I/O in Java

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I/O in Java

• Dennis Burford
• dburford_at_cs.uct.ac.za

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Input and Output

• Obtaining input
• Using BasicIo methods.
• Through a GUI
• Producing output
• Using System.out methods (System.out.println())
• Through a GUI
• Other input/output files, sockets etc.

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Streams

• Java uses the concept of Streamsfor I/O
• Data flows from a Writer to a Reader

Stream
Reader
Writer
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Why Streams?

• Stream generalizes input output
• Keyboard electronics different from disk
• Input stream makes keyboard and files seem the
  same to a Reader

Input Stream
Reader
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Simplified Keyboard Input

• Java First Contact uses BasicIo class
• String name BasicIo.readString()
• int age BasicIo.readInteger()
• Slack textbook uses Keyboard class
• String name Keyboard.readString()
• int age Keyboard.readInt()

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Simplified Keyboard Input

• Both BasicIo and Keyboard are wrapper classes
  used to hide detail.
• In reality, both classes use System.in and other
  low-level Java classes to operate.

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System class

• System class is inside java.lang package
• Contains 2 variables
• public static final InputStream in
• public static final PrintStream out
• System.in is usually connected to the users
  keyboard.
• System.out is usually connected to the users
  screen.

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Using System.in

• System.in is an InputStream
• Can use System.in many ways
• Directly (low-level access)
• Through layers of abstraction (high-level access)

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Using System.in

• read() method reads one character at a time.
• Returns an int
• Returns 1 for EOF (End of File Ctrl-Z)

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Using System.in

• public static void main(String args)
• throws java.io.IOException
• char character
• // Prompt for a character and read it
• System.out.print("Enter a character ")
• System.out.flush()
• character (char) System.in.read()
• // Display the character typed
• System.out.println()
• System.out.println("You typed " character)

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Using System.in

• int intChar System.in.read()
• while (intChar ! -1)
• // Convert to character
• char character (char) intChar
• System.out.println("Next character is "
  character)
• // Get next one
• intChar System.in.read()

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Reading Strings

• No String-reading methods in System.in
• To read strings from keyboard, first wrap
  System.in inside InputStreamReader object
• InputStreamReader ISReader
• new InputStreamReader(System.in)

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Reading Strings
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Reading Strings

• Next, wrap InputStreamReader object in
  BufferedReader object
• InputStreamReader ISReader
• new InputStreamReader(System.in)
• BufferedReader BufReader
• new BufferedReader(ISReader)

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Reading Strings

• Can combine these into a single statement

BufferedReader BufReader new
BufferedReader(new InputStreamReader(System.in))
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Reading Strings

• Can combine these into a single statement

BufferedReader BufReader new
BufferedReader(new InputStreamReader(System.in))
InputStream
InputStreamReader
BufferedReader
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Reading Strings

• Methods in BufferedReader
• read()
• readLine()
• Read a string by using readLine() method
• String str BufReader.readLine()

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Reading Strings
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Reading Strings

• InputStreamReader, BufferedReader in java.io.
• Skeleton for reading
• import java.io.
• class ClassName
• public static void main(String args)
• throws java.io.IOException
• // Create a buffered input stream and attach
  it to standard
• // input
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))
• ...

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Example Reading Users Name

• // Reads a user's first name and last name.
• // Demonstrates use of InputStreamReader,
• // BufferedReader and the readLine() method.
• import java.io.
• class ReadInputAsString
• public static void main(String args)
• throws java.io.IOException
• String firstName, lastName
• // Create an input stream and attach it to
  the standard
• // input stream
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))

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Example Reading Users Name

• // Read a line from the user as a String
• System.out.print("Enter your first name ")
• System.out.flush()
• firstName BufReader.readLine()
• // Read a line from the user as a String
• System.out.print("Enter your last name ")
• System.out.flush()
• lastName BufReader.readLine()
• // Display the strings
• System.out.println()
• System.out.println("Your name is" firstName
  lastName)

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Example Reading Users Name

• // Read a line from the user as a String
• System.out.print("Enter your first name ")
• System.out.flush()
• firstName BufReader.readLine()
• // Read a line from the user as a String
• System.out.print("Enter your last name ")
• System.out.flush()
• lastName BufReader.readLine()
• // Display the strings
• System.out.println()
• System.out.println("Your name is" firstName
  lastName)

Enter your first name Linda Enter your last
name Jones Your name is Linda Jones
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Formatting Numbers

• Alternative to Integer.parseInt() etc.
• NumberFormat
• Object factory use getInstance()
• parse() takes a string and returns a Number
• parse() throws java.text.ParseException
• General Number class
• Return value using intValue(), doubleValue() etc.

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Formatting Numbers from Kbd

• NumberFormat formatter NumberFormat.getInstance(
  )
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))
• System.out.print("Enter an integer ")
• System.out.flush()
• String response BufReader.readLine()
• Number numberObj formatter.parse(response)
• int numberInt numberObj.intValue()

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Formatting Numbers from Kbd

• NumberFormat formatter NumberFormat.getInstance(
  )
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))
• System.out.print("Enter an integer ")
• System.out.flush()
• String response BufReader.readLine()
• Number numberObj formatter.parse(response)
• int numberInt numberObj.intValue()

Combine
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Formatting Numbers from Kbd

• NumberFormat formatter NumberFormat.getInstance(
  )
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))
• System.out.print("Enter an integer ")
• System.out.flush()
• int numberInt
• formatter.parse( BufReader.readLine()
  ).intValue()

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Formatting Numbers from Kbd

• NumberFormat formatter NumberFormat.getInstance(
  )
• BufferedReader BufReader
• new BufferedReader(new InputStreamReader(S
  ystem.in))
• System.out.print("Enter an integer ")
• System.out.flush()
• int numberInt
• formatter.parse( BufReader.readLine()
  ).intValue()

Dont forget to throw or catch java.text.ParseExce
ption
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Input Streams Multiple Values

• To read several values on one line
• Use StringTokenizer object
• Breaks one string into component parts
• Must still convert numbers (if necessary)
• StringTokenizer
• Constructor takes string
• nextToken() method
• hasMoreTokens() method

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Input Streams Multiple Values

• StringTokenizer tokenizer new
  StringTokenizer("42 58")
• String token
• token tokenizer.nextToken()
• System.out.println(token) // Displays 42
• token tokenizer.nextToken()
• System.out.println(token) // Displays 58
• BETTER
• StringTokenizer tokenizer new
  StringTokenizer("42 58")
• while (tokenizer.hasMoreTokens())
• System.out.println(tokenizer.nextToken())

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FILES
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Overview of Files

• How they are accessed
• sequential data items must be accessed in the
  order in which they are stored (ie. start at the
  beginning and pass through all the items)
• direct (or random) items are accessed by
  specifying their location
• How information is represented
• text files data is stored in character form.
• binary files data is stored in internal binary
  form (faster and more compact than text files).

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Types of Files
Summer.txt
Rough winds do shake the darling buds of
May\n And Summers lease hath all too short a
date\n Sometime too hot the eye of heaven
shines,\n And oft is his gold complexion
dimmed,\n And every fair from fair sometime
declines...
Numbers.dat
Œpôw10Žélú9câÜ(3xLenfˆx¹ª(Ͻ¼øßµ