Streams Dwight Deugo ([email protected]) Nesa Matic ([email protected]) .

StreamsStreams

Dwight Deugo ([email protected])Dwight Deugo ([email protected])Nesa Matic ([email protected])Nesa Matic ([email protected])

www.espirity.com

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Additional Contributors

None as of September, 2004

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Module Overview

1. Streams

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Module Road Map

1. Streams What are streams? Stream types Character streams Byte streams Filter streams Object Serialization

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What is a Stream?

From an abstract point of view a stream is simply a sequence

From an implementation point of view you can think of a stream as a list

A stream has a start, and end, and a position

Streams can let us model systems that have state without ever using assignment or mutable data

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Stream Concept To read data serially, a Java program:

Opens a stream to a data source file remote socket

Reads the information serially To write data serially, a Java program:

Opens a stream to a data source Writes the information serially

Data source type doesn’t matter, concepts of reading and writing are the same

Once you understand the top level classes (java.io.Reader, java.io.Writer), the remaining classes are much of the same

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Stream Types Two different types of streams:

Character streams Support reading and writing of characters, text Contain 16-bit Unicode characters Supported through Reader and Writer classes

Byte streams Support reading and writing of any bytes Contain 8-but bytes Supported through InputStream and OutputStream

classes It is possible to do conversion between

character streams and byte stream InputStreamReader and OutputStreamWriter

classes can be used

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Character Streams vs. Byte Streams Character streams (reader and writer) should

be used because: They can handle any character in the Unicode

character set (while the byte streams are limited to ISO-Latin-1 8-bit bytes)

They are easier to internationalize because they are not dependent upon a specific character encoding

They use buffering techniques internally and are therefore potentially much more efficient than byte streams

Byte streams should be used for handling binary data, such as image and sound files

All stream classes are in the java.io package

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Character Streams Hierarchy

Object

OutputStreamWriter

InputStreamReader

<<abstract>>

Writer<<abstract>>

Reader

FileWriterFileReader BufferedWriter

BufferedReader

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Reader and Writer classes Parent classes for character-stream based

classes Used to read and write 16-bit character

streams Important methods for reading and

writing to streams found in these and their descendent classes include the following: int read()

int read(char buffer[]) int read(char buffer[], int offset, int length) int write(int aCharacter) int write(char buffer[]) int write(char buffer[], int offset, int length)

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FileReader and FileWriter classes

FileReader is used for reading streams of characters from a file

FileWriter is used for writing streams of characters to a file

File inputFile = new File("source.txt");

File outputFile = new File("final.txt");

FileReader in = new FileReader(inputFile);

FileWriter out = new FileWriter(outputFile);

int aCharacter;

while ((aCharacter = in.read()) != 1)

out.write(aCharacter);

in.close();

out.close();

It’s important to close the stream

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Writing and Reading Example Create a file

Read the file

FileWriter writefile = new FileWriter("source.txt");

writefile.write('A');

writefile.write("bcdefghi");

writefile.close();

Abcdefghi

FileReader inFile = new FileReader("source.txt");

inFile.skip(3); // skips next 3 chars ('A' 'b' 'c' )

char[] characterArray = new char[10];

inFile.read(characterArray ); // ['d' 'e' 'f' 'g' 'h' 'i']

String myString = new inFile.read(characterArray).trim();

inFile.close();

myString

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BufferedReader and BufferedWriter classes

Used for buffering characters as being read or written Buffer is used for storing data without

conversion Buffer size can be set Should wrap any reader/writer whose

read/write operations may be inefficientBufferedReader reader

= new BufferedReader(new DataInputStream(System.in));

String input;

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

... //do something interesting here

}

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Byte Streams HierarchyInputStrea

m

FileInputStream

FilterInputStream

DataInputStream

BufferdInputStream

ObjectInputStream

Filter streams Object streamsFilter streamsOuputStrea

m

FileOutputStream

FilterOutputStream

DataOutputStream

BufferdOutputStream

ObjectOutputStream

PrintStream

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Specialized Byte Streams File streams

Used for writing data to files and reading data from files

Object streams Used for reading and writing objects Also called object serialization

Filter streams Used for filtering data as it’s being read from

streams, or written to the streams They work with primitive data types (int, double,

boolean) They implement DataInput and DataOutput interfaces

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Filter Streams Filter data as it's being read from or written to

a stream Subclasses of the FilterInputStream a and

FilterOutputStream Constructed on another stream (the

underlying stream) Read method reads input from the underlying

stream, filters it, and passes on the filtered data to the caller

Write method writes data to the underlying stream Filtering done by the streams depends on the

stream Some streams buffer the data, some count data as

it goes by, and others convert data to another form

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Using Filter Streams…

For reading primitive data types DataInputStream class can be used

FileInputStream inputFile =

new FileInputStream("price.cat");

DataInputStream inputStream =

new DataInputStream(inputFile);

double price= inputStream.readDouble();

inputStream.close();

It’s importantto know what’s in the stream

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…Using Filter Streams

For writing primitive data types A DataOutputStream can be used

FileOutputStream outputFile =

new FileInputStream("price.cat");

DataOutputStream outputStream =

new DataInputStream(outputFile );

outputStream.writeDouble(234.56);

outputStream.flush();

outputStream.close();

Forces data to be written

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Streams in System Class System.in - standard input

An instance of BufferedInputStream class Used to read lines of text that user enters

System.out - standard output Instance of PrintStream class Used to send text to the Console

System.err - error output Instance of PrintStream class Used to send error text to the error file

BufferedReader reader

= new BufferedReader(new DataInputStream(System.in));

String input;

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

System.out.println(input);}

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Object Serialization Supported with ObjectOutputStream

class Serialized object class must implement the

Serializable interfaceGregorianCalendar calendar = new GregorianCalendar();

ObjectOutputStream out = new ObjectOutputStream

(new FileOutputStream("calendar.dat"));

out.writeObject(calendar);

out.close();

public class java.util.GregorianCalendar extents java.util.Calendar{…

public class java.util. GregorianCalendar extents java.lang.Object implements java.lang.Cloneable, java.io.Serializable{…

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Serialization Protocol Classes that perform serialization and

deserialization must implement special methods:

Object state is saved by writing the individual fields to the ObjectOutputStream

Object state is retrieved by reading the individual fields back from the ObjectInputStream

private void writeObject(java.io.ObjectOutputStream out) throws IOException

private void readObject(java.io.ObjectInputStream in)

throws IOException, ClassNotFoundException;

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Object Deserialization

Supported with ObjectInputStream class You must know the order in which things

were written in order to cast to the correct type

ObjectInputStream in = new ObjectInputStream

(new FileInputStream("calendar.dat"));

GregorianCalendar calendar = (GregorianCalendar)in.readObject();

in.close();

It’s important to know what’s in the stream for casting

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Module Summary

In this module you have learned: What streams are What are different types of streams in Java Differences between character streams

and byte streams What filter streams are Streams used in the System class How to serialize objects

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Labs Slide!

Lab: Steams