CS 424524: Introduction to Java Programming

1
CS 424/524 Introduction to Java Programming

• Lecture 13
• Spring 2002
• Department of Computer Science
• University of Alabama
• Don Yessick

2
Overview

• I/O Reading and Writing
• Using the Streams (EOF examples)
• Object Serialization
• Random Access Files
• Zip Files
• Java.Text (formatting numbers)
• Parsing Numbers from Strings

3
Streams

• Sequential Input and Output
• Pattern for stream processing
• open stream
• while more data
• read/write stream
• close stream
• IO classes have tools to supports these
  algorithms

4
Two types of streams

• Character streams
• Reader Writer model
• read() readln()
• print() println()
• (ascii / 16-bit (unicode) characters)

• Byte streams
• IOstream model
• readDouble() etc
• write() etc
• (binary 8 bit bytes only)

5
Reader vs InputStream

• Reader (unicode)
• int read()
• int read(char cbuf)
• int read(char cbuf, int offset, int length)
• InputStream (bytes)
• int read()
• int read(byte cbuf)
• int read(byte cbuf, int offset, int length)
• both provide methods for marking a location in
  the stream, skipping input, and resetting the
  current position.

6
Writer vs OutputStream

• Writer (unicode)
• int write(int c)
• int write(char cbuf)
• int write(char cbuf, int offset, int length)
• OutputStream (bytes)
• int write(int c)
• int write(byte cbuf)
• int write(byte cbuf, int offset, int length)

7
A Reader/Writer Example (EOF)

• import java.io.
• public class Copy
• public static void main(String args) throws
  IOException
• File inputFile new File(original.txt")
• File outputFile new File(copy.txt")
• FileReader in new FileReader(inputFile)
• FileWriter out new FileWriter(outputFile)
  
• int c
• while ((c in.read()) ! -1) //EOF
  condition test
• out.write(c)
• in.close()
• out.close() //best to close (flushes any
  buffers)

8
Byte Example (EOF)

• import java.io.
• public class CopyBytes
• public static void main(String args) throws
  IOException
• File inputFile new File("farrago.txt")
• File outputFile new File("outagain.txt")
  
• FileInputStream in new
  FileInputStream(inputFile)
• FileOutputStream out new
  FileOutputStream(outputFile)
• int c
• while ((c in.read()) ! -1) //EOF
  condition test
• out.write(c)
• in.close()
• out.close()

9
Encoding/Internationalization

• FileReader and FileWriter read and write 16-bit
  characters.
• These streams encode characters according to the
  default character-encoding scheme. (Most native
  file systems are based on 8-bit bytes.)
• System.getProperty("file.encoding").
• To specify an encoding, construct an
  OutputStreamWriter on a FileOutputStream and
  specify the encoding.

10
Working with Filter Streams

• Filters data being read from or written to a
  stream
• Some streams buffer data, some count data, others
  convert data to another form.
• DataInputStream DataOutputStream
• BufferedInputStream BufferedOutputStream
• LineNumberInputStream
• PushbackInputStream (Character based)
• PrintStream (This is an output stream.)

11
DataInputStream DataOutputStream

• Conceptually, data looks like this, although in
  binary form (non-ASCII)
• 19.99 12 Java T-shirt
• 9.99 8 Java Mug
• The data might have been created with the
  following
• DataOutputStream out
• new DataOutputStream( new FileOutputStream("invoi
  ce1.txt"))
• for (int i 0 i lt prices.length i )
• out.writeDouble(pricesi)
  out.writeChar('\t')
• out.writeInt(unitsi) out.writeChar('\t')
  
• out.writeChars(descsi) out.writeChar('\n')
  
• out.close()

12

• DataInputStream in
• new DataInputStream( new FileInputStream("invoice
  1.txt"))
• try //exception raised at EOF
• while (true) char chr
• price in.readDouble() in.readChar()
  //throws out the tab
• unit in.readInt() in.readChar() //throws
  out the tab
• desc new StringBuffer(20)
• char lineSep System.getProperty("line.separ
  ator").charAt(0)
• while ((chr in.readChar() ! lineSep) //end
  of line (carriage return linefeed)
• desc.append(chr)
• System.out.println("You've ordered " unit "
  units of " desc
• " at " price)
• total total unit price
• catch (EOFException e)
• System.out.println("For a TOTAL of " total)
• in.close()

13
ObjectOutputStream (Serialization)

• FileOutputStream out
• new FileOutputStream("theTime")
• ObjectOutputStream s
• new ObjectOutputStream(out)
• s.writeObject("Today")
• s.writeObject(new Date())
• s.flush()
• The writeObject method throws a
  NotSerializableException if it's given an object
  that is not serializable. An object is
  serializable only if its class implements the
  Serializable interface.

14
ObjectInputStream (Deserialization)

• FileInputStream in
• new FileInputStream("theTime")
• ObjectInputStream s
• new ObjectInputStream(in)
• String today (String)s.readObject()
• Date date (Date)s.readObject()
• ObjectInputStream stream implements the DataInput
  interface that defines methods for reading
  primitive data types

15
The Serializable Interface (majik)

• public class MySerializableClass implements
  Serializable ...
• Optional
• private void writeObject(ObjectOutputStream s)
  throws IOException
• s.defaultWriteObject()
• // customized serialization code
• private void readObject(ObjectInputStream s)
  throws IOException
• s.defaultReadObject()
• // customized deserialization code ...
• // followed by code to update the object, if
  necessary
• See also Externalizable Interface (for total
  control)

16
Random Access Files

• new RandomAccessFile(data.txt", "rw")
• int skipBytes(int) --Moves the file pointer
  forward the specified number of bytes
• void seek(long) --Positions the file pointer just
  before the specified byte
• long getFilePointer() --Returns the current byte
  location of the file pointer
• The RandomAccessFile class implements both the
  DataInput and DataOutput interfaces
• Does not extend stream class
• No support for Serialized Objects

17
java.util.zip package

• CheckedInputStream CheckedOutputStream
• An input and output stream pair that maintains a
  checksum as the data is being read or written.
• DeflaterOutputStream InflaterInputStream
• Compresses or uncompresses the data as it is
  being read or written.
• GZIPInputStream GZIPOutputStream
• Reads and writes compressed data in the GZIP
  format.
• ZipInputStream ZipOutputStream
• Reads and writes compressed data in the ZIP
  format

18
Summary of IO

• The java.io package contains classes to read and
  write data. Most implement sequential access
  streams. Some read and write bytes and some read
  and write Unicode characters. Each has a
  speciality, such as reading from or writing to a
  file, filtering data, or serializing objects.
• RandomAccessFile, implements random input/output
  access to a file. An object of this type
  maintains a file pointer indicating the current
  location from which data will be read or written.
  
• java.util.zip package contains other useful I/O
  classes.

19
Package java.text

• Classes and interfaces for handling text, dates,
  numbers, and messages independently of natural
  languages.
• Applications and applets can be
  language-independent, relying upon separate,
  dynamically-linked localized resources and
  allowing for new localizations at any time.
• Capable of formatting dates, numbers, and
  messages, parsing searching and sorting strings
  and iterating over characters, words, sentences,
  and line breaks.
• Three main groups of classes and interfaces
• Classes for iteration over text
• Classes for formatting and parsing
• Classes for string collation

20
DateFormat

• DateFormat df DateFormat.getDateInstance()
• for (int i 0 i lt a.length i)
• output.println(df.format(myDatei) " ")
• To format a number for a different Locale,
  specify it in the call to getDateInstance().
• DateFormat df DateFormat.getDateInstance(Locale.
  FRANCE)
• You can use a DateFormat to parse also.
• myDate df.parse(myString)

21
NumberFormat

• myString NumberFormat.getInstance().format(myNum
  ber)
• Or
• NumberFormat nf NumberFormat.getInstance()
• for (int i 0 i lt a.length i)
• output.println(nf.format(myNumberi) " ")
  
• Or
• NumberFormat nf NumberFormat.getInstance(Locale.
  FRENCH)
• Parsing
• myNumber nf.parse(myString)

22
More Number Formats

• getCurrencyInstance
• getPercentInstance
• DecimalNumberFormat
• Also useful
• setMinimumFractionDigits

23
Alignment

• No simple method.
• Monospaced font
• examine string length and pad with spaces
• Proportional font
• start counting pixels.
• If Decimal numbers.

24
Parsing Strings to Numbers

• From class Integer
• static int parseInt(String s)
• Parses the string argument as a signed decimal
  integer.
• static int parseInt(String s, int radix)
• Parses the string argument as a signed integer in
  the radix specified by the second argument.
• From class Float
• static float parseFloat(String s)
• Returns a new float initialized to the value
  represented by the specified String, as performed
  by the valueOf method of class Double.

25
Parsing Differences

• Formatted Strings
• use appropriate Format methods
• 1,024,512.64
• 2,345
• vs Unformatted Strings
• use static methods of appropriate class
• 1024512.64
• 2345

26
Reading Assignment

• Next Topic Threads
• Chapter 15
• Chapter 6.1