CIS 234: File Input & Output

Dr. Ralph D. WestfallMay, 2007

Data Storage Is Fundamental computers need to store data to

work with it memory (RAM) is fast but transient

data is lost when computer is turned off storage devices are slow but

persistent ("permanent storage") hard drive, diskette, CD-ROM, DVD ,

tape speeds (scroll down to L1 cache; 104)

Files on External Devices data

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

stored programs in files some early computers did not use

files for their programs (Altair)

Using Data Files with Java input/output classes in Java

import java.io.* file class constructor

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

can also include file path"A:\\Project9\\my.txt"

//note double slashes (1st=escape char)

Files Have Properties name = file identifier + extension

mydata txt parent = drive + directory(s)

A:\My Documents\Project5\ full path = parent + name

A:\My Documents\Project5\mydata.txt

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

File Organization "Hierarchy" a hierarchy is like an outline from top down (in a "flat" file):

file has records (1 or more) which have fields (1 or more) which are made up of character(s) (each is 2 bytes) made up of bits

what's above the top?

Java's File Class Methods to get data about a file

canRead() – readable (true/false)if (myData.canRead()) { some

code} canWrite() – can save to it (true/false) length() – size in bytes lastModified() – date

File Class Methods - 2 can break a full path into parts

getName() //from full path String fileName = 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 it if it's not there

Operating System Opens File sets up an area in memory to

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

in DOS, uses file allocation table (FAT) may lock file so other programs

can't use it at same time

OS Closes File may write unsaved data to output

medium frees up memory unlocks file

if it was locked while being used

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

input and output (SIO) devices defaults

input (System.in) comes from keyboard output (System.out) 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 ("flat" file organization)

COBOL works with records as a stream of data in individual bytes

better for multiplatform systems Java and C work with streams can be broken up with \n (newline

characters) into logical partitions

Input & Output Streams

InputStream myIStream;OutputStream myOStream;myIStream = System.in;myOStream = System.out;char c = myIStream.read();myOStream.write("abc");

Buffer area in memory to hold input or

output data temporarily after being read but before being

used in a program before being written to disk

buffers can improve performance program can read data from one part

of a buffer while more data is coming in from disk to another (part of) buffer

Creating an Output File two ways to associate a file with an

output stream pass filename argument to

FileOutputStream constructor pass filename argument to File

constructor, then pass file object to FileOutputStream constructor

Creating an Output File - 2FileOutputStream = new

FileOutputStream("mydata.txt"); //1st approach

File myFile = new File("mydata.txt");FileOutputStream = new FileOutputStream(myFile); // 2nd

approach

FileWriter class Java "convenience* class" for

writing character files* convenience means easier to use

constructors assume that default character encoding and buffer size are OK

Using FileWriter put code inside a try block create FileWriter object with filename can use write method to write outputs

or could create a PrintWriter object using a println() method with it (p.527-528)

be sure to use close() method after finish writing output file will lose all its data if it isn't closed

FileWriter Example (code)try{

FileWriter outs = new FileWriter("demoIo.txt" );

outs.write("Hello", 0, 5);//String, start, lengthouts.write("\n" + 1); // \n=new line outs.close(); // don't forget to use this line!!!} // view demoIo.txt with editor

[ catch (IOException e) {[some code]} ]

Review Compare and contrast memory (RAM,

cache) and storage devices (disk, CDs) Identify some types of data, etc. that

can be stored in files What is a “flat file?” What do you need at the top of a .java

file to use file data? import.java.___? Identify some file properties.

Review - 2 Identify some types of file handling

methods. Identify some parts of a file

identifier. Name some low level items in the

“file hierarchy”? What has to or might have to

happen when a file is opened? Is closed?

Review - 3 What does SIO mean? What are the default SIO devices? What are some differences between

streams and records? Which does Java use?

What is a buffer? How are buffers used in playing

videos?