Files and Steams

1
Files and Steams

• MINS 116
• Spring 2000
• Java Basics

2
Topics

• Files
• Files in Java
• Streams
• Exceptions
• URLs

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Files

• All computer operation boils down to binary.
• 0 - The circuit is open
• 1 - The circuit is closed

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Files

• All information handled by computers is
  represented in binary.
• Numbers
• Strings of information
• Images
• Java code

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Files

• Information is only viable while the computer has
  power.
• To store information beyond the operation of the
  computer, we use some form of secondary storage.

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Files

• With secondary storage, the streams of 1s and 0s
  are put on some non-volatile medium.
• Tape (paper with punched holes, magnetic)
• Disk (magnetic, optical)
• Others?

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Files, Streams and Java

• Java supports files through package java.io.
• java.io.File represents a file.
• A Stream must be associated with a Java file to
  extract information.
• Java system reserved files are System.in,
  System.out, System.err

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Files, Streams and Java

• Example
• // Open a file
• File exampleFile new File( testFile.txt)
• // Associate a stream
• FileInputStream inputStream new
  FileInputStream( exampleFile )

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Files, Streams and Java

• To correctly process a file, you need to have
  knowledge about the structure of the information
  contained in the file.
• Example
• If you open a file and read a char, but
  originally written is an integer, what results
  would you expect?

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File Types

• Sequential
• Sequential access of records in file
• Open for Read or Write access
• Random
• Fixed length records
• Wasted disk space
• Binary
• Byte stream
• A lot of program responsibility

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File Types

• In Java, File represents the file as located in
  secondary storage
• Streams, Readers, and Writers represent the
  access for reading and writing of the file.

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Stream/File Types in Java

• Sequential - all files in Java start as
  sequential.
• Random Access - class RandomAccessFile
• Binary - class ByteArrayInputStream

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Readers and Writers

• Readers - Reader provides for reading of
  character streams. Specialized readers are
• BufferedReader
• LineNumberReader
• CharArrayReader
• InputStreamReader
• FileReader
• FilterReader
• PushbackReader
• PipedReader
• StringReader

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Readers and Writers

• Writers - Writer provides for writing of
  character streams. Specialized writers are
• BufferedWriter
• CharArrayWriter
• OutputStreamWriter
• FileWriter
• FilterWriter
• PipedWriter
• StringWriter

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Streams - Input and Output

• Streams provide for inputting and outputting
  streams of bytes as opposed to the character
  strings of writers.

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Streams - Input and Output

• InputStreams
• BufferedInputStream
• ByteArrayInputStream
• DataInputStream
• FileInputStream
• FilterInputStream
• LineNumberInputStream
• ObjectInputStream
• PipedInputStream
• PushbackInputStream
• SequenceInputStream
• StringBufferInputStream

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Streams - Input and Output

• OutputStreams
• BufferedOutputStream
• ByteArrayOutputStream
• DataOutputStream
• FileOutputStream
• FilterOutputStream
• ObjectOutputStream
• PipedOutputStream
• Why more input streams than output streams?

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A brief word on exceptions...

• To deal with files and streams in Java,
  exceptions will nee to be handled.
• An exception is an asynchronous error event which
  will disrupt the flow of processing.

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A brief word on exceptions...

• Exceptions are handled in a try-catch block.
• try
• // do something which may throw an
  exception
• catch ( Exception exp )
• // handle the error condition
• ...

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A brief word on exceptions...

• If a method has the capability to throw an
  exception, your source code must either
• 1) Catch the exception in a catch block.
• 2) Throw the exception itself.

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A brief word on exceptions...

• Example (try-catch)
• File myFile null
• String path null
• try
• myFile new File ( path )
• // Begin the processing
• ...
• catch ( NullPointerException exp )
• // We end up here if since path is null

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A brief word on exceptions...

• Example (throwing)
• public void myMethod ( )
• throws NullPointerException
• File myFile null
• String path null
• myFile new File ( path )
• // Begin the processing
• ...
• Processing will never begin since new File( path
  ) will cause an exception to be thrown.

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URLs

• U - Uniform
• R - Resource
• L - Locator

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URLs

• Anybody who has used the web has used URLs.
• E.g. http//www.ecst.csuchico.edu/jlong

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URLs

• A URL provides a standardized method for locating
  resources over the web.
• URLs can reference
• http - Hyper Text Transfer Protocol
• file - Simple File access

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URLs

• Anatomy of a URL
• http//www.ecst.csuchico.edu/jlong/mins116.html
• http - information transfer protocol.
• www.ecst.csuchico.edu - host machine name.
• jlong/mins116.html - the file component.

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URLs

• Rather than using file name, which need to be
  local to the machine on which the Java code is
  executing, we can use URLs to gain access to
  information.

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URLs

• URLs provide
• Location Transparency of Information
• Standardized naming
• A input stream created from a URL need not be on
  a local machine.
• Using URLs allows our programs to be used in a
  networking environment.

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Summary

• Files are used to store persistent information on
  secondary storage devices.
• In Java, file access is performed via Streams.
• To successfully use Files and Streams, exceptions
  must be handled.
• URLs can provide location transparency for our
  information sources.