Introduction to Java 2 Programming

1
Introduction to Java 2 Programming

• Lecture 7
• IO, Files, and URLs

2
Overview

• Java I/O
• The java.io package
• Streams, Readers and Writers
• Files
• Working with Files
• URLs
• Working with Internet Resources

3
Java I/O The Basics

• Java I/O is based around the concept of a stream
• Ordered sequence of information (bytes) coming
  from a source, or going to a sink
• Simplest stream reads/writes only a single byte,
  or an array of bytes at a time
• Designed to be platform-independent
• The stream concept is very generic
• Can be applied to many different types of I/O
• Files, Network, Memory, Processes, etc

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Java I/O The Basics

• The java.io package contains all of the I/O
  classes.
• Many classes specialised for particular kinds of
  stream operations, e.g. file I/O
• Reading/writing single bytes is quite limited
• So, it includes classes which provide extra
  functionality
• e.g. buffering, reading numbers and Strings (not
  bytes), etc.
• Results in large inheritance hierarchy, with
  separate trees for input and output stream classes

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Java I/O -- InputStream
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Java I/O InputStreams
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Java I/O Using InputStreams

• Basic pattern for I/O programming is as follows
• Open a stream
• While theres data to read
• Process the data
• Close the stream

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Java I/O Using InputStreams

• I/O in Java
• InputStream in new FileInputStream(c\\temp\\my
  file.txt)
• int b in.read()
• //EOF is signalled by read() returning -1
• while (b ! -1)
• //do something
• b in.read()
• in.close()

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Java I/O Using InputStreams

• But using buffering is more efficient, therefore
  we always nest our streams
• InputStream inner new FileInputStream(c\\temp\
  \myfile.txt)
• InputStream in new BufferedInputStream(inner)
• int b in.read()
• //EOF is signalled by read() returning -1
• while (b ! -1)
• //do something
• b in.read()
• in.close()

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Java I/O Using InputStreams

• Weve omitted exception handling in the previous
  examples
• Almost all methods on the I/O classes (including
  constructors) can throw an IOException or a
  subclass.
• Always wrap I/O code in trycatch blocks to
  handle errors.

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Java I/O Using InputStreams

• InputStream in null
• try
• InputStream inner new FileInputStream(c\\tem
  p\\myfile.txt)
• in new BufferedInputStream(inner)
• //process file
• catch (IOException e)
• e.printStackTrace()
• finally
• try in.close() catch (Exception e)

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Java I/O OutputStream
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Java I/O OutputStreams
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Java I/O Using InputStreams

• Basic pattern for output is as follows
• Open a stream
• While theres data to write
• Write the data
• Close the stream

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Java I/O Using OutputStreams

• Output in Java
• OutputStream out new FileOutputStream(c\\temp\
  \myfile.txt)
• while ()
• out.write()
• out.close()

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Java I/O Using OutputStreams

• OutputStream out null
• try
• OutputStream inner new FileOutputStream(c
  \\temp\\myfile.txt)
• out new BufferedOutputStream(inner)
• //write data to the file
• catch (IOException e)
• e.printStackTrace()
• finally
• try out.close() catch (Exception e)

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But Thats Not All!

• Input/OutputStream and sub-classes were part of
  Java 1.1.
• Java 1.2 adds more classes specialised for
  character based I/O
• The stream classes are for data I/O.
• Classes for character I/O are called Readers and
  Writers
• Why have specialised classes?
• To support foreign languages

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Unicode

• Each character in the ASCII character set fits
  into a single byte
• but thats not enough for chinese, and other
  complex alphabets
• Need more than a single byte
• A Java character (char) is 2 bytes
• Java handles text using Unicode
• International standard character set, containing
  characters for almost all known languages
• And a few imaginary ones! (Klingon, Elvish)
• Inside the JVM all text is held as Unicode

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Java Text I/O

• Because byte ! character for all languages, you
  have to turn bytes into chars using a
  Input/OutputStream
• Java provides Readers and Writers to save you
  this work.
• These classes deal with streams of characters
• Read/write single character or array of
  characters
• Again there are classes specialised for
  particular purposes

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Java I/O Reader
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Java I/O Readers
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Using Readers

• Reader in null
• try
• Reader inner new FileReader(c\\temp\\myfile.
  txt)
• in new BufferedReader(inner)
• //process file
• catch (IOException e)
• e.printStackTrace()
• finally
• try in.close() catch (Exception e)

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Java I/O Writer
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Java I/O Writers
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Using Writers

• Writer out null
• try
• Writer inner new FileWriter(c\\temp\\myfile.
  txt)
• out new BufferedWriter(inner)
• //write data to the file
• catch (IOException e)
• e.printStackTrace()
• finally
• try out.close() catch (Exception e)

26
Bridging the Gap

• Sometimes you need to bridge across the two
  hierachies
• Use InputStreamReader or OutputStreamWriter
• InputStreamReader
• Reads bytes from an InputStream, and turns them
  into characters using a character encoding
• OutputStreamWriter
• Turns characters sent to the Writer into bytes
  written by the OutputStream, again using a
  character encoding.

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The File Object

• Java provides access to the file system through
  the java.io.File object
• Represents files and directories
• Has methods for working with files and
  directories
• Making directories, listing directory contents
• renaming and deleting, checking permissions, etc
• Check whether the File corresponds to a directory
  or a file with isDirectory()
• Well-featured, and intuitive
• Take a look through the javadocs
• Quick example

28
The URL Object

• Similar to File is the java.net.URL class
• Provides access to information about website
  addresses
• Most useful is a means to open an InputStream to
  a remote website
• Use the openStream() method
• Makes it very simple to retrieve files from the
  Internet.
• Throws MalformedURLException if you provide an
  illegal internet address in its constructor
• Example

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URL Object Example

• try
• URL p new URL(http//www.ldodds.com/lectures/pe
  rson.jar)
• InputStream inner p.openStream()
• BufferedInputStream in new BufferedInputStream(i
  nner)
• //process the file
• in.close()
• catch (Exception e)
• e.printStackTrace()