java.io - streams and files

marcello.thiry@gmail.comPackage java.io: streams and files

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Supplementary material

marcello.thiry@gmail.com http://ideas.scup.com/pt/files/2013/06/conte%C3%BAdo.jpg

1. Basic glossary

2. Streams and files in Java (reading and writing)

3. Text files in Java (reading and writing)

Contents.

Basic Glossary

marcello.thiry@gmail.com

if you do not know much about

charsets, code pages, encoding,

ASCII, UNICODE, etc.

Before we start…

Take a look in this article

http://www.joelonsoftware.com/articles/Unicode.html

The Absolute Minimum Every Software Developer Absolutely,

Positively Must Know About Unicode and Character Sets (No Excuses!)by Joel Spolsky

It’s a bit old, but a good start

Set of characters you can use

Charset Repertoire

ISO-8859-1 - Western Alphabet ISO-8859-5 - Cyrillic Alphabet

JIS X 0208 - Japanese Alphabet ISO-8859-7 - Greek Alphabet

A numerical value assigned to each

character in a character set

repertoire

Can be represented by one or more bytes

Code Point Code Position

A = 41hex (ASCII)

a = 61hex (ASCII)

A = 00hex 41hex (UNICODE)

= 33hex 34hex (UNICODE)

a = 00hex 61hex (UNICODE)

= 42hex F4hex (UNICODE)

A coded character set*

*Sometimes called code page

UNICODE

The way algorithm the coded

characters are stored into memory

Character Encoding

UTF-16

UTF-32

Character Encoding

http://www.w3.org/International/articles/definitions-characters/

Sequence of data elements made

available over time

Stream

Which data elements?

Byte raw binary data

Character

Primitive data type

Object

Stream

A continuous stream of bytes

stored in a file system

Stream File

http://ebiznet2u.com/wp-content/uploads/2012/07/file-viewer.jpg

Region of a physical memory

storage used to temporarily store

data while it is being moved from

one place to another

Data Buffer

Conversion of an object to a series

of bytes, which lets a program

write whole objects out to streams

and read them back again

Object Serialization

Set of routines, protocols, and

tools to access a software

component/module without the need

to know details of its

implementation

Application Programming*

Interface API

*Also used: program

Streams in Java

Characters automatically translates to and

from the local character set

Data primitive data type and String values

Objects

What is a stream in Java?

handle I/O of

Optimizes input and output by

reducing the number of calls to

the native API

And a buffered stream?

Where to use?

Network connections sockets

Blob database fields

System.in standard input

System.out standard output

To write/read into/from

https://docs.oracle.com/javase/8/docs

/api/java/io/InputStream.html

https://docs.oracle.com/javase/8/docs

/api/java/io/OutputStream.html

read() reads a single byte

read(byte[] b) reads b.length bytes into an array

read(byte[] b, int off, int len) reads len bytes into an array,

starting from the position off

skip(long n) skips discards n bytes

close() closes the stream

https://docs.oracle.com/javase/8/docs/api/java/io/InputStream.html

mark(int readlimit) marks the current position in

this input stream

reset() repositions this stream to the position at

the time the mark method was last called

https://docs.oracle.com/javase/8/docs/api/java/io/InputStream.html

And, if markSupported()...

write(int b) writes a single byte

write(byte[] b) writes b.length bytes from the array

write(byte[] b, int off, int len) writes len bytes from the array

starting at offset off

flush() forces any buffered output bytes to be written

Source

of data

Input stream for

reading BYTES from

a FILE

Binary files

UNICODE files

https://docs.oracle.com/javase/8/docs/api/java/io/FileInputStream.html

String fileName = "c:/temp/file.exe";

int byteValue;

InputStream in = new FileInputStream(fileName);

while ((byteValue = in.read()) != -1) {

System.out.format("[%2X]\n", byteValue);

in.close();

catch (IOException ex) {...}

int byteValue;

InputStream in = new FileInputStream(fileName);

while ((byteValue = in.read()) != -1) {

System.out.format("[%2X]\n", byteValue);

in.close();

catch (IOException ex) {...}

FileNotFoundException

For constructors that use a file

name as an argument

If the named file does not exist, is a

directory rather than a regular file, or

for some other reason cannot be opened

for reading

Try-with-resources

byte[] bytes = new byte[500];

int read;

try (InputStream in = new FileInputStream(fileName)) {

while ((read = in.read(bytes)) != -1) {

System.out.format("%d bytes read:\n", read);

for (int i = 0; i < read; i++) {

System.out.format("[%2X]", bytes[i]);

System.out.println();

} catch (IOException ex) {...}

Try-with-resources

byte[] bytes = new byte[500];

int read;

try (InputStream in = new FileInputStream(fileName)) {

while ((read = in.read(bytes)) != -1) {

System.out.format("%d bytes read:\n", read);

for (int i = 0; i < read; i++) {

System.out.format("[%2X]", bytes[i]);

System.out.println();

} catch (IOException ex) {...}

From this point forward, all

our examples will use the

Try-with-resources statement

Destination

of data sink

Output stream

for writing

BYTES to a FILE

Binary files

UNICODE files

https://docs.oracle.com/javase/8/docs/api/java/io/FileOutputStream.html

String fileName = "d:/downloads/mynewfile.txt";

try (OutputStream out = new FileOutputStream(fileName)) {

byte[] bytes = new byte[]{'T', 'E', 'S', 'T', 32, 0x41};

out.write(bytes);

} catch (IOException e) {...}

And if I want to

write or read

objects from a stream?

try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("user.dat"))) {

out.writeObject(new User("a", "a"));

out.writeObject(new User("b", "b"));

out.writeObject(new User("c", "c"));

out.flush();

User u;

try (ObjectInputStream in = new ObjectInputStream(new FileInputStream("user.dat"))) {

for (int i = 0; i < 3; i++) {

u = (User) in.readObject();

System.out.println(u.getLogin() + ", " + u.getPassword());

} catch (IOException | ClassNotFoundException e) {...}

Explore yourself!

And beyond!

Using text files

In Java

Source

of data

class Reader

Abstract class for reading

character streams

read(): reads a single character

read(char[]): reads characters into an array

skip(long): skips N characters

close(): closes the stream

https://docs.oracle.com/javase/8/docs/api/java/io/Reader.html

class FileReader

Reads character files

Default character encoding

Default byte-buffer size

https://docs.oracle.com/javase/8/docs/api/java/io/FileReader.html

String fileName = "temp.txt";

String line;

FileReader fileReader = new FileReader(fileName);

class BufferedReader

Usually wraps FileReader to

improve efficiency

https://docs.oracle.com/javase/8/docs/api/java/io/BufferedReader.html

Buffer size may be specified

Reading of characters, arrays, and lines

String line;

try (BufferedReader bufferedReader = new BufferedReader(fileReader)) {

while ((line = bufferedReader.readLine()) != null) {

System.out.println(line);

String line;

String fileName = “temp.txt";

String line;

} catch (FileNotFoundException ex) {...

} catch (IOException ex) {...

But what about the class

InputStreamReader?

https://docs.oracle.com/javase/8/docs/api/java/io/InputStreamReader.html

Reads bytes and decodes them into

characters using a specified charset

Bridge from byte streams to

character streams

class InputStreamReader

https://docs.oracle.com/javase/8/docs/api/java/io/InputStreamReader.html

For top efficiency, consider

wrapping within a BufferedReader

String line;

InputStreamReader inputReader = new InputStreamReader(System.in);

try (BufferedReader bufferedReader = new BufferedReader(inputReader)) {

while (!"".equals(line = bufferedReader.readLine())) {

Now we can use

FileInputStream e

InputStreamReader to

read a UNICODE file

FileInputStream in = new FileInputStream("c:/temp/fileUTF16.txt");

InputStreamReader inReader = new InputStreamReader(in, "UTF-16");

try (BufferedReader buffReader = new BufferedReader(inReader)) {

int character;

while ((character = buffReader.read()) != -1) {

System.out.print((char) character);

Destination

of data

class Writer

Abstract class for writing to

character streams

write(int): writes a single character

write(char[]): writes an array of characters

write(String): writes a string

close(): closes the stream

https://docs.oracle.com/javase/8/docs/api/java/io/Writer.html

class FileWriter

Writes in character files

Default character encoding

Default byte-buffer size

https://docs.oracle.com/javase/8/docs/api/java/io/FileWriter.html

String fileName = “temp.txt";

FileWriter fileWriter = new FileWriter(fileName);

FileWriter fileWriter = new FileWriter(fileName, false);

FileWriter fileWriter = new FileWriter(fileName, true);

String fileName = "c:/temp.txt";

try (FileWriter fileWriter = new FileWriter(fileName)) {

fileWriter.write("My first line");

fileWriter.write("\r\n"); // new line - windows

fileWriter.write("My second line");

class BufferedWriter

Usually wraps FileWriter to

improve efficiency

https://docs.oracle.com/javase/8/docs/api/java/io/BufferedWriter.html

Buffer size may be specified

Writing characters, arrays, and lines

String fileName = "c:/temp/MyFile.txt";

FileWriter writer = new FileWriter(fileName, true);

try (BufferedWriter buffWriter = new BufferedWriter(writer)) {

buffWriter.write("My first line");

buffWriter.newLine();

buffWriter.write("My second line!");

But what about the class

OutputStreamWriter?

https://docs.oracle.com/javase/8/docs/api/java/io/OutputStreamWriter.html

Characters written to it are encoded

into bytes using a specified charset

bridge from character to byte

streams

class OutputStreamWriter

For top efficiency, consider

wrapping within a BufferedWriter

https://docs.oracle.com/javase/8/docs/api/java/io/OutputStreamWriter.html

OutputStreamWriter outWriter = new OutputStreamWriter(System.out);

try (BufferedWriter buffWriter = new BufferedWriter(outWriter)) {

buffWriter.write("Printing a line on the console");

buffWriter.write("Printing a second line...\r\n");

} catch (IOException e) {}

Now we can use

FileOutputStream e

OutputStreamWriter to

write into a UNICODE file

String fileName = "c:/temp/MyNewFile.txt";

FileOutputStream out = new FileOutputStream(fileName);

OutputStreamWriter outWriter = new OutputStreamWriter(out, "UTF-16");

try (BufferedWriter buffWriter = new BufferedWriter(outWriter)) {

buffWriter.write("UNICODE text");

buffWriter.write("Some more...");

References.

Java™ Platform, Standard Edition 8 API Specification.

https://docs.oracle.com/javase/8/docs/api/overview-summary.html.

The Java™ Tutorials. https://docs.oracle.com/javase/tutorial/.

java.io - streams and files

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