IO Streams

1
I/O Streams

• A stream is a sequence of bytes that flow from a
  source to a destination
• In a program, we read data from an input stream
  and write information to an output stream
• A program can manage multiple streams at a time
• The java.io package contains many classes that
  allow us to define various streams with specific
  characteristics

2
I/O Stream Categories

• The classes in the I/O package divide input and
  output streams into other categories
• An I/O stream is either a
• character stream, which deals with text data
• byte stream, which deal with byte data
• An I/O stream is also either a
• data stream, which acts as either a source or
  destination
• processing stream, which alters or manages data
  in the stream

3
Standard I/O

• There are three standard I/O streams
• standard input defined by System.in
• standard output defined by System.out
• standard error defined by System.err
• We use System.out when we execute println
  statements
• System.in is declared to be a generic InputStream
  reference, and therefore usually must be mapped
  to a more useful stream with specific
  characteristics

4
The Keyboard Class

• The Keyboard class was written to facilitate
  reading data from standard input
• Now we can examine the processing of the Keyboard
  class in more detail
• The Keyboard class
• declares a useful standard input stream
• handles exceptions that may be thrown
• parses input lines into separate values
• converts input stings into the expected type
• handles conversion problems

5
The Standard Input Stream

• The Keyboard class declares the following input
  stream
• InputStreamReader isr
• new InputStreamReader (System.in)
• BufferedReader stdin new BufferedReader (isr)
• The InputStreamReader object converts the
  original byte stream into a character stream
• The BufferedReader object allows us to use the
  readLine method to get an entire line of input

6
Text Files

• To write data to a text file we need to create an
  object of class PrintWriter. The class includes
  two methods print and println
• To read data from a text file, we need to create
  two objects, one of class FileReader, and the
  other object of class BufferedReader.

7
Input Text Files

• Data can be read from and written to text files
  by declaring and using the correct I/O streams
• The FileReader class represents an input file
  containing character data
• To create an object of this class we need to pass
  the name of the input text file as argument
• FileReader infile new FileReader
  (myfile.data)

8
Method with Input Text File

• Import java.io.
• public class Ioerror
• public static void main (String args)
• try
• FileReader ff new FileReader
  (indata.txt)
• catch (IOException e)
• e.printStackTrace()
• // end main
• // end class

9
Reading Lines and Fields

• Reading one character at a time from an I/O
  device is very inefficient
• Normally an entire block of data is read at a
  time (1024 bytes) storing in a buffer
• Class BufferedReader is used for Buffered
  reading
• FileReader infile new FileReader
  (myfile.data)
• BufferedReader bbin BufferedReader(infile)
• This allows reading a block of data into a buffer
  

10
Reading Data into a Buffer

• Import java.io.
• public class Ioerror
• public static void main (String args)
• String line
• try
• FileReader ff new FileReader
  (indata.txt)
• BufferedReader fbuf new BufferedReader
  (ff)
• while (line fbuf.readLine()) ! null) //
  read until EOF
• System.out.println(line)
• fbuf.close()
• catch (IOException e)
• e.printStackTrace()

11
Separating Fields

• public static void main (String args)
• String line, street, city, state, zip
• StringTokenizer fields
• Address taddress
• try
• FileReader ff new FileReader
  (indata.txt)
• BufferedReader fbuf new BufferedReader
  (ff)
• while (line fbuf.readLine()) ! null)
• System.out.println(line)
• fields new StringTokenizer (line, )
• if (fields.countTokens() 4)
• street fields.nextToken()
• city fields.nextToken()
• state fields.nextToken()
• zip fields.nextToken()
• taddress new Address (street, city, state,
  zip)
• System.out.println(taddress)
• // end while

12
Writing to a Text File

• We first create an object of class FileWriter
  passing the name of the external file as argument
• Create a PrintWriter object passing the previous
  object as an argument
• FileWriter outf new FileWriter (myoutf.txt)
• PrintWriter p new PrintWriter (outf, true)

13
Writing Data to a Text File

• In a loop
• p.write(item )
• p.writeln(y)
• After the loop
• p.close()

14
Binary File Manipulation

• To open a file, we create a File object, to
  associate it to the file
• File mydatafile new File (mydat1.data)
• if (mydatafile.isFile)
• System.out.println(File Ok)
• else
• System.out.println(Its a directory)

15
Read/Write File Operations

• For R/W file operations, we need to open an input
  stream and an output stream objects
• Two such Java streams are FileInputstream and
  FileOutputStream
• To attach streams to files ofile and ifile
• FileOutputStream ostr
• FileInputStream istr
• ostr new FileOutputStream (ofile)
• istr new FileInputStream(ifile)

16
Data To and From a Stream

• Data is written directly to an output stream,
  which has been attached to a file
• ostr.write(myarray) // write byte array
• . . .
• ostr.close()
• Data is read from an input stream, which has been
  attached to a file
• istr.read(yarray)
• istr.close()

17
High-Level File Operations

• To facilitate the conversion of primitive data
  types
• A Datastream object is a layer on top of a
  FileStream object
• To create a DataOutputStream object, we first
  create a file object then a FileOutputStream
  object

18
Writing Values of Primitive Types

• To write an integer and a float value to a file
• File ofile new File (myofile.dat)
• FileOutputStream ostr
• ostr new FileOutputStream (ofile)
• DataOutputStream dostr
• dostr new DataOutputStream (dostr)
• dostr.writeInt (87) // an integer
  value
• dostr.writeFloat(1123.5F) // a float value

19
Reading Values of Primitive Types

• To read an integer and a float values from the
  file
• File ifile new File (myofile.dat)
• FileInputStream istr int intval float floatval
• istr new FileInputStream (ifile)
• DataInputStream distr
• distr new DataInputStream (istr)
• distr.readInt (intval) // an integer
  value
• distr.readFloat(floatval) // a float value

20
Writing to a Textfile
File tfile new File (otfile.dat) FileOutputSt
ream ostr ostr new FileOutputStream
(tfile) PrintWriter postr postr new
PrintWriter (ostr) dostr.println (87)
// an integer value dostr.println(1123.5F) // a
float value dostr.close() //
close file
21
Reading From a Textfile

• File ifile new File (otfile.dat)
• FileReader fhr new FileReader (ifile)
• BufferedReader bufr
• bufr new BufferedReader (fhr)
• // Read text data
• String tdata bufr.readline()
• // the string data may need to be converted to
  their primitive types (use a wrapper object)

22
Object Serialization

• Object serialization is the act of saving an
  object, and its current state, so that it can be
  used again in another program
• The idea that an object can live beyond the
  program that created it is called persistence
• Object serialization is accomplished using the
  classes ObjectOutputStream and ObjectInputStream
• Serialization takes into account any other
  objects that are referenced by an object being
  serialized, saving them too

23
Object Serialization

• The process of converting an object into a
  representation of 8-bits units.
• It is the key to providing object persistency,
  the ability to save an objects state across
  program invocation.
• This concept is essential in all network
  programming.

24
Object Persistency

• A serialized object can be stored in an 8-bit
  form (I.e., on a file), and read back and
  restored to its exact representation.
• Internal state of the object must be restored
• Internal references must also be restored.
• Classes ObjectOutputStream and ObjectInputStream
  and the Serialization interface hide most of
  these details.

25
Object I/O in Java

• Objects can be stored directly to a file
• Two streams are used
• ObjectInputStream to read objects from a file
• ObjectOutputStream to write objects to a file.
• For object I/O, the interface Serializable must
  be implemented in the class that defines the
  object

26
Using Serializable

• Consider an object of class Sched is to read or
  stored on a file.
• import java.io.
• class Sched implements Serializable
• // class members

27
Reading Objects from a File

• Carry out the following steps
• Declare and create a File object
• Declare and create a FileInputStream object
• Declare and create an ObjectInputStream object
• Invoke the readObject() method of the
  ObjectInputStream object
• Cast the input object to the appropriate object
  type

28
Example of Reading Object

• File myfile new File (myobj.dat)
• FileInputStream myistream
• myistream new FileInputStream (myfile)
• ObjectInputStream myobjstr
• myobjstr new ObjectInputStream (myistream)
• Sched mysched // object to read
• mysched (Sched) myobjstr.readObject()

29
Writing Objects to a File

• To write an object carry out the following steps
• Declare and create a File object
• Declare and create a FileOutputStream object
• Declare and create an ObjectOutputStream object
• Invoke the writeObject() method of the
  ObjectOutputStream object

30
Example of Writing an Object

• File myfile new File (myobj.dat)
• FileOutputStream myostream
• myostream new FileOutputStream (myfile)
• ObjectOutputStream myobjstr
• myobjstr
• new ObjectOutputStream (myostream)
• Sched mysched new Sched()
• myobjstr.writeObject (mysched)