Topic08 Files IO

8/11/2019 Topic08 Files IO

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CSC238 OOP

Topic 08: FILES INPUT/OUTPUT

Introduction The values of data structures in a program are lost when the

program terminates

If you want to keep the data after the program terminates, the data

JAVA program can read a data and write a data from and to a file.

This process is called File Input/Output (I/O)

To store and retrieve data on a file in JAVA, it needs :1. File

Any collection of data stored under a common name on a storage

medium other than memory is called data file, and it is referred to

as external file name.

2. File stream object

One-way transmission path that is used to connect a file stored on

a physical device such as disk to a program.

Each stream has its own mode whether the path will move thedata from a file into a program or from a program to a file


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The File class5

Every file is placed in a directory in the file system.

The complete file name consists of the directory path and the file name.

For example, c:\temp\Welcome.java is the complete file name for the fileWelcome.java on the windows operating system.

The File class contains the methods for obtaining file properties and forrenaming and deleting files. However, the File class does not contain themethods for reading and writing file contents.

e ename s a s r ng. e e c ass s a wrapper c ass or e enameand its directory path.

For example, new File(c:\\temp) creates a File object for the directoryc:\temp, and new File(c:\\temp\\test.txt) creates a File object for the filec:\\temp\\test.txt, both on Windows.

Example: File class6

import java.io.*;import java.util.*;

public class TestFileClass {public static void main(String[] args){

//create a File object" "e e = new e c: temp ct.txt ;

System.out.println("Does it exist? " + file .exists());//return true if the file or the directory represented by the File object existsSystem.out.println("Can it be read? " + file.canRead());//return true if the file represented by the File object exists and can be read

System.out.println("Can it be written? " + file.canWrite());//return true if the file represented by the File object exists and can be written

System.out.println("Is it a directory? " + file.isDirectory()); //return true if the File object represents a directorySystem.out.println("Is it a file? " + file.isFile());//return true if the File object represents a fileSystem.out.println("Is it absolute? " + file.isAbsolute());// return true if File object is created using an absolute path nameSystem.out.println("Is it hidden? " + file.isHidden());//return true if the file represented in the File object is hidden

System.out.println("What is its absolute path? " + file.getAbsolutePath());//return the complete absolute file or directory name represented by the File object

System.out.println("What is its name? " + file.getName());//return the last name of the complete directory and file name represented by the File object

System.out.println("What is its path? " + file.getPath());//return the complete directory and file name represented by the File objectSystem.out.println("When was it last modified? " + new Date(file.lastModified()));//return the time that the file was last modified

}}


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Procedure of File I/O

Open a I/O stream objects (using Fi l eReaderor Fi l eWr i t er class)

read() or wr i t e( ) methods)

Close the I/O stream objects (using cl ose( )

method)

o e:these 3 statements are of typed checked exceptions f rom

IOException class. So you need to handle or declare the

IOException exception.

Buffering

When transferring data between program and a fileusing a buffered stream, it provides a storage area thatare used by the data as they are transferred toenhance I/O speed and performance.

Buffered object stream are created from buffered stream

classes. The basic I/O stream object must be created firstbefore you create a buffered object stream.

Example :

Fi l eReader f r = new Fi l eReader ( pr i ce. t xt ) ;

Buf f er edReader br = new Buf f er edReader ( f r ) ;

Fi l eWr i t er wr = new Fi l eWr i t er ( out . t xt );

Buf f er edWr i t er br = new Buf f er edWr i t er ( f r ) ;


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Reading and Writing Character-Based Files

Character-Based Output Stream ClassesClass Type Comments Common Methods

FileWriter Basic output Basic stream used for write(), flush(), close()

Character-Based Input Stream Classes

c arac e r- ase ou pu

BufferedWriter Buffering Provides output buffering,

which typically improves

performance

write(), flush(), close()

PrintWriter Processing Provides a number of

useful output methods

flush(), close(),

print(..), println(.)

Class Type Comments Common Methods

FileReader Basic Input Basic stream used for

character-based input

read()

BufferedReader Processing Provides buffering,

which typically improves

performance

read(), readLine(), close()

Example of Writing Character-based file

import javax.swing.*;

import java.io.*;

class WriteTextFile {

public static void main(String[] args) {

try

{

String input = JOptionPane.showInputDialog("Enter the file name:");

String fileName = input.trim();

FileWriter fw = new FileWriter(fileName);

BufferedWriter bw = new BufferedWriter(fw );PrintWriter pw = new PrintWriter(bw );

for(int i=1; i


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Example of Reading Character-based file

import java.io.*;

class ReadTextFile {

public static void main ( String[] args ) {

String fileName = "thetest.txt";

int test;

Input file: thetest.txt

FileReader fr = new FileReader(fileName);

BufferedReader br = new BufferedReader(fr);

String input = null;

while( (input=br.readLine()) != null) {

test = Integer.parseInt(input);

System.out.println("Result is : "+(test));

}

br.close();

}

output

catc e ot oun xcept on e

{ System.out.println("Problem :"+e.getMessage()); }

catch (IOException ioe)

{ System.out.println("Problem :"+ioe.getMessage()); }

}

}

Reading Character-Based file using String

Tokenizer

String Tokenizer

A class that helps programmer separates individual part of a string

if the items are separated by one character, such as a comma.

It helps the programmer when the program reads textual data from

a file.

For example: Data file contains a list of grocerys list.

A stringtokenizer object can help programmer to pullout

apples,milk,butter,cakeflour,sugar,cereal

apples,butter and place them in separate variables.

The java.util.StringTokenizer class is used to break strings into tokens

(words, numbers, operators, or whatever).

Tokens: the data of interest in a string of characters. They maybe

separated by a comma, a space, or one other special character.


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Reading Character-Based file using String

Tokenizer

A StringTokenizer constructor takes a string to break into tokens and

returns a StringTokenizer object for that string. Each time its

nextToken() method is called, it returns the next token in that string. If

' ,

default.

Common methods used in String Tokenizer :

Assume that st is a StringTokenizer.

st.hasMoreTokens() -- Returns true if there are more tokens.

st.nextToken() -- Returns the next token as a String. st.countTokens() -- Returns the int number of tokens.

Example : using String Tokenizerimport java.io.*;

import java.util.*;

class ReadTextFile2 {

public static void main ( String[] args ) {

String fileName = "grocery.txt";

int num;

try {

FileReader fr = new FileReader(fileName);

Input file: grocery.txt

u = w u ;

String input = null;

int cnt=0;

while( (input=br.readLine()) != null) {

StringTokenizer st = new StringTokenizer(input, ",");cnt++;

System.out.println(cnt + " record");

while (st.hasMoreTokens()) {

String w = st.nextToken();

System.out.println(w);

}

}

br.close();

}

catch (FileNotFoundException e)

{ System.out.println("Problem :"+e.getMessage()); }

catch (IOException ioe)

{ System.out.println("Problem :"+ioe.getMessage()); }

}

}


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Scanner Class

Using Scanner for Input from file

The Scanner class is a class inj ava. ut i l , which allows the user to read

values of various types.

The Scanner looks for tokens in the input.

to en is a series o c aracters t at en s wit w at Java ca s w itespace.A whitespace character can be a blank, a tab character, a carriage return,or the end of the file.

Thus, if we read a line that has a series of numbers separated by blanks,the scanner will take each number as a separate token.

Whitespace characters (blanks or carriage returns) act as separators. The next

method returns the next input value as a string, regardless of what is keyed.


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Using Scanner for Input from file

Reading from the keyboard using Scanner

Scanner i n = new Scanner( Syst em. i n);

,with a Fi l eReader object rather than Syst em. i n.

Scanner i nFi l e = new Scanner( new Fi l eReader ( ( "i nFi l e. dat ") ) ;

Although all of the methods applied to keyboard input can be applied to file input,there are methods that are usually applied only to files.

These are the methods that ask of there are more values in the file. If there are nomore values in a file, we say that the file is at the end of the file (EOF). For

example :i nFi l e. hasNext( ) ; //returns true if inFile has another token in the file

i nFi l e. hasNext Li ne( ) ; //returns true if inFile has another line the file

Be sure to close all files. If you forget to close Syst em. i n, no harm is done, but

forgetting to close a file can cause problems.

Example : Using Scanner for Input from file

import java.io.*;import java.util.*;

class readScanner {

public static void main ( String[] args ) {//String fileName = "inFile.dat";

Input file: inFile.dat

int test;

try {

Scanner inFile = new Scanner(new FileReader("inFile.dat"));

while(inFile.hasNextLine()) {String name = inFile.next();int mark = inFile.nextInt();

System.out.println(name+" "+mark);

}inFile.close();

output

}

catch (FileNotFoundException e){ System.out.println("Problem :"+e.getMessage()); }

catch (IOException ioe){ System.out.println("Problem :"+ioe.getMessage()); }

}

}


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Exce tion handlin

Introduction

Exception handling is used to deal with runtime errors

Exception may occur for various reasons such as:

The program may attempt to open a file that doesntexist

Network connection may hang up

The program may attempt to access an out of bounds

A Java exception is an instance of a class derived

from Throwable class which contained in thejava.lang package and subclasses of Throwable arecontained in various packages


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Some of Javas Exception Classes

Handling exception

3 statements

try

Identifies a block of statements within which an exce tionmight be thrown

catch

Must be associated with a try statement and identifies ablock of statements that can handle a particular type ofexception occurs within the try block

Used to execute some code if an exception occurs or iscaught

Must be associated with a try statement and identifies ablock of statements that are executed regardless whetheror not an error occurs within the try block


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Example

Binary I/O classes

InputStream is the root for binary input classes andOutputStream is the root for binary output classes


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FileInputStream/FileOutputStream

For reading/writing bytes from/to a file

Exercise

Application of StringTokenizer class to divide one statement

into several words Write a program to read from a file name score.txt which

contains several numbers shown as follows:

Write the output to calculate the total score for each student toa text file name scoreout.txt shown as follows:

Topic08 Files IO

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