CS4812 Java

1
CS4812 Java

• Lecture 1
• File I/O Fun with Streams

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Java I/O Fun with Streams

• In general, there are streams inherent in any
  Java process
• System.in
• System.out
• System.err
• You are already familiar with most of these
  streams. E.g.,
• System.out.println (This is the out stream)

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Java IO The Big Picture

• The System.in, System.out, and System.err streams
  are building blocks for more complex IO objects
• Javas JDK 1.02 contained particularly weak IO
  design. Many classes just didnt work.
• Instead of scrapping JDK 1.02, the Sun engineers
  extended the 1.02 library with JDK 1.1
• In general, the JDK 1.1 classes provide adequate
  IO. (In some instances, one still must use JDK
  1.02.)

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Java IO -- Basic Divisions (1.0)
Java I/O is divided up based on directional flow

• InputStream
• through inheritance, all derivatives of
  InputStream have the basic method read() to
  access a byte or an array of bytes

• OutputStream
• Through inheritance, all derivatives of
  OutputStream have the basic method write() to
  write a single byte

Conceptually, the two are separate
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Types of InputStreams (1.0)
Javas InputStreams have six general flavors
Watch this class
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Decorator Classes

• Java IO uses decorator objects to provide
  layers of functionality to IO classes

Concept A decorator pattern wraps your inner
object, all using the same interface devices.
Pro/Con Flexibility with the cost of
complexity Example notice how may IO classes
feature the readLine() method.
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FilterInputStream Class

• The FilterInputStream offers a grab bag of
  methods.
• Sun this is the base class for enhancing input
  stream functionality
• Eckel they couldnt figure out where else to
  put this stuff, but it seemed like it belonged
  together TIJ (9th ed.)

Avoid it--use InputStreamReader (JDK 1.1)
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Keyboard Access Use InputStreamReader (1.1)

• For reading keystrokes, use an InputStreamReader
• Wrap with a BufferedReader decorator
• public IOHelper(boolean DEBUG)
• this.DEBUG DEBUG
• / For reading data from standard in (stdin)/
• iStreamReader new InputStreamReader(System.in
  )
• bReader new BufferedReader(iStreamReader)
• // constructor

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Hierarchy of BufferedReader
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BufferedReader in Action

• public String readLine()
• String strInput ""
• try strInput bReader.readLine()
• catch (Exception e)
• System.err.println (Keyboard error\n\t"
• e.toString())
• e.printStackTrace()
• System.exit(0)
• return strInput
• // readLine()

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The File Class (1.0)

• Another important object in Javas IO library is
  the File Class
• Misnomer the File class does not necessarily
  refer to a single file. It can represent the
  name of a single file, or the names of a set of
  files.
• The method list() returns a String array of
  file names
• Therefore, filepath might have been a better
  name
• There is no JDK 1.1 equivalent

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Using File to Obtain A Directory Listing

• public class Lister
• public static void main (String arg)
• try
• File path new File (.)
• String listing path.list()
• for(int ilisting.length i--gt 0)
• System.out.println(listingi)
• //try
• catch(Exception dammit) dammit.printStackTrace()
  
• //main
• //class

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A FilenameFilter Example

• public void printDirListing()
• final String list / MUST be final /
• final File path new File(".") / MUST be
  final /
• list path.list( new FilenameFilter()
• public boolean accept (File dir, String n)
• String f new File(n).getName().toLowerC
  ase()
• / don't list .java files /
• return f.indexOf(".class") -1
• f.indexOf(".java") -1
  
• // accept
• //FilenameFilter
• ) // anonymous inner class
• for (int i0 ilt list.length i)
• System.out.println ("\t"listi)
• // printDirListing()

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Object Serialization--eh?

• Normally, objects live only during the life of
  the program. Stop the program, and the object
  disappears
• JDK 1.1 offers the Serializable interface to
  create lightweight persistence.
• Persistent objects live beyond the life of the
  program, usually in byte from on disk or on a
  network.

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Object Serialization

• You may have heard about object persistence as
  a new Java buzzword.
• The Serializable interface provides this
  capabilities for lightweight persistence.
• The Serializable interface is lightweight because
  one must manually create/load persistent objects.
• This technique is necessary for remote method
  invocation (RMI), where objects live on another
  machine

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Object Serialization--More

• Serialization is also necessary for Java Bean
  design, where functional components have state
  information created and saved during design, not
  runtime.
• How does one serialize an object? Just have your
  class implement the Serializable interface (e.g.,
  the Dots.java class), and save approrpiately

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Saving Serialized Objects

• Simple create an OutputStream, and wrap it with
  an ObjectOutputStream.
• Twin methods
• writeObject()
• readObject()
• / requires InputStream wrapped by
  ObjectInputStream /
• Restoring objects requires a .class file!
• Therefore, dont modify your Dots.java files!

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Externalization of Objects

• JDK 1.1 also has the java.io.Externalizable
  class.
• Externalization is different from serialization.
• Th Externalizable interface extends Serializable
  and adds two methods one must implement
• public abstract void readExternal(ObjectInput in)
  throws IOException, ClassNotFoundException
• public abstract void writeExternal(ObjectOutput
  out) throws IOException

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Externalized Objects--More

• Externalized Objects are similar to serialized
  objects, except that the objects themselves
  control the field creation during reading and
  writing.
• The readExternal and writeExternal are called
  with readObject() and writeObject()
• This produces greater security, and offers the
  possibility of selective serialization
• Note Bene Constructors for externalized objects
  must be public

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Serialization the transient State

• There may be time when we want to serialize an
  object, but omit certain key features, like
  passwords or other sensitive data.
• Even objects and data identified as private get
  serialized. How does one keep data out of
  externalized objects?
• Solution use the transient keyword
• private transient String password pssst.
• Data and methods identified as transient are not
  serialized.

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Networking Basics

• This class is not about networking, but in order
  to use the java.net. package and classes, youll
  have to be familiar with some networking
  concepts.
• The following slides cover simple networking
  terminology for purposes of p1.

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Basic Client-Server Relations

• At a fundamental level, all networked
  applications divide into either client or server
  services. Client and server applications
  communicate with each other through some type of
  communications link--often an agreed protocol for
  sharing information.
• It is often helpful to conceive of the network as
  a series of layers, representing different
  levels of abstraction.

• At the top level, we have applications--web
  browsers, ftp programs, telnet applications.
• This top-level software utilizes a transport
  layer (TCP protocol)
• Which in turn uses a network layer (IP protocol
  e.g., IPv4)
• Which in turn uses a link layer (ethernet
  protocol)

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The User-Process Level

• For the most part, we will be utilizing the
  user-process level, and rely on TCP/IP protocols.
  
• Defined The Transport Control Protocol (TCP) is
  a connection-based protocol that provides
  reliable flow of data between two computers.
• If time permits, we may take a look at UDP (user
  datagram protocol) communications as well. (In
  short UDP has no guarantees that information
  will arrive, but it is considerably faster.)

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Ports Sockets What?

• Generally, computers possess only a single
  physical connection to a network. All
  information arrives and departs through this
  connection. To many different applications to
  use this same connection, computers create
  logical groupings, or ports.
• Together with an IP address--a unique number
  assigned to a machine on a network--a port allows
  one to identify a specific process on a specific
  machine.
• A socket is a software abstraction that provides
  a terminus to a connection between machines its
  a point of abstraction that allows for users to
  address a connection between machines.

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URLs How To

• An essential part of any socket is the URL. A
  URL has two main components
• Other components include
• Host Name -- the name of the machine hosting
  the resource
• Filename -- the pathname to the file on the host
• Port Number -- the port number to which to
  connect (typically optional).
• Reference -- a reference to a named
  anchor within a resource that usually identifies
  a specific location within a file
  (typically optional).

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Creating an URL

• The easiest way to create a URL in Java is to
  start with a String
• try
• URL myURL new URL (http//www.cc.gatech.edu)
  
• catch (MalformedURLException e)
• System.err.println (This method
  e.toString())
• URLs can also be created relative to an existing
  URL
• try
• URL firstURL new URL(http//www.foo.com/top_le
  vel)
• URL secondURL
• new URL (firstURL, lower_level)
• catch (Exception e)

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Using a URL

• URLs contain many useful methods, including
• getProtocol() returns the protocol
  identifier component
• of the URL (ftp/http, e.g.).
• getHost() returns the host name component of
  the URL.
• getPort() returns the port number component of
  the URL (or -1 if not set).
• getFile() returns the filename component of
  the URL.
• getRef() returns the reference component of
  the URL.

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import java.net. import java.io. public
class URLReader public static void main
(String args) throws Exception
URL myURL new URL("http//www.blarg.foo.org/")
BufferedReader in new
BufferedReader(new InputStreamReader
(myURL.openStream())) String
strInput while ((strInput in.readLine())
! null) System.out.println(strInput)
in.close()
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Connecting to a URL
One can also use openConnection() to
connect to a URL. This creates a communication
link between your Java program and the URL over
the network. For example try
URL myURL new URL("http//www.blarg
.foo.org/") myURL.openConnection()
catch (MalformedURLException e) catch
(IOException e)
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Extracting the Stream from a URL

• import java.net.
• import java.io.
• public class URLConnectionReader
• public static void main
• (String args) throws Exception
• URL myURL
• new URL("http//www.cc.gatech.edu/")
• URLConnection uc myURL .openConnection()
• BufferedReader in new BufferedReader
• (new InputStreamReader
• (uc.getInputStream()))
• / see getOutputStream() /
• String inputLine
• while ((inputLine in.readLine()) ! null)
• System.out.println(inputLine)
• in.close()