File IO

1
File I/O

• Chapter 10

2
Chapter Contents

• Chapter Objectives
• 10.1 Introductory Example Weather Data Analysis
• 10.2 Java/s I/O System Readers, Writers, Streams
• 10.3 Exceptions
• 10.4 More About I/O Streams
• 10.5 Example Scanning for a Virus
• 10.6 Example Student Data Retrieval
• 10.7 Graphical/Internet Java Student Data
  Retrieval GUI
• Part of the Picture Data Base Systems

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Chapter Objectives

• Study Java's I/O with text and binary files
• Examine role of exceptions in I/O
• Observe use of files in GUI programs in context
  of an information retrieval problem
• Learn about the role of files in database
  management systems

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Need for File Access

• Files saved for word processors, spreadsheet
  programs, etc.
• Saved on secondary memory
• hard disks, floppy disks, CD-ROMs, etc.
• Requirements information stored in secondary
  memory
• can be retrieved in the future
• kept separate from other documents, programs, etc.

5
Intro Example Weather Data Analysis

• Problem large amounts of weather-related data is
  recorded and processed
• year's worth of atmospheric pressure readings
• taken every 15 minutes
• stored in a text file pressure.dat
• Weatherman needs minimum, maximum, and average
  of the data
• Data must be read from file, statistics
  calculated, results written to a text output file

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Solution Overview

• Note source code, Figure 10.1
• Names of input and output files are given to the
  program via the args parameter
• input file is args0
• output file is args1
• Command to open the input fileBufferedReader in
  new BufferedReader ( new
  FileReader (args0))
• Reading text from input filevalueString
  in.readLine()

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Solution Overview

• Opening text output filePrintWriter out new
  PrintWriter ( new
  BufferedWriter( new
  Filewriter (args1)))
• Writing text values to a file out.println ( . .
  . )
• Closing the filesin.close() out.close()

8
Handling Exceptions

• Many things can go wrong when dealing with files
• requested input file does not exist
• input file has invalid data
• output file unavailable
• When such an error occurs, the method in which
  the problem happened "throws an exception"
• Java can "catch" the exception if it happens in a
  "try block"
• Note the try block in the source code

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Handling Exceptions

• A try block is followed by one or more "catch
  blocks"
• Note the source codecatch (Exception
  anException) Controller.fatal ( )
• This catch block will
• determine the kind of exception
• report the problem
• terminate the program

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10.2 Java's I/O System Readers, Writers, and
Streams

• All input and output in Java is accomplished by
  classes called streams
• Input streams provide ways to move bytes of data
  from an input device to a program
• Output streams move the data in the opposite
  direction

Executing Program
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Predefined Streams

• System.in
• InputStream object, usually for the keyboard
• System.out
• a buffered PrintStream object, usually the screen
  or an active window
• System.err
• an unbuffered PrintStream object usually
  associated with the screen or console window

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Wrapping Classes

• InputStream class provides methods for reading
  bytes only
• to read at a higher level we must "wrap"
  System.in with another class
• Example BufferedReader classBufferedReader
  myReader new (BufferedReader ( new
  InputStreamReader( System.in))
• Now we can send myReader either
• the .read() message for a single char value or
• the .readLine() message for an entire line of text

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Using Readers and Writers

• Reader and Writer classes provide support for
  char (16-bit Unicode) I/O
• InputStream and OutputStream provide support for
  byte I/O
• The Java language is still growing and "maturing"
• extensions of Reader and Writer classes have yet
  to be fully accepted
• author of this text suggests Use Reader or
  Writer class whenever possible, use Stream class
  only when there is no Reader/Writer class that
  fits

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Using a Reader

• Wrap FileReader in BufferedReader
  classBufferedReader inFile new
  BufferedReader ( new FilReader
  (input_filename) )
• Now the BufferedReader object (inFile) can be
  sent a .readLine() message
• A BufferedReader is so named because it buffers
  the input
• this improves program performance

15
Using a Reader

• How to know when all the data in a file has been
  read?
• the readLine() method returns the value
  nullvalueString inFile.readLine()while
  (valueString ! null) // process
  valueString valueString inFile.readLine()
  
• Also possible to check for other things such as
  an empty line with no textif (valueString.equals(
  "")) continue

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Using a Reader

• Sequence of steps for using text input
• Build a BufferedReader by wrapping one around a
  FileReader
• Use an input loop to read lines of text from the
  BufferedReader
• Convert those lines to numeric values if
  necessary
• Close the BufferedReader

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Using a Writer

• When writing values to a text file, author
  suggests use a Writer class
• increase FileWriter capabilities by wrapping it
  in BufferedWriter and PrintWriter classes
• PrintWriter outFile new PrintWriter(new
  BufferedWriter ( new FileWriter (file_name) ) )

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10.3 Exceptions

• Easy for things to go wrong in a program
• faulty logic
• bad input from user, etc.
• Java classifies all these as "exceptional
  occurrences"
• provides Exception class
• tells something abnormal occurred

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Try-Catch Blocks

• To signal an abnormal occurrence
• method can throw an exception
• throw occurs inside a try block
• try block followed by one or more catch blocks
• Form
• try statement list
• catch (Exception_Type1 variable_name1)
• catch (Exception_type2 variable_name2)
• finally

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Try-Catch Blocks

• After catch block executed
• execution continues in finally block or
• if no finally , continues to next statements
  following catch
• If no matching exception type is found
• program terminates
• If no exception thrown
• catch blocks skipped
• execution continues in finally or in
  statements following catch block

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Exception Hierarchy
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Try/Catch Strategy

• Wrap all calls to methods that throw exceptions
  in single try block
• followed by single catch block .. or
• followed by sequence of catch blocks, one for
  each type of exception
• Wrap each call to a method that throws an
  exception in its own try block
• a catch block exists for each particular
  exception thrown

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Throwing Exceptions

• Method that throws an exception must explicitly
  state this in its heading
• public void read(BufferedReader aReader)
  throws EOFException
• Now calling method must invoke this method using
  try-catch blocks to handle the potential
  exception

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10.4 More About I/O Streams

• Data can be stored in a file as text or in binary
  format
• same format used to store values in memory
• takes less space to store an integer in binary
  format than in text format
• Java Stream classes provide byte level I/O
• FileOutputStream
• FileInputStream
• Also BufferedOutputStream and BufferedInputStream

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Writing Primitive types

• Java provides DataOutputStream and
  DataInputStream classes
• Note methods provided, Table 10.1
• note number of bytes used to store the data
• Consider Data Stream Demonstration, Figure 10.3
  in text note the features
• wrapping of Stream I/O classes
• use of try-catch blocks
• specific methods used to write, then read
• sample run

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Writing Sequence of Values

• Consider a group of values of different types
  that are written to a file
• they must be read in the same order that they
  were written
• Possible to create a class
• that group of values is the private data of the
  class
• specific methods are provided for an object of
  the class to write/read those values to/from a
  file
• the read and write methods receive the file
  handles as parameters

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10.5 Example Scanning for a Virus

• What is a virus?
• software that hides itself within other
  executable programs
• that executable program is the "host" for the
  virus
• the virus program tries to "infect" other
  programs
• the virus can cause problems that range from
  annoying to malicious
• A virus that prints an annoying message stores
  that string value within itself
• we will write a program that searches through a
  file to find that text, or signature

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Virus-Detection Algorithm

• Repeat the following
• read a line from a file
• if no line read due to EOF, terminate
• otherwise scan the line for specified string of
  text
• if that text found, stop reading from file
• Display message whether or not target string was
  found signifying virus found/not found
• Note source code, Figure 10.4

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Simulated Virus Scanning

• Note use of command line arguments
• first argument is the name of the file to be read
• second argument is the target string or virus
  signature
• Observe other features studied in this chapter
• use of try/catch
• wrapping of classes for file read

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10.6 Example Retrieving Student Information

• Problem Information Retrieval
• Records of information about students consist of
• studentID, a 9 digit integer
• first name, last name as strings
• studentYear as a string
• credits and GPA as reals
• We seek a program which will
• read a sequence of students from students.txt
• prompt for studentID, read from keyboard
• search for match, display if found

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Objects
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Student Class

• Operations needed include
• initialize itself with default values
• initialize itself with explicitly supplied
  values
• read its attributes from a BufferedReader store
  them in itself
• convert itself to a String (for output)
• access each of its attributes
• mutate any of its attributes
• Note methods provided for these capabilities in
  source code of Figure 10.5

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Program Algorithm

• Build students as a StudentSequence
• initialized from an input file
• name of file received in args0
• Repeatedly do the following
• prompt for and read studentID
• search students for target studentID, return its
  position
• if search successful display student at
  positionotherwise display error message

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Class StudentSequence

• Note source code, Figure 10.6
• Reads numberOfStudents as first value in the file
• Constructor declares myStudents, an array of
  Student
• Use numberOfStudents as limiting value in for
  loop
• stores incoming values in myStudents array
• Provides linear search method, find()

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Student Information Retrieval

• Note source code, Figure 10.7
• Program declares students, object of type
  StudentSequence
• constructor loads values from file
• Forever loop used to
• prompt for, receive as input student id
• call .find method to determine position of that
  student in the array
• print results (found or not)

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10.7 Graphical/Internet JavaStudent Data
Retrieval GUI

• Implement student data retrieval problem internet
  GUI context
• Much of work already done can be reused
• Behavior

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Implementation

• Methods for GUIStudentRetreiver
• constructor
• action handler
• enter initial state
• enter found an ID state
• enter not found state
• Attributes
• labels
• entry fields
• panes for panels

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Event-Handler Method

• public void actionPerformed(ActionEvent event)
• String idString myIdField.getText()
• if (idString.equals("")) enterInitialState()
  
• else int id Integer.parseInt(idString)
• int position myStudents.find(id)
• if (position gt 0)
• enterGoodIDState(myStudents.getStudent(positio
  n))
• else
• enterErrorState("No student found with that
  ID")

Note source code, Figure 10.8
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Part of the PictureDatabase Systems

• Businesses must make decisions
• Based on data
• accuracy and detail help make better decisions
• large data sets stored on computers
• Data sets maintained by Database Management
  Systems (DBMS)

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DBMS Facilities

• High level views of the data
• Access routines
• query language
• Support for large databases
• as large as 1000 gigabytes (one terabyte)
• Security
• data is sensitive or proprietary
• Data sharing
• protect data when more than one person wishes to
  access/ modify same piece of data
• Data integrity

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Relational Model

• Database viewed as a set of tables, called
  relations
• one row for each entry
• column for each attribute or field of the
  database
• SQL, "Structured Query Language"
• provides commands for manipulate data
• ExampleSELECT FROM Employee WHERE Rate 10.5