Java Programming Fourth Edition - Rutgers Universityszhou/351/ch16.pdf · 2010-11-23 · Java...

Java Programming Fourth Edition

Chapter 16 File Input and Output

Java Programming, Fourth Edition 2

Objectives

•  Understand computer files •  Use the File class •  Understand data file organization and streams •  Use streams •  Write to and read from a file


Objectives (continued)

•  Write formatted file data •  Read formatted file data •  Use a variable filename •  Create and use random access files •  Write records to a random access file


Objectives (continued)

•  Read records from a random access file •  Read and write objects to and from files


Understanding Computer Files

•  Temporary storage –  Computer memory or random access memory

(RAM) –  Volatile

•  Permanent storage –  Not lost when computer loses power –  Nonvolatile

•  Computer file –  Collection of information stored on nonvolatile device

in computer system


Understanding Computer Files (continued)

•  Permanent storage devices –  Hard disks –  Floppy disks –  Zip disks –  USB drives –  Reels or cassettes of magnetic tape –  Compact discs

•  Data files –  Contain facts and figures



•  Program files or application files –  Store software instructions

•  Root directory •  Folders or directories •  Path

–  Complete list of disk drive plus hierarchy of directories in which file resides



•  Work with stored files in application –  Determine whether and where file exists –  Open file –  Read data from file –  Write information to file –  Close file


Using the File Class

•  File class –  Gather file information –  java.io package File someData = new File("Data.txt");

File someData = new File("C:\\Java\\Chapter.16\\Data.txt");


Selected File Class Methods


The FileDemo Class


Understanding Data File Organization and Streams

•  Retain data for any significant amount of time –  Save on permanent, secondary storage device

•  Businesses store data in hierarchy –  Character –  Field –  Record –  Files

•  Sequential access file –  Each record stored in order based on value in some

field


Data Hierarchy


Understanding Data File Organization and Streams (continued) •  Open file

–  Create object –  Associate stream of bytes with it

•  Close file –  Make it no longer available to application –  Should always close every file you open

•  Stream –  Bytes flowing into program from input device –  Bytes flow out of application to output device –  Most streams flow in only one direction


File Streams


Using Streams

•  InputStream and OutputStream –  Abstract classes that contain methods for performing

input and output •  System.out

–  PrintStream object –  Defined in System class

•  System.err –  Usually reserved for error messages


Description of Selected Classes Used for Input and Output


Using Streams (continued)

•  FileWriter class –  Convenience class for writing character files

•  Convenience class –  Designed to make programmer’s job easier

•  System.in –  BufferedInputStream object –  Captures keyboard input

•  Buffer –  Memory location used to hold data temporarily


Using Streams (continued)

•  BufferedInputStream class –  Allows keyboard data to be entered at command line –  Held until user presses Enter


Writing to and Reading from a File

•  Assign file to InputStream or OutputStream –  Construct FileOutputStream object –  Assign it to OutputStream

•  Associate File object with output stream –  Pass filename to constructor of FileOutputStream class

–  Create File then pass File object to constructor of FileOutputStream class

•  FileOutputStream constructor –  Might throw FileNotFoundException


Reading from a File

•  Read data from file –  Create File object and assign it to input stream


Writing Formatted File Data

•  DataOutputStream objects –  Write binary data to OutputStream –  Data not readable in text editor

•  Not stored as characters –  Formatted correctly for its type

•  DataOutput interface –  Implemented by DataOutputStream


Writing Formatted File Data (continued)

•  Create DataOutputStream –  Assign FileOutputStream object to it –  Data stored in file –  Chaining stream objects out = new DataOutputStream(new FileOutputStream ("someFile"));


Reading Formatted File Data

•  DataInputStream objects –  Read binary data from InputStream

•  DataInput interface –  Implemented by DataInputStream –  Each read method correctly reads type of data

indicated by name •  Create DataInputStream object to read from file

–  Use chaining technique in = new DataInputStream(FileInputStream("someFile"));


Reading Formatted File Data (continued)

•  Detect end of file –  Check for return value of –1 from read() method –  Catch EOFException


Using a Variable Filename

•  Instead of hard-coding filename –  More flexible to use variable filename –  Same application can process different data sets –  Pass filename to method that opens file –  First element of args array passed to main()

method becomes filename java ReadEmployeeFileFrame EmpData.dat


First Part of the ReadEmployeeFileFrame Constructor Using a String

Argument


A main() Method that Uses the First Element of the args Array as the

Filename


Creating and Using Random Access Files

•  Sequential access files –  Access records sequentially from beginning to end –  Good for batch processing

•  Same tasks with many records one after the other –  Inefficient for many applications

•  Realtime applications –  Require immediate record access while client waits


Creating and Using Random Access Files (continued)

•  Random access files –  Records can be located in any order –  Also called direct access files

•  Instant access files –  Records must be accessed immediately

•  RandomAccessFile class –  Create random access files

RandomAccessFile myFile = new RandomAccessFile("C:\\Temp\\SomeData.dat","rw");


Creating and Using Random Access Files (continued)

•  RandomAccessFile class (continued) –  Contains read(), write(), and close() –  seek() method

•  Select beginning position within file •  Places file pointer at selected location myFile.seek(200);

•  File pointer, or file position pointer –  Automatically created variable –  Holds byte number of next file position to be used


Writing Records to a Random Access File

•  Access a particular record myFile.seek((n-1) * 50);

•  Place records into file based on key field •  Key field

–  Field that makes record unique from all others


Sizes of Data Types


Reading Records From a Random Access File

•  Process random access file –  Sequentially –  Randomly


Accessing a Random Access File Sequentially

•  ReadRandomStudentFile application –  Reads through 1000-record StudentData.dat file

sequentially in a for loop (shaded) •  Reads int-double pairs

–  When ID number value is 0 •  No user-entered record stored at that point •  Application does not bother to print it


Accessing a Random Access File Randomly

•  Display records in order based on key field –  Do not need to create random access file –  Wastes unneeded storage –  Instead sort records

•  Benefit of using random access file –  Retrieve specific record from file directly –  Without reading through other records


Accessing a Random Access File Randomly (continued)

•  ReadRandomStudentFile2 application –  Calculates student record’s position in data file –  Positions file pointer at correct location to begin

reading


Reading and Writing Objects to and from Files

•  Serialization –  Java’s built-in mechanism for manipulating objects

as streams of bytes •  Serializable interface

–  Endows your class with ability to be serialized implements Serializable

•  After serialized object written into file –  Can be read from file and deserialized


The Employee Class


Reading and Writing Objects to and from Files (continued)

•  Deserialization –  Process of recreating an object in computer memory

after it is retrieved from a file •  Scanner object

–  Text reader can pull primitive types and strings from input data streams

–  Works by dividing input into pieces called tokens •  Single, small elements of text


Selected Scanner Class Methods


You Do It

•  Using the File class to examine file status •  Comparing two file dates •  Using InputStream and OutputStream objects •  Writing to an output file •  Reading data from a file •  Creating a file of sequential data records


You Do It (continued)

•  Adding a constructor that opens a file •  Adding an actionPerformed() method •  Adding a main() method •  Reading from a sequential access file •  Creating a random access file •  Accessing records randomly


Summary

•  Files –  Objects stored on nonvolatile, permanent storage

•  File class –  Gather file information

•  Java views file as a series of bytes –  Views stream as object through which input and

output data flows •  DataOutputStream class

–  Accomplish formatted output


Summary (continued)

•  DataInputStream objects –  Read binary data from InputStream

•  Random access files –  Records can be located in any order –  RandomAccessFile class

•  Write objects to files if they implement Serializable interface

Java Programming Fourth Edition - Rutgers Universityszhou/351/ch16.pdf · 2010-11-23 · Java...

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