION)
  • file chooser.getSelectedFile()
  • directory chooser.getCurrentDirectory()
  • System.out.println("Directory "
    directory.getName())
  • System.out.println("File selected to
    open " file.getName())
  • System.out.println("Full path name "
    file.getAbsolutePath())
  • else
  • JOptionPane.showMessageDialog(null, "Open
    File dialog canceled")

21
  • System.out.println("\n\n")
  • status chooser.showSaveDialog(null)
  • if (status JFileChooser.APPROVE_OPTION)
  • file chooser.getSelectedFile()
  • directory chooser.getCurrentDirectory()
  • System.out.println("Directory "
    directory.getName())
  • System.out.println("File selected for saving
    data " file.getName())
  • System.out.println("Full path name "
    file.getAbsolutePath())
  • else
  • JOptionPane.showMessageDialog(null, "Save
    File dialog canceled")

22
Sample output from running the TestJFileChooser
program
23
Reading and Writing Files
  • Once a file is opened by associating a File
    object to it, we can access the file.
  • To read data from or write data to a file, we
    must create one of the Java stream objects and
    attach it to the file.
  • A stream is a sequence of data items, usually
    8-bit bytes.
  • Java has two types of streams an input stream
    and an output stream.

24
Reading and Writing Files
  • An input stream has a source from which the data
    items come, and an output stream has a
    destination to which the data items are going.
  • FileOutputStream and FileInputStream are two
    stream objects that facilitate file access.
  • FileOutputStream allows us to output a sequence
    of bytes values of data type byte.

25
Writing Data to a File
  • To send output to a file, do the following
  • Create and open a File object
  • File output new File(C/datafiles, stuff)
  • Associate a new FileOutputStream object with the
    File object
  • FileOutputStream outs new FileOutputStream(outpu
    t)
  • Write the desired data to the FileOutputStream
    object
  • byte snacks 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
  • outs.write(snacks)
  • Close the File stream
  • outs.close()

26
The importance of closing a file
  • If the stream object is not closed, then data may
    get lost due to data caching
  • Data is saved in blocks of bytes to reduce the
    time it takes to save all of our data.
  • The operation of saving data as a block is called
    data caching.
  • To carry out data caching, part of memory is
    reserved as a data buffer or cache, which is used
    as a temporary holding place.

27
The importance of closing a file
  • Data are first written to a buffer. When the
    buffer becomes full, the data in the buffer are
    actually written to a file.
  • If there are any remaining data in the buffer and
    the file is not closed, those data will be lost.

28
Reading Data from a File
  • To read data into a program, we reverse most of
    the steps in the output routine.
  • We use the read method of FileInputStream to read
    in an array of bytes.
  • First we create a FileInputStream object
  • File inFile new File(sample1.data)
  • FileInputStream inStream new FileInputStream(inF
    ile)
  • Then we must declare and create an array to hold
    the data
  • int filesize (int) inFile.length()
  • byte byteArray new bytefilesize

29
Reading Data from a File
  • We use the length method of the File class to
    determine the size of the file.
  • This allows us to create an array of bytes whose
    size is the size of the file.
  • Finally, we read the data into the array of
    bytes
  • inStream.read(byteArray)

30
Low-level File I/O
  • The read and write methods of the FileInputStream
    and FileOutputStream classes read and write
    entire arrays of bytes, as we have seen
  • We can output data other than bytes if we can
    type cast them into bytes.
  • To read the data back, we use the read method.
  • Depending on the data type we converted the data
    from, we may need to type cast back into the
    original data type.

31
High-level File I/O
  • Using DataOutputStream allows us to output Java
    primitive data type values by converting the
    values into a sequence of bytes.
  • The argument to the DataOutputStream constructor
    is a FileOutputStream object.
  • A DataOutputStream object does not get connected
    to a file directly instead, its role is to
    provide high-level access to a file by handling
    the data type conversions.

32
Relationship between File and output Stream
objects
33
High-Level File I/O
  • To read data back from the file, we reverse the
    operation.
  • We use three objects File, FileInputStream, and
    DataInputStream.
  • Data must be read in the order in which it was
    written otherwise, the results will be
    unpredictable.

34
Relationship between File object and input Stream
objects
35
High-Level File I/O
  • FileOutputStream and DataOutputStream objects
    produce a binary file in which the contents are
    stored in the format (binary format) in which
    they are stored in the main memory.
  • Data may be stored in ASCII format instead of
    binary format this requires the use of a
    different set of objects

36
High-Level File I/O
  • With ASCII format, all data are converted to
    string data.
  • A file whose contents are stored in ASCII format
    is called a text file.
  • To generate a textfile we use a PrintWriter is an
    object .
  • PrintWriter supports only two output methods,
    which should look familiar
  • print
  • println (for print line)

37
High-Level File I/O
  • An argument to the methods may be any primitive
    data type.
  • The methods convert the parameter to string and
    output this string value.
  • The constructor of PrintWriter, like that of
    DataOutputStream, requires an output stream as
    its argument.

38
High-Level File I/O
  • To read data from a text file, we use the
    FileReader and BufferedReader objects.
  • We first associate a BufferedReader object to a
    file.
  • File inFile new File(sample3.data)
  • FileReader fileReader new FileReader(inFile)
  • BufferedReader brdr new BufferedReader(fileRea
    der)
  • Then we read data, using the readLine method of
    BufferedReader.
  • String str bufReader.readLine()
  • Finally, we convert the String to a primitive
    data type as necessary (using wrapper classes as
    weve done before).

39
File I/O and Objects
  • To write objects to a file, we use
    ObjectOutputStream.
  • To read objects from a file, we use
    ObjectInputStream.

40
File I/O and Objects
  • In this example, we will write Person objects to
    a file.
  • The first step is to modify the Person class
    definition to allow ObjectOutputStream and
    ObjectInputStream to perform object I/O.
  • We modify the definition by adding the phrase
    implements Serializable to it.
  • import java.io.
  • class Person implements Serializable
  • // the rest is the same

41
File I/O and Objects
  • To save objects to a file, we first create an
    ObjectOutputStream object
  • File outFile new File(objects.dat)
  • FileOutputStream outFileStream new
    FileOutputStream(outFile)
  • ObjectOutputStream outObjectStream new
    ObjectOutputStream(outFileStream)

42
File I/O and Objects
  • To save a Person object, we execute
  • Person person new Person(Mr. Man, 20, M)
  • outObjectStream.writeObject (person)
  • Different types of objects may be saved to a
    single file.
  • We can also mix objects and primitive data type
    values in the same file.

43
File I/O and Objects
  • To read objects from a file, we use
    FileInputStream and ObjectInputStream.
  • We use the method readObject to read an object.
  • Because we can store any types of objects to a
    single file, we must type cast the object read
    from the file.

44
File I/O and Objects
  • The readObject method can throw a
    ClassNotFoundException (wrong type casting) in
    addition to an IOException.
  • Either exception may be caught or propagated.
  • If a file contains objects from different
    classes, they must be read in the correct order
    and the matching type casting must be applied.
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