Two Ways to Store Data in a File

1
Two Ways to Store Data in a File

• Text format
• Binary format

2
Text Format

• Information stored as a sequence of characters
• Characters are stored as their ASCII equivalent
  - int value 12345 stored as 1 2 3 4
  5
  
• 5 bytes 91 92
  93 94 95
• A text file is readable by humans.
• Java classes Reader and Writer (and their
  subclasses) are for use with text files.

3
Text I/O

• In Java, characters are stored using UNICODE.
• Reader objects read characters in the current
  O.S. format, and convert them to UNICODE storage
  format.
• Writer objects convert the UNICODE stored data to
  character encoding used by the current O.S.
• Reader and Writer are abstract classes

4
Text Input
READER class is an abstract class which is
extended by InputStreamReader which is


extended by FileReader extended by
BufferedReader
Reader class provides abstract methods which will
be implemented specifically by all
subclasses InputStreamReader object reads bytes
from an input stream and decodes them into
characters, using the platforms charset (ASCII
for Windows). These characters are
converted to UNICODE for storage.
read() reads one character (number of bytes
depends on charset) FileReader object inherits
InputStreamReader capabilities, but the stream
is a file. BufferedReader object is a
wrapper for any InputStreamReader (including
FileReader), and provides buffering (ie. handles
multiple char reads and provides a string)
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Text one char at a time

• Create a InputStreamReader object
• Constructor takes an input stream, which
  may
• be keyboard, network connection, any
  input source
• InputStreamReader in new
  InputStreamReader(System.in)
• InputStreamReader extends the Reader class,
  implementing input methods specifically for files
• Use its read method to read a single character
• returns the next char as an int
• or the integer -1 at end of input
• Test for -1 to determine if a char was read

6
Reading one char at a time.

• InputStreamReader reader new
• InputStreamReader(System.in)
• char c //Java chars are UNICODE
• int next reader.read() //byte read and
• if (next ! -1)
• c (char)next()
• // process c
• next reader.read()
• //See demo testISReader.java

7
Reading one line at a time.

• InputStreamReader in new
• InputStreamReader(System.in)//input
  is from keyboard
• BufferedReader bin new BufferedReader(in)
• String val
• val bin.readLine() // get one string from
  keyboard
• while(val ! null) //while user does not
  enter CTRL Z
• System.out.println( val )
• System.out.flush()
  //output any lines waiting to be printed
• val bin.readLine()
• //See demo testBISReader.java

8
Text one char at a time from a file

• Create a FileReader object
• FileReader extends the Reader class, implementing
  input methods specifically for files
• Use its read method to read a single character
• returns the next char as an int
• or the integer -1 at end of input
• Test for -1 to determine if a char was read
• Close the file when done

9
Reading one char at a time from file.

• FileReader reader new
• FileReader("input.txt")
• char c //Java chars are UNICODE
• int next reader.read() //byte read and
• if (next ! -1)
• c (char)next()
• next reader.read()
• reader.close()
• // see testFileReader.java

10
Reading Text Line by Line
Create a BufferedReader object (pass a
FileReader object to constructor) objects of
type BufferedReader can group characters
buffer them method readLine() available, to
provide file data 1 line at a time (the method
handles reading the characters from the
FileReader for you) readLine() returns the next
line of file (as a String), or null if none exists
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//Reads first line from file named input.txt //
line is expected to contain a double
value FileReader reader new FileReader("input.t
xt") BufferedReader in new
BufferedReader(reader) String inputLine
in.readLine() double x Double.parseDouble(inp
utLine)
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//Reads and all lines from file // and writes
them to console import java.io public class
demo public static void main(String
args)throw IOException FileReader reader
new FileReader("input.txt") BufferedReader in
new BufferedReader(reader) String line
in.readLine() While (line ! null)
System.out.println(line) line
in.readLine() in.close()
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Text Output
WRITER class is an abstract class which is
extended by OutputStreamWriter which is


extended by FileWriter extended by
PrintWriter
Writer class provides abstract methods which will
be implemented specifically by all
subclasses OutputStreamWriter object writes
bytes to an output stream and encodes them
into characters, using the platforms charset
(ASCII for Windows). These characters are
converted from stored UNICODE
write(char) writes one character (number of bytes
written depends on charset) Object
is buffered, may be flushed FileWriter object is
inherits OutputStreamWriter capabilities, but the
stream is a file PrintWriter object is a
wrapper for any OutputStreamWriter, and provides
buffering (ie. handles multiple char
writes and provides a string)
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Write a Character to File
FileWriter class implements Writer methods
specifically for files. FileWriter writer
new FileWriter("output.txt")
char c a' writer.write(c)
writer.close()
15
Writing Strings to Text Files

• A PrintWriter object handles the unbuffering
  of data for output file writer
• Create PrintWriter object (pass FileWriter
  object to constructor)
• PrintWriter class provides println method
  which accepts String and uses FileWriter to print
  one char at a time.
• FileWriter writer new FileWriter(output.txt)
• PrintWriter out new PrintWriter(writer)

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• //use PrintWriter object to output data to file
  output.txt
• FileWriter writer new FileWriter(output.txt)
• PrintWriter out new PrintWriter(writer)
• out.println(29.95)
• out.println(new Rectangle(5,10,15,25))
• out.println("Hello, World!")

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StringTokenizer Class

• When reading a line of text, we get a single long
  string. Suppose our line of text looked
  something like
• JohnDoe161998
• In other words, the string contained know
  delimiters, and we wanted to access the
  pieces between these delimiters.
• Java.util package provides a class to help here
  StringTokenizer
• Methods StringTokenizer(String theline,
  String delimiters)
• boolean hasMoreTokens()
• String nextToken()
• int countTokens()
• see file CountTHE.java

18
Binary Format

• More compact and efficient
• int 12345 stored using binary representation
• 00000000 00000000 0010000 0011100
• 00 00 48
  57
• 4 bytes
• Java abstract classes InputStream and
  OutputStream (and their subclasses) provide
  methods for reading and writing these types of
  files

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Binary File I/O

• In Java, an object from which we can read a
  sequence of bytes is called an input stream.
• An object to which we can write a sequence of
  bytes is called an output stream.
• I/O streams have beginning, end, and are read
  sequentially.
• InputStream and OutputStream are abstract
  classes.

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• InputStream class has an abstract method
• abstract int read()
• // reads and returns one byte
• An abstract method is an method which MUST be
  implemented by any extending class. The idea is
  that the extending class provides specifics
• OutputStream class has an abstract method
• abstract void write(int b)
• // writes one byte to output
• Java provides many stream classes which extend
  from InputStream and OutputStream
• that let you work with data in the forms that
• you normally use.

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Byte Streams
InputStream is extended by
FileInputStream which is used for byte based
input from a
file OutputStream is extended by
FileOutputStream which is used for byte based
output to
a file Both of these classes implement read and
write methods as specified in their abstract
super classes. see file ByteIO.java
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Input Byte Streams
InputStream is extended by FilterInputStream
FilterInputStream is extended
by DataInputStream
FilterInputStream acts as a wrapper for an
InputStream objects, which it uses as its basic
source of data. Methods of FilterInputStream
objects simply pass requests to the InputStream
objetct. Each subclass of FilterInputStream
transforms the data along the way, or provides
some additional functionality. For example,
DataInputStream wraps a InputStream object and
assembles bytes read into numerical types.
readInt, readDouble, readBoolean are just some of
the methods available with a DataInputStream
object See file DataByteIO.java
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Input Byte Streams
InputStream is extended by FilterInputStream
FilterInputStream is extended
by BufferedInputStream
Another subclass of FilterInputStream is
BufferedInputStream. BufferedInputStream
wraps a InputStream object and provides
buffering for efficiency. BufferedInputStream
overloads read so that in addition to one byte
at a time, one byte array can be read.
// buffered
input BufferedInputStream bin new
BufferedInputStream(new InputStream(file.txt))
// add another layer so that
DataInputStream object will not do individual
reads // for each byte, but use the
buffered reads DataInputStream dbin new
DataInputStream(bin)
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Output Byte Streams
FilterOutputStream extends OutputStream
(Subclasses may be superclasses themselves)
PrintStream extends FilterOutputStream
FilterOutputStream acts as a wrapper for an
OutputStream object, which it uses as its basic
depository of data. Methods of
FilterOutputStream simply pass requests to the
OutputStream objetct. PrintStream wraps a
OutputStream and provides an accurate display of
data types to the output, in addition to
buffering. PrintStream also provides the
overloaded println, which accepts String and
byte parameters, as well as byte. System.out,
is an object of type PrintStream, which is
provided by the System class in the java.lang
input. System.out wraps the OutputStream
object representins the console.
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• FILE OBJECTS provide methods which allow the
  creation, deletion, and other file
  manipulation/status methods
• FileInputStream and FileReader constructors are
  overloaded to accept a File object.
• The file object will allow you to check the
  status of a file before you open it!!
• File f new File(in.txt)
• if ( ! f.exists() )
• System.out.println(file does not exist)
• else if ( ! F.canRead() )
• System.out.println(file cannot be read)
• else
• FileInputStream in new FileInputStream(f)
  
• BufferedInputStream bufIn new
  BufferedInputStream(in)
• int bb bufIn.read()

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A JFileChooser Dialog
27
File Dialogs

• Use JFileChooser to let a user supply a file name
  through a file dialog
• Construct a file chooser object
• Call its showOpenDialog or showSaveDialog method
• (Specify null or the user interface
  component over which to pop up the dialog )
• If the user chooses a fileJFileChooser.APPROVE_O
  PTION is returned
• If the user cancels the selectionJFileChooser.CA
  NCEL_OPTION is returned
• If a file is chosen, use GetSelectedFile method
  to obtain a File object describing the file

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Code to Use a JFileChooser
JFileChooser chooser new JFileChooser()
FileReader in if (chooser.showOpenDialog(null)
JFileChooser.APPROVE_OPTION)
File selectedFile chooser.getSelectedFile()
in new FileReader(selectedFile) else
in null // see file testDialog.java
29
Mark and Reset
The abstract classes InputStream and Reader
provide void mark (int byteLimit)
Marks the current position in this input
stream. A subsequent call to the reset method
repositions this stream at the last marked
position so that subsequent reads re-read the
same bytes. byteLimit is the number of
bytes that can be read before this mark becomes
invalid void reset() repositions
stream to position when mark was last
called boolean markSupported () indicates if a
stream supports these methods
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int intList intList
new int100
//this file contains one number 66
FileReader file new FileReader("number.txt"
) BufferedReader ifile new
BufferedReader(file)
// all array elements are
initialized to 66 ifile.mark(5)
for (int i 0 ilt100 i)
String sVal ifile.readLine()
ifile.reset() intListi
Integer.parseInt(sVal)
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Object Output

• Another byte (binary) I/O class which extends
  from OutputStream is ObjectOutputStream
• ObjectOutputStream class can save entire objects
  to disk !
• Objects that are written to an object stream must
  belong to a class that implements the
  Serializable interface.
• class Coin implements Serializable
• ...
• Serializable interface has no methods.

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Serializable

• Objects that are written to an object stream must
  belong to a class that implements the
  Serializable interface.
• class Coin implements Serializable
• ...
• Serializable interface has no methods.

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ObjectOutputStream methods
Stream does not just write objects .. write( )
- writes a byte writeInt(int) - writes a 32 bit
int writeDouble(double) - writes a 64 bit
double writeChar(char) - writes a 16 bit
char writeObject(Object) - writes the specified
object to the output
stream (if Serializable) close() - closes
stream
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Writing an Object to a File
OutputStream os new FileOutputStream("data.
txt") ObjectOutputStream out new
ObjectOutputStream(os)
ArrayListltBoxgt boxlist new ArrayListltBoxgt ()
boxlist.add(new Box(5))
boxlist.add(new Box(17)) boxlist.add(new
Box(20)) out.writeObject(boxlist)
out.close()
35
Object Input

• Another byte (binary) I/O class which extends
  from InputStream is
• ObjectInputStream
• ObjectInputStream class can read from a file,
  written by ObjectOutputStream
• (Data must by read with respect to datatype and
  order with which it was written)

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ObjectInputStream methods
read( ) - reads a byte readInt() - reads a 32
bit int readDouble() - reads a 64 bit
double readChar() - reads a 16 bit
char readObject( ) - reads the specified object
to the input stream (if
Serializable) close() - closes stream
37
Reading from File containing Objects
public static void main(String args) throws
IOException,ClassNotFoundException
InputStream is new FileInputStream("data.txt")
ObjectInputStream in new
ObjectInputStream(is)
ArrayListltBoxgt boxlist (ArrayListltBoxgt)
in.readObject() for (int i 0
ilt boxlist.size() i)
System.out.println(boxlist.get(i))
in.close()
38
Random .vs. Sequential Access

• Sequential access
• A file is processed a byte at a time.
• Random access
• Allows access at arbitrary locations in the
  file