CS 162 Spring 2004

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CS 162 - Spring 2004

• Chapter 15
• Streams

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Streams versus Files

• Streams are an abstraction that generalizes the
  idea of files
• A stream is simply a sequence of characters
• Could be from a file,
• A network connection
• A buffer,
• Or someplace else

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Binary versus Text Format

• Files are traditionally thought of as in one of
  two different format
• Binary format. Just a string of uninterpreted
  bits (or bytes, which are generally more
  efficient to access)
• Text files. Storage is same, but the
  interpretation of bits as is text

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Storing 12,345

• The value 12,345 stored as binary would be
• 00 00 48 57 (48 256 57 12,345)
• Stored as text would be
• 49 50 51 52 53
• Which is the ASCII representation of each
  character

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Unicode

• Java has generalized the traditional view of the
  char data type
• An 8-bit char (as in C, C) is fine for Roman
  alphabets, not for rest of world (Chinese, Thai,
  etc)
• The 16-bit Unicode character is a newer more
  international standard

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Files Streams and Readers

• Java I/O is actually composed of two related
  libraries
• The Stream library is used to read 8-bit
  character values. Used for binary I/O
• The Reader/Writer library is used to work with 16
  bit character values. Used for text I/O. Layered
  on top of the Stream library.

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Reading from a stream

• FileInputStream inputStream new
  FileInputStream("input.dat")
• Int next inputStream.read()
• byte b
• If (next ! -1)
• b (byte) next

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Reading from Reader

• FileReader reader new FileReader("input.dat")
• Int next reader.read()
• Char c
• If (next ! -1)
• c (char) next

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Writing/Closing a file

• FileWriter writer new FileWriter("out.put")
• Char c '!'
• Writer.write( c )
• Writer.close() // important, makes changes

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Wrappers

• File streams and reader/writers provide only the
  basic facility for reading and writing a single
  character.
• Other useful functionality is provided by
  wrappers - classes that wrap around basic files
  and provide new methods

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Printing

• An example wrapper, PrintWriters
• PrinterWriters provide a number of useful
  multi-character printing routines, I.e. print and
  println, also conversion from primitives
• FileWriter writer new FileWriter("out.put")
• PrintWriter out new PrintWriter(writer)
• Out.println("Hello world") out.println(42)

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Another wrapper, BufferedReader

• Another useful wrapper, BufferedReader provides
  the method readLine
• FileReader fread new FileeReader("in.txt")
• BufferedReader in new BufferedReadeer(fred)
• String inputLine in.readLine()

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Readers are actually Wrappers

• In fact, readers/writers are actually wrappers
  around files. FileReader constructor is short for
• File f new File("raw.data")
• FileReader fr new FileReader(f)

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Input Stream Hierarchy

• InputStream
• ByteArrayInputStream
• FileInputStream
• FilterInputStream
• DataInputStream (e.g., System.in)
• BufferedInputStream (reads an array more
  efficiently)
• LineNumberInputStream (keeps track of line
  numbers)
• PushbackInputStream (can unread characters)
• PipedInputStream (can connect to piped output
  stream)
• ObjectInputStream
• SequenceInputStream

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Output Stream Hierarchy

• OutputStream
• ByteArrayOutputStream
• FileOutputStream
• FilterOutputStream
• DataOutputStream
• BufferedOutputStream
• PrintStream (e.g. System.out)
• PipedOutputStream
• ObjectOutputStream

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Reader Hierarchy (there's a similar one for
Writer)

• Reader
• BufferedReader
• LineNumberReader
• CharArrayReader
• InputStreamReader
• FileReader
• FilterReader
• PushbackReader
• PipedReader
• StringReader

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File Dialogs

• If you have a graphical application there is an
  easy way to create a file dialog
• JFilChooser chooser new JFileChooser()
• FileReader in null
• If (chooser.showOpenDialog(null)
  JFileChooser.APPROVE_OPTION)
• File selectedFile chooser.getSelectedFile()
• in new FileReader(selectedFile)

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Object Streams

• Complex objects are often difficult to store
• They have internal state
• Two names can reference the same value
• They mix primitive/object values
• Writing your own code to store/read such values
  can be tedious and error prone.

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Writing Objects

• Java makes storing even complex objects almost
  trivial
• Declare objects as Serializable
• Write to an ObjectOutputStream or read from an
  objectInputStream
• Notice this is stream (binary I/O) not text files

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Writing to a Stream

• Coin c
• ObjectOutputStream out new ObjectOutputStream(ne
  w FileOuputStream("coins.dat")
• Out.writeObject( c )
• It's that simple.

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Reading an Object

• ObjectInputStream in new ObjectInputStream(new
  FileInputStream("in.put"))
• Coin c (Coin) in.readObject()
• Its's that simple. Note Can throw cast exception.

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Questions?

• Using Files and Readers in Assignment 3.
• Read Chapter 16 and Chapter 3 from my book Intro
  to OOP (from course webpage).