CS4812Java

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CS4812--Java

• Lecture 2
• File I/O Fun with Streams

2
Java I/O Fun with Streams

• Last week, we noticed that there were three types
  of basic streams in Java
• 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 -- Basic Divisions (1.0)

• 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

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Types of InputStreams (1.0)
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Decorator Classes--eh?

• Java IO uses the concept of 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 we 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(iStreamRe
  ader)
• // 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 ("Error
  reading from keyboard\n\t"
• e.toString())
• e.printStackTrace()
• System.exit(0)
• return strInput
• // readLine()

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The File Class (1.0)

• 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 would be a better name
• There is no JDK 1.1 equivalent

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A Directory Listing

• public class ls
• public static void main (String arg) try
• File path new File (.)
• String listing path.list()
• for(int countlisting.length count--gt0)
• System.out.println(listcount)
• catch(Exception e)e.printStackTrace()

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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().toLowerCase()
• // don't list
  .java files
• return
  f.indexOf(".class")-1
• 
  f.indexOf(".java")-1
  
• // accept
• //FilenameFilter
• ) // new listing for path with
  anonymous inner class
• for (int i0 ilt list.length i)
• System.out.println
  ("\t"listi)
• // printDirListing()

13
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--Oh, yea. That stuff

• You may have heard about object persistence as
  a new Java buzzword.
• The Serializable interface is lightweight because
  you must manually create/load persistent objects.
• 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? Just have your class impelement the
  Serializable interface
• E.g., the Equation.java class

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Saving Serialized Objects

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

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Externalization of Object

• JDK 1.1 also has the java.io.Externalizable
  class. This interface extends Serializable and
  adds two methods you 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 selective
  serialization
• 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
  password or other sensitive data.
• Even objects and data identified as private get
  serialized
• Solution use the transient keyword
• private transient String password pssst.
• Data and methods identified as transient are not
  serialized

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String Parsing--Tips

• You might consider the following sequence of
  steps to prepare your String equations
• / Given String eq . . . /
• eq removeY(eq)
• eq removeWhiteSpaces(eq)
• eq convertTrig(eq)
• eq insertParens(eq)
• eq insertOperands(eq)
• eq insertSpaces(eq)

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Removing the Y portion

• Reasoning
• We know all equations will start with the
  expression y .
• We cant count on spacing, so we shouldnt just
  lop off the left two characters.
• How do we do we separate equations out in
  mathematics? -- look for the sign.
• Everything to the left of the sign may be
  considered junk, since were always solving for
  y.
• java.lang.String.substring() method might be
  useful

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Removing the Y portion

• We can find where in the String the sign is
  located using
• java.lang.String.indexOf()
• Then just take the substring starting at the
  index 1
• Remember Strings elements are numbered from zero!

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Removing the Y portion

• /
• removeY (String) -- remove the "y" portion
  of the equation, based on location of sign
• All characters to the left of the sign
  are removed
• _at_param String eq -- the equation that might
  contain 'y '
• _at_return String eq -- the equation without the
  y expression
• /
• public String removeY(String eq)
• if (eq.indexOf("")!-1)
• return eq.substring(eq.indexOf("")
  1, eq.length())
• else return eq
• // removeY

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A peak at the insertOperands() method

• We assume that our operands and function calls
  are all single char expressions, and that all
  white space has been purged
• Lets look at an example to develop the logic of
  inserting operands
• 3s(x)5x-xx / the y is omitted! /
• We want this to read
• 3 s (x) 5 x x x

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insertOperands() -- logic

• We can use two pointers to traverse the length of
  the String current and previous chars. The
  current char is added to a new String, except
  that occasional are added, depending on the
  relationship between the current and previous char

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insertOperands() -- logic

• 3s(x)5x-xx

x /x adjacent
fcn adjacent
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insertOperands() method

• We therefore need helper methods to
• identify a char as a function (s,S,c,C,t,T,l,L)
• identify a char as a number (0-9 and ,)
• public boolean isNumber(char c)
• return (Character.isDigit(c) c'.'
  c'E' c'e'
• c'P' c'p') // isNumber
• identify a char as an operand (,,,-,/, etc.)

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insertOperands() -- Logic Table