File Input & Output

Objectives
 After you have read and studied this chapter, you should
be able to
 Use File object to get info about a file
 Include a JFileChooser object in your program to let the user
specify a file.
 Write bytes to a file and read them back from the file, using
FileOutputStream and FileInputStream.
 Write values of primitive data types to a file and read them back
from the file, using DataOutputStream and DataInputStream.
 Write text data to a file and read them back from the file, using
FileWriter,PrintWriter and FileReader,BufferedReader
 Read a text file using Scanner

File
 File is regarded as a collection of bytes
 When a file is read, computer delivers some of
those bytes to the program.
 When a file is written, computer accepts some
bytes from the program and saves them in part
of the file.
 Computer makes no distinction between eg.
image files and text files. Its all bytes to the
hardware. What those bytes are used for is up
to the software.

Types of File
 All information in any file is kept in binary form.

Types of File
 File can be categorized as text (ASCII) file or binary file.
 A text file is a file that contains bytes that represent:
 characters 'A' through 'Z'
 characters 'a' through 'z'
 characters '0' through '9'
 the space character
 punctuation and symbols like . , : ; " + - $ (and others)
 a few control characters that represent end of lines, tabs and some
other things.
using ASCII Codes
 A binary file is a file that contains bytes that represent
others (eg. numbers, image, audio, formatted text etc)

The File Class The File class (from java.io). can be used to obtain info about file
 To do this, we must first create a File object
 A File object can represent a file or a directory
File inFile = new File(“sample.dat”);
File inFile = new File
(“C:/SamplePrograms/test.dat”);
Creates File object for
the file sample.dat in the
current directory.
the file sample.dat in the
current directory.
the file test.dat in the
directory
C:SamplePrograms
using the generic file
separator / and
providing the full
pathname.
the file test.dat in the
directory
C:SamplePrograms
using the generic file
separator / and
providing the full
pathname.

Some File Methods
if ( inFile.exists( ) ) {
if ( inFile.isFile() ) {
File directory = new
File("C:/JavaPrograms/Ch12");
String filename[] = directory.list();
for (int i = 0; i < filename.length; i++) {
System.out.println(filename[i]);
}
To see if inFile is
associated to a real file
correctly.
To see if inFile is
associated to a real file
correctly.
To see if inFile is
associated to a file. If
false, it is a directory.
Also, can test directly if it
is a directory.
To see if inFile is
associated to a file. If
false, it is a directory.
Also, can test directly if it
is a directory.
List the name of all files
in the directory
C:JavaProjectsCh12
List the name of all files
in the directory
C:JavaProjectsCh12
if ( inFile.isDirectory() ) {

Some File Methods
if ( inFile.length( ) ) {
if ( inFile.canRead() ) {
To see the size of the file
in bytes represented by
inFile
To see the size of the file
in bytes represented by
inFile
To see if inFile is
associated to a file that
exist & can be read
To see if inFile is
exist & can be read
if ( inFile.canWrite() ) { To see if inFile is
exist & can be written
To see if inFile is
exist & can be written
if ( inFile.getName() ) { To get the name of the
file represented by inFile
To get the name of the
file represented by inFile

The JFileChooser Class
 A javax.swing.JFileChooser object allows
the user to select a file.
JFileChooser chooser = new JFileChooser( );
chooser.showOpenDialog(null);
JFileChooser chooser = new JFileChooser("D:/JavaPrograms/Ch12");
chooser.showOpenDialog(null);
To start the listing from a specific directory:

Getting Info from
JFileChooser
int status = chooser.showOpenDialog(null);
if (status == JFileChooser.APPROVE_OPTION) {
JOptionPane.showMessageDialog(null, "Open is clicked");
} else { //== JFileChooser.CANCEL_OPTION
JOptionPane.showMessageDialog(null, "Cancel is clicked");
}
File selectedFile = chooser.getSelectedFile();
File currentDirectory = chooser.getCurrentDirectory();

I/O Streams
 To read data from or write data to a file,
we must create one of the Java stream
objects and attach it to the file.
 A stream is a sequence of data items,
usually 8-bit bytes.
 Java has two types of streams: an input
stream and an output stream.
 An input stream has a source form
which the data items come, and an
output stream has a destination to which

I/O Streams
 IO streams are either character-oriented or byte-
oriented.
 Character-oriented IO has special features for
handling character data (text files).
 Byte-oriented IO is for all types of data (binary files)
IO class
Hierarchy
in java.io
package

Streams for Byte –level Binary File I/O
 FileOutputStream and
FileInputStream are two stream objects
that facilitate file access.
 FileOutputStream allows us to output a
sequence of bytes; values of data type
byte.
 FileInputStream allows us to read in an
array of bytes.

Sample: Byte-level Binary
File Output//set up file and stream
File outFile = new File("sample1.data");
FileOutputStream
outStream = new FileOutputStream( outFile );
//data to save
byte[] byteArray = {10, 20, 30, 40,
50, 60, 70, 80};
//write data to the stream
outStream.write( byteArray );
//output done, so close the stream
outStream.close();

Sample: Byte-level Binary
File Input//set up file and stream
File inFile = new File("sample1.data");
FileInputStream inStream = new FileInputStream(inFile);
//set up an array to read data in
int fileSize = (int)inFile.length();
byte[] byteArray = new byte[fileSize];
//read data in and display them
inStream.read(byteArray);
for (int i = 0; i < fileSize; i++) {
System.out.println(byteArray[i]);
}
//input done, so close the stream
inStream.close();

Streams for Data-Level Binary File I/O
 FileOutputStream and
DataOutputStream are used to output
primitive data values
 FileInputStream and DataInputStream
are used to input primitive data values
 To read the data back correctly, we
must know the order of the data stored
and their data types

Setting up
DataOutputStream A standard sequence to set up a DataOutputStream
object:

Sample Output
import java.io.*;
class Ch12TestDataOutputStream {
public static void main (String[] args) throws IOException {
. . . //set up outDataStream
//write values of primitive data types to the stream
outDataStream.writeInt(987654321);
outDataStream.writeLong(11111111L);
outDataStream.writeFloat(22222222F);
outDataStream.writeDouble(3333333D);
outDataStream.writeChar('A');
outDataStream.writeBoolean(true);
outDataStream.close();
}
}

Setting up DataInputStream A standard sequence to set up a DataInputStream object:

Sample Input
import java.io.*;
class Ch12TestDataInputStream {
. . . //set up inDataStream
//read values back from the stream and display them
System.out.println(inDataStream.readInt());
System.out.println(inDataStream.readLong());
System.out.println(inDataStream.readFloat());
System.out.println(inDataStream.readDouble());
System.out.println(inDataStream.readChar());
System.out.println(inDataStream.readBoolean());
//input done, so close the stream
inDataStream.close();
}
}

Reading Data Back in Right
Order The order of write and read operations must match in
order to read the stored primitive data back correctly.

Reading & Writing Textfile
 Instead of storing primitive data values
as binary data in a file, we can convert
and store them as a string data.
This allows us to view the file content using
any text editor
 To write data as a string to text file, use
a FileWriter and PrintWriter object
 To read data from textfile, use
FileReader and BufferedReader classes
From Java 5.0 (SDK 1.5), we can also use
the Scanner class for reading textfiles

Sample Writing to Textfile
import java.io.*;
class Ch12TestPrintWriter {
//set up file and stream
File outFile = new File("sample3.data");
FileWriter outFileStream
= new FileWriter(outFile);
PrintWriter outStream = new PrintWriter(outFileStream);
//write values of primitive data types to the stream
outStream.println(987654321);
outStream.println("Hello, world.");
outStream.println(true);
outStream.close();
}
}

Sample Reading from
Textfileimport java.io.*;
class Ch12TestBufferedReader {
//set up file and stream
FileReader fileReader = new FileReader(inFile);
BufferedReader bufReader = new BufferedReader(fileReader);
String str;
str = bufReader.readLine();
int i = Integer.parseInt(str);
//similar process for other data types
bufReader.close();
}
}

Sample Reading Textfile
using Scanner
import java.io.*;
class Ch12TestScanner {
//open the Scanner
Scanner scanner = new Scanner(inFile);
//get integer
int i = scanner.nextInt();
//similar process for other data types
scanner.close();
}
}

try{
// read line one by one till all line is read.
Scanner scanner = new Scanner(inFile);
while (scanner.hasNextLine()) { //check if there are more line
String line = scanner.nextLine();
System.out.println(line);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
Sample Reading Textfile using Scanner
Code fragments shows how to read the whole contents of a file
Use hasNextLine() & nextLine() methods

File Input & Output

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