CIS 234: File Input & Output

introduction

Data Storage Is Fundamental computers need to store data to

work with it memory (RAM) is fast but transient

lost when computer turned off storage devices are slow but

persistent diskette, hard drive, tape, CD-ROM,

DVD

Files on External Devices data

accounting data in "flat" files data in database files reports in word processor files images in graphics files

stored programs in files

Using Data Files with Java input/output classes in Java

import java.io.* file class constructor

File myData = new File("my.txt");// on same directory as class file

can also include file path"A:\\Project9\\my.txt" //note double slashes (escape char)

Files Have Properties name – file identifier + extension

mydata txt parent – drive + directory(s)

A:\My Documents\Project8\ path – parent + name

A:\My Documents\Project8\mydata.txt

File Properties - 2 readable – by the program writeable – can save data into it length – file size in bytes last modified – date last saved

File Class Methods canRead() – readable (true/false)

if (myData.canRead()) { some code}

canWrite() – can save to it (true/false)

length() – size lastModified() – date

File Class Methods - 2 getName()

String fn = myData.getName(); getPath() //if identified when created getParent() //if identified when created exists() – check to see if it's there

don't try to read if not there

File Organization "Hierarchy" a hierarchy is like an outline from top down:

file has records (1 or more), which have fields, which are made up of characters, which are made up of bits

OS Opens File sets up an area in memory to

receive data sends message to storage device finds location of file data on media

file allocation table in DOS may lock file so other programs

can't use it

OS Closes File may write unsaved data to medium frees up memory unlocks file

Input and Output Devices Java, C, etc. programs use

"standard" input and output devices

defaults input comes from keyboard output goes to screen

can override defaults e.g., send output to a file or printer

"Streams" two ways to deal with data

1 record at a time COBOL works with records

as a stream of data better for multiplatform systems Java and C work with streams

Creating an Output File two ways to associate file with an

output stream pass filename argument to

FileOutputStream constructor pass filename argument to File

constructor, then pass file object to FileOutputStream constructor

Read/write text files

Read – Input FileReader infile = new FileReader(“data.txt”); BufferedReader in = new BufferedReader(infile);

in.readLine(); // test end of data (inStr != null)

Write – output FileWriter outStream = new FileWriter(“data.txt”); PrintWriter out = new PrintWriter(outStream);

Out.println( “myData” + “|” + name);