IO Basics

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I/O Basics
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I/O

• There are several places to read information
• Hard Drive
• Network
• Keyboard
• There are several places to write information
• Hard Drive
• Network
• Printer or Screen
• When working with I/O several things throw
  exceptions!

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The General Modelfor Input
KeyBoard
Files
The Net
The Keyboard, Files and Network work at the
byte-level!
Convert toCharacters
Convert toStrings
We want to work with Strings, not bytes!
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InputStreams
KeyBoard
Files
The Net

• To read from the Keyboard, start with System.in
• To read from files, start with a FileInputStream
• To read from the network, create a Socket then
  get its InputStream!

System.in
Sockets
FileInputStream
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InputStreams

• All InputStreams have the ability to read
• int read ( )
• Reads the next byte of data from the input
  stream
• int read (byte b)
• Reads some number of bytes from the input
  stream into an array of bytes
• Several different read methods

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Bad ExampleWorking with the Keyboard

• import java.io.
• public class BadExample
• public static void main (String args )
• int userVal 0
• System.out.println (Enter your name)
• // Try reading their name Jeff
• try
• userVal System.in.read( )
• catch (IOException e)
• // Prints out only 74 the ASCII value of
  J
• System.out.println (userVal)

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Bad Example 2Better, but still bad!

• import java.io.
• public class BadExample2
• public static void main (String args )
• int userVal 0
• System.out.println (Enter your name)
• while (userVal ! -1)
• // Try reading their name Jeff
• try
• userVal System.in.read( )
• catch (IOException e)
• // Prints out only 74, 101, 102, 102, 10
• System.out.println (userVal)
• // Note the 10 is from when they hit Enter
  or Return

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The Problem

• We have the ability to read in numbers from the
  user, but want to work with Strings!
• We need to convert the numbers to characters
  (using an InputStreamReader)
• Then, we need to convert the characters into
  Strings (using a BufferedReader)
• This way, we only have to work with the
  BufferedReader, and not System.in or the
  InputStreamReader!

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InputStreamReaders

• To instantiate an InputStreamReader, you need to
  tell it where it is getting its data from!
• In this case, the ISR is getting data from the
  Keyboard (System.in), SO
• InputStreamReader isr
• isr new InputStreamReader (System.in)

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BufferedReaders

• BufferedReaders read a series of characters and
  hold on (buffer) them.
• BufferedReaders have a readLine( ) method! (no
  more working with bytes)
• To instantiate a BufferedReader, you need to tell
  it where it is reading data from
• BufferedReader br
• // Tell br to read from our InputStreamReader
• br new BufferedReader (isr)

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Better Example

• import java.io.
• public class Echo
• public static void main (String args )
• InputStreamReader isr
• BufferedReader br
• String userVal
• System.out.println (Enter your name)
• try
• // Tell isr to read from the Keyboard
• isr new InputStreamReader (System.in)
• // Tell br to read from isr
• br new BufferedReader (isr)
• userVal br.readLine( )
• catch (IOException e)
• // Prints out the users name
• System.out.println (userVal)

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SO

• When we read from br, it reads from isr
• When br reads from isr, isr reads from System.in
  (the Keyboard)
• Basically, a lot of translation is going on!

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Reading from Files

• Work with the FileInputStream class
• Remember, FileInputStream alone works with bytes!
• Uses the exact same model as working with
  Keyboard (still need ISR and BR)
• Create a FileInputStream by telling it the name
  of the file
• FileInputStream fis
• fis new FileInputStream (bob.txt)

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Reading from Files

• import java.io.
• public class More
• public static void main (String args )
• FileInputStream fis
• InputStreamReader isr
• BufferedReader br
• String userVal
• try
• fis new FileInputStream (bob.txt)
• // Tell isr to read from the fis
• isr new InputStreamReader (fis)
• br new BufferedReader (isr)
• userVal br.readLine( )
• while (userVal ! null)
• System.out.println (userVal)
• userVal br.readLine( )
• catch (IOException e)

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The ModelOutput

• Note System.out is a PrintWriter, and is already
  set up at runtime

System.out
FileOutputStream
Socket (network)
PrintWriter
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OutputStreams

• All OutputStreams have the ability to write
• void write (int b)
• Writes the specified byte to this output stream
• void write (byte b)
• Writes b.length bytes from the specified byte
  array to this output stream

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FileOutputStreams

• Opposite of FileInputStream, FOSs write to files!
• To create, pass the name of the file, and whether
  or not to append to the end of the file
• Same problem as System.in and FileInputStream
  works with bytes only!
• FileOutputStream fos
• // Connect to bob.txt and destroy what was there
• fos new FileOutputStream (bob.txt, false)

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The PrintWriter

• PrintWriters have the ability to print Strings
  directly to lower-level classes
• System.out is a PrintWriter that prints to the
  screen!
• Supports both print( ) and println ( ) methods
  (plus several others)
• To create, need to pass it who it will print to
• PrintWriter pw
• pw new PrintWriter (fos)

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Flushing and Closing

• Because it is more efficient to send big block of
  text than individual characters, there can be
  problems.
• Most of the time, you need to tell the
  PrintWriter to flush, meaning send what you
  have
• Quote Dont forget to flush!
• Also, you should close your streams when you are
  finished with them

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Writing to a File

• import java.io.
• public class FileExample
• public static void main (String args )
• FileOutputStream fos
• PrintWriter pw
• try
• // Hook fos up to the file bob.txt
• fos new FileOutputStream (bob.txt)
• // Tell pw to write to fos, which writes to
  bob.txt
• pw new PrintWriter (fos)
• // Write data
• pw.println (Bob was here)
• pw.println (and hes mad)
• // Thanks mom!
• pw.flush( )
• pw.close( )
• catch (IOException e)

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Extra Credit The Poor Mans Web Browser

• Reading a web page from the internet is almost
  the same as reading from a file.
• We create a Socket, which is a connection between
  our machine and another
• Then, get the InputStream (for reading a web
  page) and OutputStream (to send the request for a
  page)
• Need to import java.net.
• To create a Socket, you need to specify a machine
  to hook up to, and a port
• Socket s
• s new Socket (kahuna.clayton.edu, 80)

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• import java.net.
• import java.io.
• public class PoorMansWebBrowser
• public static void main (String args )
• Socket s
• OutputStream os
• InputStream is
• InputStreamReader isr
• BufferedReader br
• PrintWriter pw
• String htmlLine
• try
• s new Socket ("kahuna.clayton.edu",
  80) // Create the socket
• os s.getOutputStream()
  // Get the OutputStream
• is s.getInputStream()
  // Get the InputStream
• isr new
  InputStreamReader (is) // isr writes to the
  net!
• br new BufferedReader
  (isr) // br writes to isr
• pw new PrintWriter
  (os) // pw writes to the net!
• pw.println ("GET
  /index.html HTTP/1.0 \n\n") // send request for
  page

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Summary

• Input always works the same way
• Start out with byte stream (keyboard, files, net)
• Convert bytes into chars (InputStreamReader)
• Convert chars into Strings (BufferedReader)
• Output always works the same way
• Start with byte stream (files or net)
• Print Strings to that stream (PrintWriter)

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