Java Programming

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Lecture 6. Java I/O

Cheng-Chia Chen

Java I/O

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Contents

1. Overview of I/O Streams

2. Using the Data Sink Streams 1. How to Use File Streams

2. How to Use Pipe Streams

3. Using the Processing Streams 1. How to Concatenate Files

2. Working with Filtered Streams 1. How to Use DataInputStream and DataOutputStream

2. Writing Your Own Filtered Streams

3. Object Serialization 1. Serializing Objects

2. Providing Object Serialization for Your Classes

4. Working with Random Access Files 1. Using Random Access Files

2. Writing Filters for Random Access Files

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I/O Streams

A stream is a sequence of bytes (or data or objects) that flow from a source to a destination

In a program, we read information from an input stream and write information to an output stream

for output

for input

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procedure for reading/writing data to/from an input/output stream

general program schema for for reading and writing data from/to an Input/output stream: // for reading: 1. open an input stream // open 2. while there is more information // reading read next data from the stream 3. close the stream. // close // for writing 1. open an output stream // open 2. while there is more information // writing write data to the stream 3. close the stream. // close

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I/O Stream Categories

The java.io package contains many classes that allow us to define various streams with specific characteristics

The classes in the I/O package divide input and output streams into many categories

An I/O stream is either a character stream, which deals with text data byte stream, which deal with byte data

An I/O stream is also either a data stream, which acts as either a source or destination processing stream, which alters or manages information in the stream

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classification of Java I/O streams and their naming conventions By I/O Directions:

for Input => Input for output => Output

By datatype of stream content char data => Reader / Writer byte data => InputStream / OutputStream other primitive data(int, long, float,…) => DataInputStream /DataOutputStream Objects => ObjectInputStream /ObjectOutputStream

By characteristic of stream source / destination 1. for final device : data sink stream file, byte/char array, StringBuffer, pipe purpose: serve as the source/destination of the stream. 2. for intermediate process : processing streams Buffering, Filtering, Byte2Char, Char2Byte, etc. purpose: alters or manages information in the stream

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The java.io package

InputStream

OutputStream

Reader

Writer

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java.io.InputStream and its subclasses

gray => data sink stream white => processing stream // require other streams

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The Java.io.OutputStream and its subclasses

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java.io.Reader and its subclasses

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java.io.Writer and its subclasses

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Basic methods provided in java input classes

Reader and InputStream define similar APIs but for different data types.

Reader contains methods for reading characters and arrays of characters: int read() // read one char, casted as an int. int read(char cbuf[]) // read chars into cbuf, return #chars read. int read(char cbuf[], int offset, int length)

InputStream defines the same methods but for reading bytes and arrays of bytes: int read() int read(byte cbuf[]) int read(byte cbuf[], int offset, int length)

Both classes provide methods for marking a location in the stream, skipping input, and resetting the current position.

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Basic methods provided in java output streams

Writer defines these methods for writing character, string and arrays of characters void write(int c) // 16 low-ordered bits of c is written void write(char[] cbuf [, int off, int len ] ) void write(String s [, int off, int len ] )

OutputStream defines the same methods but for bytes: void write(int c) // 8 low-ordered bits of c is written void write(byte cbuf[]) void write(byte cbuf[], int offset, int length)

All of the streams--readers, writers, input streams, and output streams— are automatically opened when created ! can be closed by calling its close() method, or the garbage collector. will throw IOException on failure of IO operations.

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Standard I/O

There are three standard I/O streams: standard input – defined by System.in standard output – defined by System.out standard error – defined by System.err

We use System.out when we execute println statements System.out and System.err are of the type PrintStream.

System.in is declared to be a generic InputStream reference, and therefore usually must be mapped to a more useful stream with specific characteristics eg: to read char instead of bytes => InputStreamReader cin = new InputStreamReader(System.in);

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java.io.InputStream public abstract class InputStream extends Object Constructor : InputStream() // default no-arg constuctorMethod Summary int available()

Returns the number of bytes that can be read (or skipped over) from this input stream without blocking by the next read(…) .

void close() Closes this input stream and releases any system resources associated with the

stream. abstract int read( [ byte[] b [, int offset, int length ]] )

Reads the next byte of data from the input stream. Reads some number of bytes from the input stream and stores them into the buffer

array b. long skip(long n)

Skips over and discards n bytes of data from this input stream.

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Methods for marking operaqtions

void mark(int readlimit) Marks the current position in this input stream. readlimit is the number of bytes that can be read without invalidating

this mark operation.

void reset() Repositions this stream to the position at the time the mark method

was last called on this input stream.

boolean markSupported() Tests if this input stream supports the mark and reset methods.

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java.io.Reader public abstract class Reader extends Object Abstract class for reading character streams.

The only methods that a subclass must implement are read(char[], int, int) and close().

Most subclasses, however, will override some of the methods defined here in order to provide higher efficiency, additional functionality, or both.

Field Summary protected Object lock

The object used to synchronize operations on this stream. use lock instead of this or synchronized method for efficient time-critical operations

Constructor Summary protected Reader()

Create a new character-stream reader whose critical sections will synchronize on the reader itself.

protected Reader(Object lock) Create a new character-stream reader whose critical sections will synchronize on the

given object.

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Methods summary of java.io.Reader

abstract void close() // Close the stream. int read() // Read a single character. abstract int read( [ char[] cbuf [ ,int off, int len ] ] )

Read characters into an array.

boolean ready() // replacement of available() in java.InputStream Tell whether this stream is ready to be read [ without causing I/Oblocking in next read()].

long skip(long n) // Skip characters. void mark(int limit)

Mark the present position in the stream. limit is the number of chars that can be read without causing failure of the following reset().

void reset() // Reset the stream. boolean markSupported()

Tell whether this stream supports the mark() operation.

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Java.io.OutputStream

public abstract class OutputStream extends Object the superclass of all classes representing an output stream of bytes.

An output stream accepts output bytes and sends them to some sink. Subclasses must always provide at least a method that writes one byte of output.

Constructor Summary OutputStream() // will automatically open this stream for writing

Methods summary void close() // Closes this output stream and releases system resources. void flush()

Flushes this output stream and forces any buffered output bytes to be written out.

void write(byte[] b [, int off, int len ] ) Writes len bytes from the specified byte array starting at offset off to this output

stream.

abstract void write( int b) // Writes the specified byte to this output stream.

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java.io.Writer

public abstract class Writer extends Object Abstract class for writing to character streams.

Subclass must implement write(char[], int, int), flush(), and close(). subclasses may override some of the methods defined here in order to provide

higher efficiency, additional functionality, or both.

Field Summary protected Object lock

The object used to synchronize operations on this stream.

Constructor Summary protected Writer()

Create a new character-stream writer whose critical sections will synchronize on the writer itself.

protected Writer(Object lock) Create a new character-stream writer whose critical sections will synchronize on the

given object.

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java.io.Writer method summary

abstract void close() // Close the stream, flushing it first. abstract void flush() // Flush the stream. abstract void write(char[] cbuf [, int off, int len])

Write a portion of an array of characters.

void write(int c) // Write a single character. void write(String str) // Write a string. void write(String str, int off, int len)

Write a portion of a string.

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JDK ‘s implementation of write( int )

protected Object lock; // for synchronization

private char[] writeBuffer; // for chars to be output

private final int writeBufferSize = 1024;

public void write( int c) throws IOException {

synchronized (lock) {

if (writeBuffer == null){

writeBuffer = new char[writeBufferSize];

}

writeBuffer[0] = (char) c;

write(writeBuffer, 0, 1);

} } Subclasses that intend to support efficient single-character output

should override this method.

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2. The Data Sink Streams

Types of data sinks: memory[array, strings], files, or pipes Naming convention: <sinkType><I/O;CharOrByte>

Sink type character stream byte stream

arrayCharArrayReader, CharArrayWriter

ByteArrayInputStream, ByteArrayOutputStream

StringStringReader, StringWriter

StringBufferInputStream

(deprecated)

PipePipedReader, PipedWriter

PipedInputStream, PipedOutputStream

FileFileReader, FileWriter

FileInputStream, FileOutputStream

Input

Output

Input

Output

Input

Output

Input

Output

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Summary of the data sink streams

FileReader, FileWriter; FileInputStream, FileOutputStream Collectively called file streams; used to read from or write to a file on

the native file system.

CharArrayReader, CharArrayWriter; ByteArrayInputStream, ByteArrayOutputStream Use these streams to read from and write to memory array. create these streams on an existing array and then use the read and

write methods to read from or write to the array. Constructors: <Type>( {char | byte } [] [, int offset, int length ] ) Ex: CharArrayWriter cwr = new CharArrayWriter( new char[100] ) ; CharArrayWriter cw = new CharArrayWriter( new char[1024], 256,512 ) ; ByteArrayInputstream bi = new ByteArrayInputStream ( new byte[] {\011,\022, \0xff} );

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String Streams

StringReader, StringWriter, StringBufferInputStream StringReader : used to read characters from a String. StringWriter : used to write to a String. StringWriter collects the

characters written to it in an implicit StringBuffer, which can then be converted to a String.

StringBufferInputStream : similar to StringReader, except that it reads bytes from a StringBuffer. (deprecated since it cannot correctly convert a char to bytes; use StringReader instead)

Constructors: StringReader(String); StringWriter( [int initSize] ) // initial buffer size; use toString() to get

the written String StringBufferInputStrean(String) // deprecated ! Only the low eight bits

of each character in the string are used by this class.

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Piped Streams

PipedReader, PipedWriter; PipedInputStream, PipedOutputStream

Implement the input and output components of a pipe. Pipes are used to channel the output from one program (or thread) into

the input of another. PipedReader and PipedInputStream get data from the output of another

thread, while PipedWriter and PipedOutputStream output data for processing by other thread.

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2.1 How to Use File Streams

File streams are the easiest streams to understand. Create (and open )the file stream:

<FileStream> f = new <FileStream>( <file> ); where <FileStream> is any of FileReader, FileWriter, FileInputStream,

and FileOutputStream and <file> is either a string for file name, a File object, or a FileDescriptor

object (low-level handle to open file or socket).

EX: FileReader reader1 = new FileReader(“example1.txt”); File file1 = new File(“d:\\java\\examples\\ex1.java”); FilerWriter writer1 = new FileWriter(file1); FileInputStream in1 = new FileInputStream(“ex2”); FileOutputStream out1 = new FileOutputStream ( java.io.FileDescriptor

out );

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A simple file-copy program Copy the contents of a file named input.txt into a file called output.txt: import java.io.*; public class Copy { public static void main(String[] args) throws IOException { File inputFile = new File(“input.txt"); File outputFile = new File("output.txt");

FileReader in = new FileReader( inputFile ); FileWriter out = new FileWriter( outputFile );

int c; while ((c = in.read()) != -1) out.write(c); in.close(); out.close(); } }

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Copy bytes

import java.io.*;public class CopyBytes { public static void main(String[] args) throws IOException { File inputFile = new File(“input.txt"); File outputFile = new File("output.txt");

FileInputStream in = new FileInputStream(inputFile); FileOutputStream out = new FileOutputStream(outputFile);

int c; while ((c = in.read()) != -1) out.write(c); in.close(); out.close(); }}

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2.2 How to Use Pipe Streams

A pipe is a channel for receiving data from one program (or thread) and sending the received data to the input of another.

PipedReader, PipedWriter ; PipedInputStream, PipedOutputStream implement the input and output components of a pipe.

for p2, p is an PipedWriter ( or PipedOutputStream ) for p1, p is a PipedReader ( or PipedInputstream )

p : a pipe input of program p1program p2: output

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Pipe example

a simple program to reverse, sort and reverse words in a file. Note: without pipe, programmers would need 2 additional

temporary files/buffers to store the intermediate results.

pipe 1 pipe 2

sink(words)

sink(rhymedWords)

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The program

import java.io.*;public class RhymingWords { public static void main(String[] args) throws IOException { FileReader words = new FileReader("words.txt");

// do the reversing and sorting Reader rhymedWords = reverse(sort(reverse(words)));

// write new list to standard out BufferedReader in = new BufferedReader( rhymedWords ); String input; while ((input = in.readLine()) != null) out.println(input); in.close(); }

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the reverse method

public static Reader reverse(Reader source) throws IOException {

BufferedReader in = new BufferedReader(source);

PipedWriter pipeOut = new PipedWriter();

PipedReader pipeIn = new PipedReader( pipeOut );

// PipedReader pipeIn = new PipedReader( ); // alternative codes for

// PipedWriter pipeOut = new PipedWriter( pipeIn ); // the above two lines

PrintWriter out = new PrintWriter(pipeOut); // wrap pipeOut for easy write

new ReverseThread(out, in).start(); return pipeIn; }

reverse Thread

in(source)

out(pipeOut)

pipeOut

pipeIn

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// source is a PipedReaderpublic static Reader sort(Reader source) throws IOException { BufferedReader in = new BufferedReader(source);

PipedWriter pipeOut = new PipedWriter(); PipedReader pipeIn = new PipedReader(pipeOut); PrintWriter out = new PrintWriter(pipeOut);

new SortThread(out, in).start(); return pipeIn; }}

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ReverseThread.java

import java.io.*;public class ReverseThread extends Thread { private PrintWriter out; private BufferedReader in; public ReverseThread(PrintWriter out, BufferedReader in) { this.out = out; this.in = in; } public void run() { // entry point of a thread (invoked by start()), // like main() of a java program if (out != null && in != null) { try { String input; while ((input = in.readLine()) != null) { out.println(reverseIt(input)); out.flush(); } out.close(); } catch (IOException e) { err.println("ReverseThread run: " + e); } } } private String reverseIt(String source) { …} // reverse source }

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SortThread.javaimport java.io.*;public class SortThread extends Thread { private PrintWriter out = null; private BufferedReader in = null; public SortThread( PrintWriter out, BufferedReader in ) { this.out = out; this.in = in; } public void run() { int MAXWORDS = 50; if (out != null && in != null) { try { String[] listOfWords = new String[MAXWORDS]; int numwords = 0; // read words from in into listOfWords while ((listOfWords[numwords] = in.readLine()) != null) numwords++; quicksort(listOfWords, 0, numwords-1); // sort for (int i = 0; i < numwords; i++) out.println(listOfWords[i]); // output out.close(); } catch (IOException e) { System.err.println("SortThread run: " + e); } } }

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quicksort

private static void quicksort(String[] a, int lo0, int hi0) { int lo = lo0; int hi = hi0; if (lo >= hi) return; String mid = a[(lo + hi) / 2]; while (lo < hi) { while (lo<hi && a[lo].compareTo(mid) < 0) lo++; while (lo<hi && a[hi].compareTo(mid) > 0) hi--; if (lo < hi) { String T = a[lo]; a[lo] = a[hi]; a[hi] = T; lo++; hi--; } } if (hi < lo) { int T = hi; hi = lo; lo = T; } quicksort(a, lo0, lo); quicksort(a, lo == lo0 ? lo+1 : lo, hi0); }}

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3. Using the processing streams

Using Streams to Wrap Other Streams : The reverse method contains some other interesting code; in

particular, these two statements: BufferedReader in = new BufferedReader(source); ... PrintWriter out = new PrintWriter(pipeOut);general format: WrappedClass w = new WrappedClass( rawObject )

where rawObjwect is an obejct of type RawClass which does not provide the needed method, while Wrapped class is an extension of rawClass with intended methods provided.

In revserse() method: source is a [file | pipe] Reader, which does not provide the readLine()

method. pipeOut is a [piped] writer, which does not provide the convenient

println() method.

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3.1 Types of the Processing Streams

process character stream byte stream

BufferingBufferedReader, BufferedWriter

BufferedInputStream, BufferedOutputStream

FilteringFilterReader, FilterWriter

FilterInputStream, FilterOutputStream

B2C and C2BInputStreamReader, OutputStreamWriter

StringBufferInputStream

(deprecated)

Concatenation SequenceInputStream

Object SerializationObjectInputStream, ObjectOutputStream

Data ConversionDataInputStream, DataOutputStream

Counting LineNumberReader LineNumberInputStream

printing PrintWriter PrintStream

Peeking Ahead PushbackReader PushbackInputStream

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3.2 Summary of the processing streams

Buffered Streams Buffer data while reading or writing, thereby reducing the number of

accesses required on the original data source. typically more efficient than similar nonbuffered streams; suggest

always wrap a buffer stream around any stream such as a file stream whose read/write operations are costly.

BufferedReader f = new BufferedReader(new FileReader(“data.txt”)); BufferedOutputStream b = new BufferedOutputStream ( new FileOutputStream(“dataout”));

raw Input Stream program raw Output Stream

buffer bufferBufferedInputStream BufferedOutputStream

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Filter Streams

Filter streams contains some other input stream as its basic source of data, possibly

transforming the data along the way or providing additional functionality.

Abstract classes, same as their parent streams (without new methods). They define the interface for filter streams, which filter data as it's

being read or written.

underlying Input Stream program underlying Output Stream

FilterInputStream

Transformation +additionalfunctionality

FilterOutputStream

transformation +additional functionality

(InputStream/Writer etc)

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Byte2Char and char2Byte

A reader and writer pair that forms the bridge between byte streams and character streams.

default encoding from system property : “file.encoding” Ex:

InputStreamReader reader = new InputStreamReader( new FileInputStream(“data.txt”) , “Big5” ); OutputStreamWriter writer = new OutputStreamWriter( new FileOutputStream(“out.txt”));

InputStream program OutputStream

InputStreamReaderencoding decoding

OutputStreamWriter

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SequenceInputStream Concatenates multiple input streams into one input stream. ex: SequenceInputStream sin = new SequenceInputStream(s1,s2);

ObjectInputStream and ObjectOutputStream Used to serialize objects.

DataInputStream and DataOutputStream Read / write primitive Java data types in a machine-independent

format. subclass of filter stream

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LineNumberReader, LineNumberInputStream Keeps track of line numbers while reading.

PushbackReader, PushbackInputStream Two input streams each with a character (or byte) pushback buffer. Sometimes, when reading data from a stream, you will find it useful to

peek at the next item in the stream in order to decide what to do next. However, if you do peek ahead, you'll need to put the item back so that it can be read again and processed normally.

PrintWriter, PrintStream Contain convenient printing methods. public void print(Type), and println([Type]), where the argument Type

can be any type, either primitive or not. These are the easiest streams to write to, so you will often see other

writable streams wrapped in one of these. c.f. PrintWriter(wrapping a writer) can not write byte[ ].

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How to Concatenate Files using the sequenceInputStream a concatenation utility that sequentially concatenates files together in the

order they are listed on the command line. The main program

import java.io.*;

public class Concatenate {

public static void main(String[] args) throws IOException {

ListOfFiles mylist = new ListOfFiles(args);

// mylist is an enumeration of objects of type InputStream

SequenceInputStream s = new SequenceInputStream(mylist);

int c;

while ((c = s.read()) != -1) System.out.write(c);

s.close();

} }

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ListOfFiles.java

import java.util.*; import java.io.*;public class ListOfFiles implements Enumeration { private String[] listOfFiles; private int current = 0; public ListOfFiles(String[] listOfFiles) { this.listOfFiles = listOfFiles; } public boolean hasMoreElements() { if (current < listOfFiles.length) return true; else return false; } public Object nextElement() { InputStream in = null; if (!hasMoreElements()) throw new NoSuchElementException("NoMoreFiles."); String nextElement = listOfFiles[current]; current++; try { in = new FileInputStream(nextElement); } catch (FileNotFoundException e) { … }; } return in; } }

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The SequenceInputerStream class

public class SequenceInputStream extends InputStream

Constructor Summary SequenceInputStream(Enumeration e)

e is an Enumeration of objects whose runtime type is InputStream.

SequenceInputStream(InputStream s1, InputStream s2)

Method Summary : All overriding those from inputStream. int available(), void close(), int read(), int read(byte[] b [, int off, int len]) …

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Working with Filtered Streams

You attach a filtered stream to another stream to filter the data as it's read from or written to the original stream.

Subclasses of either FilterInputStream or FilterOutputStream: DataInputStream and DataOutputStream : primitive data bytes BufferedInputStream and BufferedOutputStream : LineNumberInputStream : (deprecated) PushbackInputStream : PrintStream :

Subclasses of FilterReader and FilterWriter: BushbackReader, LineNumberReader, …

Note: FilterReader/Writer use a character stream as its underlying stream while FilterInputStream/OutputStream use a byte stream as its underlying stream.

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Writing Your Own Filtered Streams

Steps Create a subclass of FilterInputStream and FilterOutputStream. Override the read and write methods. Override and/or define any other methods that you might need. Make sure the input and output streams work together.

Example: Use Adler32 to implement a CheckedInputStream and

CheckedOutputStream to ensure that the data read match those written in other program [ now included in java.util.zip package]

Four classes and one interface make up this example program: 1. CheckedOutputStream, CheckedInputStream. 2. The Checksum interface and the Adler32 class compute a checksum

for the streams. 3. The CheckedIOTest class defines the main method for the program.

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CheckedOutputStream

import java.io.*;

public class CheckedOutputStream extends FilterOutputStream {

private Checksum cksum;

public CheckedOutputStream(OutputStream out, Checksum cksum) {

super(out); this.cksum = cksum; }

public void write(int b) throws IOException {

out.write(b); cksum.update(b); }

public void write(byte[] b) throws IOException {

out.write(b, 0, b.length); cksum.update(b, 0, b.length); }

public void write(byte[] b, int off, int len) throws IOException {

out.write(b, off, len); cksum.update(b, off, len); }

public Checksum getChecksum() { // additional functionality

return cksum; }}

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the CheckedInputStream

import java.io.FilterInputStream, java.io.InputStream, java.io.IOException;public class CheckedInputStream extends FilterInputStream { private Checksum cksum ; public CheckedInputStream(InputStream in, Checksum cksum) { super(in); this.cksum = cksum; } public int read() throws IOException { int b = in.read(); if (b != -1) { cksum.update(b); } return b; } public int read(byte[] b) throws IOException { int len; len = in.read(b, 0, b.length); if (len != -1) { cksum.update(b, 0, len); } return len; } public int read(byte[] b, int off, int len) throws IOException { len = in.read(b, off, len); if (len != -1) { cksum.update(b, off, len); } return len; } public Checksum getChecksum() { return cksum; }}

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Checksum and Adler32

public interface Checksum {

public void update(int b);

public void update(byte[] b, int off, int len);

public long getValue(); // return checksum value

public void reset(); }

public class Adler32 implements Checksum {

private int value = 1; // state of the checksum

private static final int BASE = 65521; // largest prime < 65536

// NMAX is the largest n with 255n(n+1)/2 + (n+1)(BASE-1)<= 2^32-1

private static final int NMAX = 5552;

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public void update(int b) { int s1 = value & 0xffff; int s2 = (value >> 16) & 0xffff; s1 += b & 0xff; s2 += s1; value = ((s2 % BASE) << 16) | (s1 % BASE); }public void update(byte[] b, int off, int len) { int s1 = value & 0xffff; int s2 = (value >> 16) & 0xffff; while (len > 0) { int k = len < NMAX ? len : NMAX; len -= k; while (k-- > 0) { s1 += b[off++] & 0xff; s2 += s1; } s1 %= BASE; s2 %= BASE; } value = (s2 << 16) | s1; }public void reset() { value = 1; }public long getValue() { return (long) value & 0xffffffff; }}

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The test

import java.io.*;public class CheckedIOTest { public static void main(String[] args) throws IOException { Adler32 inChecker = new Adler32(); Adler32 outChecker = new Adler32(); CheckedInputStream in = null; CheckedOutputStream out = null; try { in = new CheckedInputStream( new FileInputStream(“input.txt"), inChecker); out = new CheckedOutputStream(new FileOutputStream("output.txt"), outChecker); } catch (FileNotFoundException e) { … } } catch (IOException e){…} } int c; while ((c = in.read()) != -1) out.write(c); System.out.println("Input stream check sum: " + inChecker.getValue()); System.out.println("Output stream check sum: " + outChecker.getValue()); in.close(); out.close(); }}

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The java.io.DataInputStream class

public class DataInputStream extends FilterInputStream implements DataInput

// allow to read primitive data in machine indep. way Constructor Summary

DataInputStream(InputStream in) Method Summary

int read(byte[] b [, int offset , int length ] ) // read to array b void readFully(byte[] [int,int]) // throws EOFException if not fully read <type> read<Type>( ) // read one data item <type> is one of boolean, byte, short, int, long, float, double, char String readUTF() // read a string in UTF8 format (instead of

unicode/native) String readLine() // read next line, deprecated int readUnsigned<Type>() // <Type> is either Byte or Short int skipBytes(int n) // always returns n if no exception

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java.io.DataOutputStream public class DataOutputStream extends FilterOutputStream implements

DataOutputConstructor Summary DataOutputStream(OutputStream out) Method summary void flush(); int size() // return # of bytes written into this stream void write(byte[] b [, int off, int len ]) void write<Type>(<type> v) throws IOException;

<type> is any of int, long, float, double. void write<Type>(int v) throws IOException;

<type> is one of boolean, byte, short or char. void write<Type>( String s)

<Type> is either Bytes or Chars; for Bytes only low-bytes of s are written Writes out the string to the underlying output stream as a sequence of bytes or chars.

void writeUTF(String str) Write str in UTF8 format; (first 2 byte = #bytes written) != str.size() in general.

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How to Use DataInputStream and DataOutputStream

import java.io.*;public class DataIOTest { public static void main(String[] args) throws IOException { // write the data out DataOutputStream out = new DataOutputStream(new FileOutputStream("invoice1.txt")); double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 }; int[] units = { 12, 8, 13, 29, 50 }; String[] descs = { "Java T-shirt", "Java Mug", "Duke Juggling Dolls", "Java Pin", "Java Key Chain" }; for (int i = 0; i < prices.length; i ++) { out.writeDouble(prices[i]); out.writeChar('\t'); out.writeInt(units[i]); out.writeChar('\t'); out.writeChars(descs[i]); out.writeChar('\n'); } out.close();

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Transparency No. 58

// read it in againDataInputStream in = new DataInputStream(new

FileInputStream("invoice1.txt")); double price; int unit; String desc; double total = 0.0; try { while (true) { price = in.readDouble(); in.readChar(); // throws out the tab unit = in.readInt(); in.readChar(); // throws out the tab desc = in.readLine(); System.out.println("You've ordered " + unit + " units of " + desc + " at $" + price); total = total + unit * price; } } catch (EOFException e) { } System.out.println("For a TOTAL of: $" + total); in.close(); }}

Java I/O

Transparency No. 59

How read/writeUFT8() work

Note: There is no writeUnsignedByte() or writeUnsignedShort() in DataOutputStream.

writeByte(-128) => readByte() = -128 and

readUnsignedByte() = 128. Modified UTF8 format (also used in .class for representing strings)

'\u0001' ~ '\u007F' represented by a single byte: 0xxx xxxx (1~127)

‘\u0000’ and '\u0080' ~ '\u07FF' : 110x xxxx 10xx xxxx (0,128~2047)

'\u0800' ~ '\uFFFF' : 1110 xxxx 10xxxxxx 10xxxxxx (2048~65535) Note: \u0000 is represented by 2 bytes:1100 0000 1000 0000 instead of 0000 0000. when writeUTF(String) the first 2 bytes are an unsigned short indicating

the number of bytes required to write the string.

Java I/O

Transparency No. 60

File Management

Have learned how to read/write data from a file, But how about file management besides read/write ?

create new file/directory, set/get file properties (access permission, last update date, owner, file

name, full path etc.)

The class java.io.File Represents a file on the native file system. encapsulates the functionality that you will need to work with the file

system on a machine. You can create a File object for a file on the native file system and then

query the object for information about that file (such as its full pathname).

Java I/O

Transparency No. 61

Path Name vs Abstract path name

Pathname [strings] are system-dependant strings for identifying directories or files in a system, while abstract pathnames are system independent strings for identifying directories or files.

pathname = sys-dep-prefix ( DirName FileSpearator)*

[ DirName | FileName ] FileSeparator is ‘/’ for UNIX and ‘\’ for WIN32. Default determined by “file.separator” system property; given at field File.separator

abstract pathname = [ sys-dep-prefix ] DirName * [ DirName | FileName ] where sys-dep-prefix is an optional system-dependent prefix string, Unix: => “/” for UNIX root directory. win32: => “Driver:”, “Driver:\" , or "\\" for a Win32 UNC(universal Naming Convention)pathname, and A sequence of zero or more string names.

Each name in an abstract pathname except for the last denotes a directory; the last name may denote either a directory or a file.

Java I/O

Transparency No. 62

relative vs absolute pathnames

Pathnames and abstract pathname can be either absolute or relative.

An absolute pathname contains full information to locate the file or directory that it denotes.

A relative pathname, in contrast, must be interpreted in terms of information taken from some other base directory pathname.

absolute_pathname = base_directory relative_pathname where by default base_directory is the current user directory, which

can be got by the system property “user.dir”.

Java I/O

Transparency No. 63

jva.io.File

public class File extends Object implements Serializable, Comparable

File constructors : File(String pathname) // pathname can be relative or absolute

Creates a new File instance by converting the given system dependant pathname string into an system independent abstract pathname.

File(File dir, String child) // child is interpreted as relative Creates a new File instance from a parent abstract pathname and a

child pathname string. f1 = new File(“a/b/”, “c:\\d\\e”) ; // returns a\b\c:\d\e, / doesn’t care!

File(String parentDir, String child) Creates a new File instance from a parent pathname string and a child

pathname string.

Java I/O

Transparency No. 64

File constructor examples

// create File using absolute pathnameFile barDir = new File( “C:\\tmp\\bar" ); // bar need not exist in advance!barDir.exists(); // return false iff C:\temp\bar does not exist.File fooFile = new File( “/tmp/foo.txt" ); // ok for WIN32 and UNIXbarFile.getPath(); // return \tmp\foo.txt for WIN32

// create a file with a relative path : File f1 = new File( "foo" ); // f.getPath() returns “foo”.String dir = System.getProperty(“user.dir”); // suppose it returns “c:\java”File f2 = new File( dir, “foo”); // f2.getPath() returns c:\java\fooFile userDir = new File( dir );File f3 = new File(userDir, “foo”);f2.equals(f3); // return truef2.equals(f1); // return falsef2.equals( f1.getAbsoluteFile()); // return true

Java I/O

Transparency No. 65

Field Summary

static String pathSeparator The system-dependent path-separator character, represented as a

string for convenience. ( = “path.separator” system property) ; for WIN32 and : FOR UNIX

static char pathSeparatorChar The system-dependent path-separator character.

static String separator The system-dependent default name-separator character, represented

as a string for convenience. ( = “file.separator” system property) \ for WIN32 and / for UNIX

static char separatorChar The system-dependent default name-separator character.

Java I/O

Transparency No. 66

Method Summary // class methods static File createTempFile(String prefix, String suffix [, File dir])

Creates an empty file in the dir directory, using the given prefix and suffix to generate its name.

suffix == null =>use “tmp” as default; | prefix | > 2. dir = null or not given => use system property “java.io.tmpdir”, which is /tmp or

/var/tmp on UNIX and c:\temp on WIN32 static File[] listRoots()

List the available filesystem roots.

Ex:File[] fs = File.listRoots();for ( int i = 0; i < fs.length(); i++) System.out.println(fs[i].getPath()); the output:>A:\>C:\>D:\

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Transparency No. 67

instance methods

// File properties getter/setter boolean exists(), isDirectory(), isFile() , isHidden() boolean isAbsolute()

Tests whether this abstract pathname is absolute. boolean canRead(), canWrite() // current application can read/write ? long length() // the length of the file denoted by this; 0 for directory. long lastModified()

Returns the time that the file denoted by this abstract pathname was last modified. boolean setLastModified (long time) boolean setReadOnly() boolean equals(Object obj)

Tests this abstract pathname for equality with the given object. int hashCode()

Computes a hash code for this abstract pathname. int compareTo(File pathname), compareTo(Object o)

Compares two abstract pathnames lexicographically.

Java I/O

Transparency No. 68

pathname/ File transformations File getAbsoluteFile() ; String getAbsolutePath() ;

Returns the absolute pathname string of this abstract pathname. File getCanonicalFile() ; String getCanonicalPath() ;

Returns the canonical form of this abstract pathname. new File(“/a/b/../”).getCanonicalPath() // return C:\a but C:\a\b\.. for getAbsolutePth()

String getParent() ; File getParentFile() Returns the pathname string or abstract pathname of this abstract pathname's

parent, or null if this pathname does not name a parent directory. String getName() // Returns the name of the file/directory denoted by this. String getPath() // File pathname String

Converts this abstract pathname into a pathname string. boolean renameTo(File dest)

Renames the file denoted by this abstract pathname. String toString() : Returns the pathname string of this abstract pathname.

URL toURL() Converts this abstract pathname into a file: URL. c:\a\b\c.txt file:/c:/a/b/c.txt \\host\sh\b\c.txt file://host/sh/b/c.txt

Java I/O

Transparency No. 69

Directory and File operations

// File operations boolean createNewFile() // atomically check and create new file if not

existing. boolean delete() // delete this file or directory void deleteOnExit() // delete when on system exits

// directory operations boolean mkdirs() , mkdir()

Creates the directory named by this Fie, mkdirs() will also create any necessary nonexistent parent directories.

String[] list(), list(FilenameFilter filter) Returns an array of strings naming the files and directories in the directory denoted

by this abstract pathname that satisfy the specified filter.

File[] listFiles(), listFiles(FileFilter filter), listFiles(FilenameFilter filter) Returns an array of abstract pathnames denoting the files and directories in the

directory denoted by this abstract pathname that satisfy the specified filter.

Java I/O

Transparency No. 70

FilenameFilter and FileFilter

public interface FilenameFilter {public boolean accept(File dir, String name); }

public interface FileFilter { public boolean accept(File dir); } Implementation of listFiles(FilenameFilter)public File[] listFiles( FilenameFilter filter) {

String ss[] = list(); if (ss == null) return null;ArrayList v = new ArrayList();for (int i = 0 ; i < ss.length ; i++) if (( filter == null) || filter.accept(this, ss[i]) )

v.add(new File( this.getpath() , ss[i])); return (File[])(v.toArray(new File[0]));

}

Java I/O

Transparency No. 71

Example

Find all java source files lastly modified in one day in current user directory

File userDir = new File (System.getProperty(“user.dir”));int now = new Date().getTime(); // now is current time in ms.String[] rlts = dir.list( new FilenameFilter(){ boolean accept( File dir, String name) { File f = new File (dir, name); // dir bound to this at

runtime return name.endWith(“.java”) && (now - f.lastModified() < 1000*60*60*24) ; } });for(int i = 0; i < rlts.length; i++) System.out.println(rlts[i]);

Java I/O

Transparency No. 72

Object Serialization

The process of reading and writing objects is called object serialization

ObjectInputStream and ObjectOutputStream are streams used to read/write objects.

when to use object serialization: Remote Method Invocation (RMI)--communication between objects via

sockets Lightweight persistence--storage of an object for use in a later

invocation of the same program What a Java programmer need to know about object

serialization: How to serialize objects by writing them to an ObjectOutputStream and

reading them in again using an ObjectInputStream. how to write a class so that its instances can be serialized.

Java I/O

Transparency No. 73

Serializing Objects

How to Write to an ObjectOutputStream ? straight-forward! Ex:

FileOutputStream out = new FileOutputStream("theTime");

ObjectOutputStream s = new ObjectOutputStream(out);

s.writeInt( 12345 );

s.writeObject("Today");

s.writeObject(new Date());

s.flush();

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Transparency No. 74

Notes about ObjectOutputStreams

If an object refers to other objects, then all of the objects that are reachable from the first must be written at the same time

ObjectOutputStream stream implements the DataOutput interface that defines many methods for writing primitive data types, such as writeInt, writeFloat, or writeUTF. You can use these methods to write primitive data types to an

ObjectOutputStream.

The writeObject method throws a NotSerializableException if it's given an object that is not serializable. An object is serializable only if its class implements the Serializable

interface. Serializable is a marker interface: you just declare you implement it but

do not need to implement any additional method.

Java I/O

Transparency No. 75

How to Read from an ObjectInputStream

EX:

FileInputStream in = new FileInputStream("theTime");

ObjectInputStream s = new ObjectInputStream(in);

int d = s.readInt();

String today = (String)s.readObject();

Date date = (Date)s.readObject();

ObjectInputStream stream implements the DataInput interface that defines methods for reading primitive data types. Use these methods to read primitive data types from an ObjectInputStream.

The order and types of objects read in must be the same as they were written out using ObjectOutputStream.

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Transparency No. 76

The java.io.datainput interface

public abstract interface java.io.DataInput { // Methods

public abstract <type> read<Type>();

// <type> is any of 8 primitive types : byte, short, char, int, long, float, dobule, boolean.

public abstract void readFully(byte[] b [, int off, int len ] );

public abstract String readLine();

public abstract int readUnsignedByte();

public abstract int readUnsignedShort();

public abstract String readUTF();

public abstract int skipBytes(int n);}

Java I/O

Transparency No. 77

The java.io.ObjectInput Interface

public abstract interface java.io.ObjectInput extends java.io.DataInput {

// Methods

public abstract int available();

public abstract void close();

public abstract int read();

public abstract int read(byte[] b);

public abstract int read(byte[] b, int off, int len);

public abstract Object readObject();

public abstract long skip(long n);

}

Java I/O

Transparency No. 78

java.io.ObjectInputStream

public class java.io.ObjectInputStream extends java.io.InputStream implements java.io.ObjectInput { // Constructors public ObjectInputStream(InputStream in); // Public Instance Methods; implementation of ObjectInput readObject(), readInt() ,…

// Read the non-static and non-transient fields of the current class from this stream. may only be called by readObject() of the object to be deserialized.

public final void defaultReadObject();

public synchronized void registerValidation(ObjectInputValidation obj, int prioity); ObjectInputValidation.validateObject() can be used to validate read-in object. There can be more than one validation objects registered, each called back according

to prioity ( 0 first ). See this reference for an explanation of objectInputValidation

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Transparency No. 79

// Protected Instance Methods

protected final boolean enableResolveObject(boolean enable);

protected void readStreamHeader();

protected Class resolveClass(ObjectInputStreamClass v);

protected Object resolveObject(Object obj);

}

Java I/O

Transparency No. 80

The DataOutput interface

public abstract interface java.io.DataOutput { // Methods

public abstract void write(byte[] b);

public abstract void write(byte[] b, int off, int len);

public abstract void write(int b);

public abstract void write<Type>(<type> v);

where <Type> is any of Boolean, Byte, Char, Short, Int, Long, Float, Double., and corresponding <Type> is int, int, int, int, long, float and double.

public abstract void writeUTF(String str);

}

Java I/O

Transparency No. 81

The DataOutput Stream

public abstract interface java.io.ObjectOutput extends java.io.DataOutput {

// Methods

public abstract void close();

public abstract void flush();

public abstract void write(int b);

public abstract void write(byte[] b);

public abstract void write(byte[] b, int off, int len);

public abstract void writeObject(Object obj);

}

Java I/O

Transparency No. 82

the ObejctOutputStream

public class java.io.ObjectOutputStream extends java.io.OutputStream implements java.io.ObjectOutput { // Constructors public ObjectOutputStream(OutputStream out); // Instance Methods; implementation of ObjectOutput. …. // Write the non-static and non-transient fields of the current class to this

stream. may only be called by writeObject() of the object to be serialized. public final void defaultWriteObject(); public void flush(); public void reset();// Protected Instance Methods; used only if you need to extend this class. protected void annotateClass(Class cl); protected void drain(); protected final boolean enableReplaceObject(boolean enable); protected Object replaceObject(Object obj); protected void writeStreamHeader();}

Java I/O

Transparency No. 83

Providing Object Serialization for Your Classes

Implementing the Serializable Interface :

package java.io; public interface Serializable {};

Customizing Serialization Implementing the Externalizable Interface Protecting Sensitive Information

Java I/O

Transparency No. 84

Customizing Serialization

The default serialization mechanism (ObjectInputStream.readXXX() + ObjectOutputStream.writeXXX())

will read/write the type and content of every target object, and recursively all types and contents of all subobjects.

Sometimes it is needless to read/write all fields of an object: ex: public class Point { int x,y; float r, theta; } For point object, we may want to save x and y only.

Solution: use the transient modifier : pubilc class Point {int x,y; transient float r, theta ; } Every transiant field will not be read/written by during object

serialization. Problem: How to recover r, theta from read x and y ?

Customizing serialization!!

Java I/O

Transparency No. 85

public class Point implement Serializable {

int x,y;

transiant float r, theta

private readObject(ObjectInputStream oin ) {

oin.defaultReadObject() ; // this will set x and y only.

r = Math.sqrt( x * x + y * y );

theta = (float ) Math.arcTan( (double) y / x );

}

}

Java I/O

Transparency No. 86

More Example

public class LabeledPoint implements Serializable { private String label; private transient Point2D.Double point;

private void writeObject(ObjectOutputStream out) throws IOException { out.defaultWriteObject(); // this will write label only!! out.writeDouble(point.getX()); // manually write contents of point out.writeDouble(point.getY()); } private void readObject(ObjectInputStream in) throws IOException { in.defaultReadObject(); // this will set label only! double x = in.readDouble(); double y = in.readDouble(); // manually read previously written data! point = new Point2D.Double(x, y); // reconstruct point!} }

Java I/O

Transparency No. 87

Read/Write Externalizable objects

Serialization is somewhat slow because the JVM must discover the structure of each object.

If you are concerned about performance and if you need to read /write a large number of objects of a particular class, you should use the Externalizable interface.

public interface Externalizable {

public void readExternal(ObjectInput ) ;

pubilc void writeExternal(ObjectOutput);

} Notes:

Target class should implement this interface. both read/write methods are public. should also read/write ancestor classes data Object<I/O>Stream will use either method if they can find them in target classes.

Java I/O

Transparency No. 88

Example

public class Employee implements Serializable, Externalizable { String name; double salary; transiant Date hireDay ;public void writeExternal(ObjectOutput s ) { s.writeUTF(name); s.writeDouble(salary); s.writeLong( hireDay.getTime() ); }public void readExternal(ObjectInput s) throws IOException { name = s.readUTF(); salary = s.readDouble(); hireDay = new Date(s.readLong()); }} e = new Employee(); … ; out = new ObjectOutputStream(…); out.writeObject(e) ; … ; in = new ObjectInputStream(…) ; Employee e1 = in.readObject(); assert e.equlas(e1);

Java I/O

Transparency No. 89

References for Java Object Serialization

Java Object Serialization Specification ObjectInputStream ObjectOutputStream Serialize objects not serializable Serialization and magic methods

Java I/O

Transparency No. 90

RandomAccessFile class

reads data from and writes data to a file. The file is specified using a File object or a String that

represents a pathname. constructors take a mode parameter that specifies whether the

file is being opened solely for reading, or for reading and writing.

can throw a SecurityException if the application does not have permission to access the specified file using the given mode.

supports random access to the data in the file; the seek() method allows you to alter the current position of the

file pointer to any location in the file. Implements both the DataInput and DataOutput interfaces, so it

supports reading and writing of all the primitive data types.

Java I/O

Transparency No. 91

class summary

public class java.io.RandomAccessFile extends java.lang.Object implements java.io.DataInput, java.io.DataOutput { // Constructors; mode is either “r” or “rw” public RandomAccessFile(File file, String mode); public RandomAccessFile(String name, String mode);

if “rw” mode and fie not exists => create it!

// Instance Methods // implementatin of DataInput and DataOutput + public final FileDescriptor getFD(); public native long getFilePointer(); public native long length(); pubilc void setLength(int); // set the length of the file, resulting in

extension/truncation of the file content. public native void seek(long pos);}