Io18 File Handeling

7/28/2019 Io18 File Handeling

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Input and Output

Text and Binary I/OChapter 19

Introduction to Java Y.Daniel Liang 1


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File Input and Output

Programs to run are in the FileIO folder:

1. ScannerDriver.java

2. ScannerFile.javaScannerTextfile.txt (input file)

ScannerOutputfile.txt (output file)

3. Inventory.java (Driver)

InventoryItem.java

input file: inventory.txtIntroduction to Java Y.Daniel Liang 2


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Programs to run contd.

4. TestPrint.java

5. TestPrintWriter.java Team #1

output file: pw.txt

6. TestFileStream Team #2

7. TestDataStream Team #3

output file: temp.datIntroduction to Java Y.Daniel Liang 3


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Programs to run contd.

8. Copy.java Team #4

Welcome.java (source file)Temp. java (target file)

9. TestObjectStreamForArray Team #5



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Objectives

To understand how I/O is processed in Java

To distinguish between text I/O and binary

I/O

To read data from the console using the

Scanner class

To read data from a file using the Scanner

class Introduction to Java Y.Daniel Liang 5


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Objectives contd.

To write data to a file using the PrintWriterclass

To read and write bytes using FileInputStream and FileOutputStream

To read and write primitive values andstrings using DataInputStream/

DataOutPutStream



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Objectives contd.

To store and restore objects usingObjectOutputStream and ObjectInputStream

and to understand how objects are serialized

and what kind of objects can be serialized

To use the Serializable interface to enable

objects to be serializable

To know how to serialize arrays



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Input and Output

Introduction

Data held in variables, arrays, and objects aretemporary. Data are lost when the program

ceases processing

To save the data created during the life of the

program, you must save the data in a file onto a

disk, CD, or some other storage device .

Then the file can be transported and can be

read later by other programs



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ntro uct onRedirection of Input and Output

It is tedious to test programs by typing data in for every test run.

Testing is far easier if the program reads input from a file.

You can prepare the file once and reuse it for many tests.

The command line interfaces of most operating systems provide a way

to link a file to the input of a program, as if all the characters in the

file had actually been typed by the user .


t t


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ntro uct onRedirection of Input and Output example:

java ClassName < data.txt > output.txt

Use input redirection to avoid repetitive typing during testing.

Use output redirection to save your program output in a file.



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Introduction contd.

Data are stored in two basic formats:

text and binary

Text file is represented in human-readable form

Examples oftext files that you can read:

Files that are created with a simple text exitor ,

such as Windows Vista NotePad, as well as Java

source code, and HTML Files



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Introduction contd.

Data stored in a binary file are represented in binary

form. You cannot read binary files. The files are

designed to be read by programs.

Binary files consists of a series of bits

Text file consist of a series of characters.



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Introduction contd.

The advantages of binary files are that:

1. They are more efficient to process than text

files. Text I/O requires encoding and decodingwhereas binary I/O does not.

2. Binary files are independent of the encoding

scheme on the host machine and thus are

portable.



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Introduction to Java Y.Daniel Liang

The Scanner class

The simplest tool for reading text files is theScanner class.

Class java.util.Scanner

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to Get Input from the Console Using the Scanner Class

Run ScannerDriverprogram in the FileIO Folder

next(): reading a string. A string delimeter is a space

nextByte(): reading an integer of the byte type

nextShort(): reading an integer of the short type

nextInt(): reading an integer of the int type

nextLong(): reading an integer of the long type

nextFloat(): reading a number of the float type

nextDouble(): reading number of the double type

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To Get Input from a text file Using the Scanner Class

Run ScannerFileProgram in the FileIO Folder

The action of reading data from a

file is called file input.

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f f


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To read text data from a disk file, we use the FileReaderand

BufferedReaderobjects.

We first construct a FileReader object

with the name of the file Then we associate a BufferedReaderobject to the file.

Then we read data, using the readLine method of the

BufferedReader class.

Finally, we convert the String to a primitive data type as

necessary.

Run Programs: InventoryItem (Driver)Inventory

Use input file: inventory.txt

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Sample code for processing a text file:

String file = customer.txt;

String line;try{

FileReader fr = new FileReader (file);

BufferedReader inFile = new BufferedReader(fr);

line = inFile.readLine();

while(line != null && count < MAX) {tokenizer = new StringTokenizer(line);

name = tokenizer.nextToken();

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C td


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Contd.

try{

custNumber = Long.parseLong (tokenizer.

nextToken());balance = Double.parseDouble(tokenizer.

nextToken());

phone = tokenizer.nextToken();

custsArray[count++] = new Customer(name,

custNumber, balance, phone);

} // end inner try block

catch (NumberFormatException e) { }

line = inFile.readLine();

}// end while block

inFile.close();

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PrintWriter is an object we use to generate an output text

file.

PrintWriter supports only two output methods:

print

println (for print line)

Run program TestPrinterWriter

output file: pw.txt

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Constructors of the

PrintWriter

PrintWriter(Writer out)

PrintWriter(Writer out, booleanautoFlush)

PrintWriter(OutputStream out)

PrintWriter(OutputStream out, boolean

autoFlush)

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PrintWriter Methods


void print(Object o)

void print(String s)

void println(String s)

void print(char c)

void print(char[] cArray)

void print(int i)

void print(long l)

void print(float f) void print(double d)

void print(boolean b)

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St


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StreamsIn Java, all I/O is handled in streams.

Astream is an abstraction of the continuous one-way flow of data. The programreceives data thru the input stream and sends data thru the output stream.

Any source of input or destination for output is called a stream.

Program

Output Stream

File

Input Stream

Introduction to Java Y.Daniel Liang 23Introduction to Java Y.Daniel Liang


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Streams

All streams except random-access filestreams flow in only one direction;therefore if you want to input and output,you need two separate stream objects.

Streams are objects. Stream objects havemethods that read and write data , flush the

stream, close the stream, and count thenumber of bytes in the stream, etc.

Bi I/O


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Binary I/O

Text I/O requires encoding and decoding. The JVM converts a

Unicode to a file specific encoding when writing a character and

coverts a file specific encoding to a Unicode when reading acharacter.

Binary I/O does not require conversions. When you write a byte to

a file, the original byte is copied into the file. When you read a byte

from a file, the exact byte in the file is returned.Text I/O program

The Unicode of

the characterEncoding/

Decoding

Binary I/O program

A byte is read/written(b)

(a)

e.g.

"199"

The encoding of the character

is stored in the file

0x31

e.g.

199 00110111

00110001 00111001 00111001

0x39 0x39

0xC7

The same byte in the file


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Binary I/O Classes

The design of the Java I/O classes is a good example ofapplying inheritance, where common methods are generalized

in superclasses, and sub classes provide specialized methods.

See next slide. It list some of the classes for performingbinary I/O.

InputStream is the root for binary input classes.

OutputStream is the root for binary output classes


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27Introduction to Java Y.Daniel Liang 27

Binary I/O Classes

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream


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Stream Classes

The stream classes can be categorized into two types: bytestreams and character streams.

The InputStream/OutputStream class is theroot of all byte stream classes,

the Reader/Writer class is the root of all characterstream.

The subclasses ofInputStream/OutputStream are analogous to thesubclasses ofReader/Writer.


To Read and Write Binar Data from /To a Disk File


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To Read and Write Binary Data from /To a Disk File

FileInputStream and FileOutputStream- all methods in these

classes are inherited from InputStream and OutputStream

(java.io. FileInputStream and java.io.FileOutputStream )

Create a FileInputStream:

FileInputStream inputStream = new

FileInputStream inputStream (

input.bin);

To write Binary Data to a Disk File:

FileOutputStream outputStream = new

FileOutputStream outputStream (

output.bin);

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I tSt


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InputStream

java.io.InputStream

int read(byte[] b) throws IOException

abstract int read() throws IOException

void close() throws IOException

int available() throws IOException

long skip(long n) throws IOException

Note: The abstract InputStream class defines

the methods for the input stream of bytes

O t tSt


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OutputStream

java.io.OutputStream

abstract void write(int b) throws

IOException

void write(byte[] b) throws IOException

void close() throws IOException

void flush() throws IOException

Note: The abstract OutputStream class

defines the methods for the output

stream of bytes.


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FileInputStream/FileOutputStream

FileInputStream/FileOutputStream associates abinary input/output stream with an external file.

All the methods inFileInputStream/FileOuptputStream are inheritedfrom its superclasses.

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream


DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream


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FileInputStream

To construct a FileInputStream, use the following

constructors:

public FileInputStream(String filename)

public FileInputStream(File file)

A java.io.FileNotFoundException would occur if you attempt to

create a FileInputStream with a nonexistent file.


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FileOutputStreamTo construct a FileOutputStream, use the following constructors:

public FileOutputStream(String filename)

public FileOutputStream(File file)

public FileOutputStream(String filename, boolean append)

public FileOutputStream(File file, boolean append)

If the file does not exist, a new file would be created. If the file alreadyexists, the first two constructors would delete the current contents inthe file. To retain the current content and append new data into the

file, use the last two constructors by passing true to the appendparameter.


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FileOutputStream

The program TestFileStream uses binary I/O towrite ten bytes values from 1 to 10 to a filenamed temp.dat

TestFileStream Run
http://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestFileStream.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestFileStream.html


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FilterInputStream/FilterOutputStream

Filter streams are streams that filter bytes for some purpose. The basic byte input stream providesa read method that can only be used for reading bytes.

If you want to read integers, doubles, or strings, you need a filter class to wrap the byte inputstream. Using a filter class enables you to read integers, doubles, and strings instead of bytes andcharacters. FilterInputStream and FilterOutputStream are the base classes for filtering data.

When you need to process primitive numeric types, use DatInputStream and DataOutputStream

to filter bytes.

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream


DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream


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DataInputStream/DataOutputStreamDataInputStream reads bytes from the streamand converts them into appropriate primitive

type values or strings.

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream


DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

DataOutputStream converts primitive type values

or strings into bytes and output the bytes to the

stream.


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DataInputStream

DataInputStream extends FilterInputStreamand implements the DataInput interface.

java.io.DataInput

+readBoolean(): boolean

+readByte(): byte

+readChar(): char

+readFloat(): float

+readDouble(): float

+readInt(): int

+readLong(): long

+readShort(): short

+readLine(): String

+readUTF(): String

Reads a Boolean from the input stream.

Reads a byte from the input stream.

Reads a character from the input stream.

Reads a float from the input stream.

Reads a double from the input stream.

Reads an int from the input stream.

Reads a long from the input stream.

Reads a short from the input stream.

Reads a line of characters from input.

Reads a string in UTF format.

InputStream

FilterInputStream

DataInputStream

+DataInputStream(in: InputStream)


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DataOutputStreamDataOutputStream extends FilterOutputStream and implements the

DataOutput interface.java.io.DataOutput

+writeBoolean(b: Boolean): void

+writeByte(v: int): void

+writeBytes(s: String): void

+writeChar(c: char): void

+writeChars(s: String): void

+writeFloat(v: float): void

+writeDouble(v: float): void

+writeInt(v: int): void

+writeLong(v: long): void

+writeShort(v: short): void

+writeUTF(s: String): void

Writes a Boolean to the output stream.

Writes to the output stream the eight low-order bits

of the argument v.

Writes the lower byte of the characters in a string to

the output stream.Writes a character (composed of two bytes) to the

output stream.

Writes every character in the string s, to the output

stream, in order, two bytes per character.

Writes a float value to the output stream.

Writes a double value to the output stream.

Writes an int value to the output stream.

Writes a long value to the output stream.

Writes a short value to the output stream.

Writes two bytes of length information to the output

stream, followed by the UTF representation ofevery character in the string s.

OutputStream

FilterOutputStream

DataOutputStream

+DataOutputStream(

out: OutputStream)


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Characters and Strings in Binary I/O

A Unicode consists of two bytes. ThewriteChar(char c) method writes the Unicode ofcharacter c to the output.

The writeChars(String s) method writes theUnicode for each character in the string s to the

output.


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Characters and Strings in Binary I/OWhy UTF-8? What is UTF-8?

UTF-8 (8-bit UCS/Unicode Transformation Format) is a codingscheme that allows systems to operate with both ASCII and Unicodeefficiently. Most operating systems use ASCII.

Java uses Unicode. The ASCII character set is a subset of theUnicode character set. Since most applications need only the ASCIIcharacter set, it is a waste to represent an 8-bit ASCII character as a

16-bit Unicode character.


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Characters and Strings in Binary I/OWhy UTF-8? What is UTF-8?

The UTF-8 is an alternative scheme that stores a character using 1, 2,or 3 bytes. ASCII values (less than 0x7F) are coded in one byte.

Unicode values less than 0x7FF are coded in two bytes. OtherUnicode values are coded in three bytes.


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Using DataInputStream/DataOutputStream

Data streams are used as wrappers on existing input and output

streams to filter data in the original stream. They are created using thefollowing constructors:

public DataInputStream(InputStream instream)

public DataOutputStream(OutputStream outstream)

The statements given below create data streams. The first statementcreates an input stream for file in.dat; the second statement creates anoutput stream for file out.dat.

DataInputStream infile =

new DataInputStream(new FileInputStream("in.dat"));

DataOutputStream outfile =

new DataOutputStream(new FileOutputStream("out.dat"));


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DataInputStream and DataOutputStream read and write data primitive

types values and strings in a machine-independent fashion.

Thereby enabling you to write a data file on one machine and read iton another machine that has a different operating system or file

structure.

An application uses a data output stream to write data that can later beread by a program using a data input stream.


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The TestDataStream program writes student names and

scores to a file named temp.dat and reads the data back fromthe file.

TestDataStream Run

Order and Format
http://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestDataStream.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestDataStream.html


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Checking End of File

TIP: If you keep reading data at the end of a stream, an EOFExceptionwould occur. So how do you check the end of a file? You can useinput.available() to check it. input.available() == 0 indicates that it isthe end of a file.

Order and Format

See program TestDataStream

CAUTION: You have to read the data in the same order and sameformat in which they are stored. For example, since names are writtenin UTF-8 using writeUTF, you must read names using readUTF.


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BufferedInputStream/BufferedOutputStream

Reduces the number of reads and writes

Using buffers to speed up I/O

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream


DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

BufferedInputStream/BufferedOutputStream does not contain new

methods.

All the methods BufferedInputStream/BufferedOutputStream are inherited

from the InputStream/OutputStream classes.


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Constructing


// Create a BufferedInputStreampublic BufferedInputStream(InputStream in)

public BufferedInputStream(InputStream in, int bufferSize)

// Create a BufferedOutputStream

public BufferedOutputStream(OutputStream out)

public BufferedOutputStream(OutputStream out, int bufferSize)

i


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Constructing


Improve the performance of the TestDataStream program byadding buffers in the streams in lines 6-7 and 21-22, as follows:

DataOutputStream output = new DataOutputStream(

new BufferedOutputStream(new

FileOutputStream(temp.dat));

DataInputStream input = new DataInputStream(

new BufferedInputStream(newFileInputStream(temp.dat));

C S di C il


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Case Studies: Copy File

This case study develops a program that copies files. The user needs

to provide a source file and a target file as command-line argumentsusing the following command:

java Copy source target

The program copies a source file to a target file and displays thenumber of bytes in the file. If the source does not exist, tell the userthe file is not found. If the target file already exists, tell the user thefile already exists.

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Case Studies: Copy File

The program uses the File class to check whether the source file

and the target file exist.

The program copies a Java source file to an identical Java file

Copy Run

bj /

Optional
http://localhost/var/www/apps/conversion/tmp/scratch_5/html/Copy.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/Copy.html


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Object I/O

DataInputStream/DataOutputStream enables you to perform I/O for

objects in addition to primitive type values and strings.ObjectInputStream/ObjectOutputStream enables you to perform I/Ofor objects in addition for primitive type values and strings.

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream


DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

p

Obj S


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ObjectInputStream

ObjectInputStream extends InputStream andimplements ObjectInput and ObjectStreamConstants.

java.io.ObjectInput

+readObject(): Object Reads an object.

java.io.InputStream

java.io.ObjectInputStream

+ObjectInputStream(in: InputStream)

java.io.DataInput

ObjectStreamConstants

Obj O S


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ObjectOutputStream

ObjectOutputStream extends OutputStream andimplements ObjectOutput and ObjectStreamConstants.

java.io.ObjectOutput

+writeObject(o: Object): void Writes an object.

ava.io.OutputStream

java.io.ObjectOutputStream

+ObjectOutputStream(out: OutputStream)

java.io.DataOutput

ObjectStreamConstants

U i Obj S


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Using Object Streams

You may wrap an ObjectInputStream/ObjectOutputStream on any

InputStream/OutputStream using the following constructors:

// Create an ObjectInputStream

public ObjectInputStream(InputStream in)

// Create an ObjectOutputStream

public ObjectOutputStream(OutputStream out)

TestObjectOutputStream Run

TestObjectInputStream Run

Object Streams
http://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectOutputStream.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectInputStream.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectInputStream.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectOutputStream.html


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Object StreamsThe ObjectInputStream class can save entire object out to a

disk, and the C class can read them back in.

Objects are saved in binary format; therefore, you use streams

and not writers.

For example, you can write a Bank Account object to a file:

BankAccount bk = new ;

ObjectOutputStream out = new ObjectOutputStream (

new FileOutputStream(bank.dat));

Out.writeObject(bk);

Object Streams


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Object StreamsUse Object streams to save and restore all instance fields of an

object automatically.

When reading the object back in, you use the readObject

method of the ObjectInputStream class. That method returns

an Object reference, so you need to remember the types of the

objects that you saved and use a cast:

ObjectInputStream in = new ObjectInputStream (

new FileInputStream(bank.dat));

BankAccount bk = (BankAccount) in.readObject( );

The readObject method can throw a ClassNotFoundException-

it is a checked exception, so you need to catch or declare it.

The Serializable Interface


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The Serializable InterfaceNot all objects can be written to an output stream. Objects that can be

written to an object stream is said to beserializable. A serializable

object is an instance of the java.io.Serializable interface. So the classof a serializable object must implement Serializable.

The Serializable interface is a marker interface. It has no methods, so

you don't need to add additional code in your class that implementsSerializable.

public MyClass implements Serializable{

// Definition of the class

}

Implementing this interface enables the Java serialization mechanism

to automate the process of storing the objects and arrays.

Th t i t K d


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The transient Keyword

If an object is an instance of Serializable, but it containsnon-serializable instance data fields, can the object be

serialized?

The answer is no.

To enable the object to be serialized, you can use the

transient keyword to mark these data fields to tell the JVMto ignore these fields when writing the object to an object

stream.

Th t i t K d t


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The transient Keyword, cont.

Consider the following class:

public class Foo implements java.io.Serializable {

private int v1;

private static double v2;

private transient A v3 = new A();

}

class A { } // A is not serializable

When an object of the Foo class is serialized, only variable v1 is

serialized.Variable v2 is not serialized because it is a static variable, and

variable v3 is not serialized because it is marked transient. If v3 werenot marked transient, a java.io.NotSerializableException would occur.

S i li i A


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Serializing Arrays

An array is serializable if all its elements are serializable.

So an entire array can be saved using writeObject into a file

and later restored using readObject. Listing 16.12 stores an

array of five int values an array of three strings, and an

array of two JButton objects, and reads them back to

display on the console. Note : java.io.File implements

Comparable and Serializable

TestObjectStreamForArray Run

More about the Serializable Interface

Th f i bj i ll d i li i b
http://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectStreamForArray.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/html/TestObjectStreamForArray.html


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The process of saving objects to a stream is called serialization because

each object is assigned a serial number on the stream. If the same object

is saved twice, only the serial number is written out the second time.

When the objects are read back in, duplicate serial numbers are restored

as reference to the same object.

Why dont all classes implementSerializable

?

For security reasons, some programmers may not want to serialize

classes with confidential contents. Once a class is serializable, anyone

can write its objects to disk and analyze the disk file.

There are also some classes that contain values that are meaningless oncea program exists, such as operatingsystem-specific font descriptors.

These values should not be serialized.

R d A Fil


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Random Access Files

All of the streams you have used so far are known as

read-only orwrite-only streams.

The external files of these streams aresequentialfiles that

cannot be updated without creating a new file.

It is often necessary to modify files or to insert newrecords into files.

Java provides the RandomAccessFile class to allow a fileto be read from and writen to at random locations.

R d A Fil


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RandomAccessFile

Creates a RandomAccessFile stream with the specified File object andmode.

Creates a RandomAccessFile stream with the specified file name

string and mode.

Closes the stream and releases the resource associated with the stream.

Returns the offset, in bytes, from the beginning of the file to where the

next read or write occurs.

Returns the length of this file.

Reads a byte of data from this file and returns 1 an the end of stream.

Reads up to b.length bytes of data from this file into an array of bytes.

Reads up to len bytes of data from this file into an array of bytes.

Sets the offset (in bytes specified in pos) from the beginning of the

stream to where the next read or write occurs.Sets a new length of this file.

Skips over n bytes of input discarding the skipped bytes.

Writes b.length bytes from the specified byte array to this file, startingat the current file pointer.

Writes len bytes from the specified byte array starting at offset off to

thi fil

DataInput DataInput

java.io.RandomAccessFile

+RandomAccessFile(file: File, mode:String)

+RandomAccessFile(name: String,mode: String)

+close(): void

+getFilePointer(): long

+length(): long

+read(): int

+read(b: byte[]): int

+read(b: byte[], off: int, len: int) : int

+seek(long pos): void

+setLength(newLength: long): void

+skipBytes(int n): int

+write(b: byte[]): void

+write(byte b[], int off, int len)+write(b: byte[], off: int, len: int):

Io18 File Handeling

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