Jan. 20041 Java I/O Yangjun Chen Dept. Business Computing University of Winnipeg.
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Transcript of Jan. 20041 Java I/O Yangjun Chen Dept. Business Computing University of Winnipeg.
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Jan. 2004 1
Java I/O
Yangjun Chen
Dept. Business ComputingUniversity of Winnipeg
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Jan. 2004 2
Outline: I/O Streams
• I/O Streams- Byte stream: Input Stream and Output Stream
- Filter Stream- Buffered Stream- Data Stream- Print Stream- File Stream
- Character Stream: Reader and Writer- Input Stream Reader and Output Stream Writer- Buffered Reader/Writer- File Reader/Writer
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Jan. 2004 3
I/O Streams
• What is a stream?- A stream is a sequence of data of undetermined length.- Input streams move data into a Java program usually from an external
source.- Output streams move data from a Java program to an external target.
• Java Streams- A Java stream is composed of discrete bytes (characters) of data.
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Jan. 2004 4
ObjectObject
InputStreamInputStream
Byte Streams
OutputStreamOutputStream
FileInputStreamFileInputStream
FilterInputStreamFilterInputStream
FileOutputStreamFileOutputStream
FilterOutputStreamFilterOutputStream
BufferedInputStreamBufferedInputStream
DataInputStreamDataInputStream
BufferedOutputStreamBufferedOutputStream
DataOutputStreamDataOutputStream
PrintStreamPrintStream
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Jan. 2004 5
ObjectObject
ReaderReader
Character Streams
writerwriter
BufferedReaderBufferedReader
InputStreamReaderInputStreamReader FileReaderFileReader
BufferedWriterBufferedWriter
OutputStreamWriterOutputStreamWriter
PrintWriterPrintWriter
FileWriterFileWriter
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Jan. 2004 6
InputStream Class
• java.io.InputStream is an abstract class for all input streams.• It contains methods for reading in raw bytes of data from input
stream: key board, files, network client.
- public abstract int read()- public int read (byte[] buf)- public int read(byte[] buf, int offset, int length)
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Jan. 2004 7
InputStream Class
- public long skip(long n). skip n number of bytes
- public int available( ). how many bytes can be read before blocking
- pcublic void close( )- public synchronized void mark (int readlimit)
. bookmark current position in the stream- public boolean markSupported( )- public synchronized void reset( )
. rewind the stream to the marked position• All but the last two methods throw an IOException.
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Jan. 2004 8
The read( ) method
• The basic read() method reads a single unsigned byte of data and returns the integer value of the unsigned byte.
• This is a number between 0 and 255
• Returns a -1 if the end of a stream is encountered.
• The method blocks until input data are available, the end of stream is detected or an exception is thrown.
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Jan. 2004 9
The read( ) method
int[] data = new int[10];for (int i =0; i <data.length, i++)
data [i]= System.in.read( );}
• This code reads in 10 bytes from the System.in input stream and stores it in the int array data.
• Notice that although read() reads in a byte, it returns a value of type int. If you want the raw byte, cast the int into a byte.
BufferedInputStreamBufferedInputStream
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Jan. 2004 10
The read( ) method
• read() has a possibility of throwing an exception.try {
int data[] = new int[10] ;for (int i=0; i<data.length; i++) {int datum = System.in.read();if (datum == -1) break;data[il = datum;}//for
}//try catch (IOException e) {
System.err.println(e);}
End of streamEnd of stream
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Jan. 2004 11
The read( ) method
• The value of -1 is returned when the end of stream is reached. This can be used as a check for the stream end.
• Remember that read() blocks. So if there is any other important work to do in your program, try to put your I/O in a separate thread.
• read() is abstract method defined in InputStream. This means you can't instantiate InputStream directly: work with one of it's subclasses instead.
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Jan. 2004 12
Echo Example(l)
import java.io.*,
public class Echo {public static void main(String[] args){echo(System.in);}//main
public static void echo(InputStream is) {try {
for (int j = 0; j < 20; j++) {int i = is.read( );
BufferedInputStreamBufferedInputStream
An instance of a subclassAn instance of a subclassof InputStreamof InputStream(remember: upcasting)(remember: upcasting)
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Jan. 2004 13
Echo Example(2)
// -1 returned for end of streamif (i == -1)
break;char c = (char) i ;System.out.print(c);
}//for loop}//trycatch (IOException e){
System.err.println();}//catchSystem.out.println( );
}//echo method}//Echo class
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Jan. 2004 14
Reading Multiple Bytes
• Since accessing I/O is slow in comparison to memory access, limiting the number of reads and writes is essential.
• The basic read() method only reads in a byte at a time.• The following two overloading read() methods read in multiple
bytes into an array of bytes.- public int read(byte b[])- public int read(byte b[], int offset, int length)
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Jan. 2004 15
Reading Multiple Bytes
• The first method tries to read enough bytes to fill the array b[].
try {byte[ ] b = new byte[10];int j = Svstem.in.read(b);}
catch (IOException e){ }
• This method blocks until data are available just like the read() method.
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Jan. 2004 16
Reading Multiple Bytes
• The second method reads length bytes from the input stream and stores them in the array b[] starting at the location offset.
try {//what does this loop do byte[] b = new byte[100]; int offset = 0;while (offset < b.length) {
int bytesRead = System.in.read(b, offset, b.length - offset);if (bytesRead == -1) break;offset += bytesRead; }//while
catch (IOException e){}
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Jan. 2004 17
Closing Input Streams
• For well behaved programs, all streams should be closedbefore exiting the program.
• Allows OS to free any resources associated with the stream.• Use the close() method
- public void close() throws IOException• Not all streams have to be closed.
- System.in does not have to be closed.
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Jan. 2004 18
Closing Input Streams
try {URL u = new URL(“http://java.sun.com”);InputStream in = u.openStream();/ read from stream ... in.close();
}catch (IOException e){}
• Once an input stream has been closed, you can no longer read from it. Doing so will cause an IOException to be thrown.
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Jan. 2004 19
Reading from File Input Streamsimport java.io.*;class FileInputStreamDemo {
public static void main(String args[]) {try {//Create a file input stream
FileInputStream fis = new FileInputStream(args[0]);//read 12 byte from the fileint i;while ((i = fis.read()) != -1)
{System.out.println(i);}//Close file output streamfis.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 20
Reading from Buffered Input Streamsimport java.io.*;class FileBufferedStreamDemo {
public static void main(String args[]) {try {//Create a file input stream
FileInputStream fis = new FileInputStream(args[0]);//Create a buffered input streamBufferedInputStream bis = new BufferedInputStream(fis);//read 12 byte from the fileint i;while ((i = bis.read()) != -1)
{System.out.println(i);}//Close file output streamfis.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 21
Reading from Data Input Streams
import java.io.*;class DataInputStreamDemo {
public static void main(String args[]) {try {//Create a file input stream
FileInputStream fis = new FileInputStream(args[0]);//Create a data input straemDataInputStream dis = new DataInputStream(fis);//read and display dataSystem.out.println(dis.readBoolean());System.out.println(dis.readByte());
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Jan. 2004 22
Reading from Data Input Streams
System.out.println(dis.readChar());System.out.println(dis.readDouble());System.out.println(dis.readFloat());System.out.println(dis.readInt());System.out.println(dis.readLong());System.out.println(dis.readShort());//Close file input streamfis.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 23
Output Streams
• java.io.OutputStream class sends raw bytes of data to a target such as the console, a file, or a network server.
• Methods within this class are:- public abstract void write(int b)- public void write(byte b[])- public void write(byte b[], int offset, int length)- public void flush()- public void close()
• All methods throw an IOException
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Jan. 2004 24
Output Streams
• The write() methods sends raw bytes of data to whomever is listening to the stream.
• Sometimes for performance reasons, the operating system buffers output streams.
• When the buffer fills up, the data are all written at once.• The flush() method will force the data to be written whether
the buffer is full or not.
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Jan. 2004 25
Writing to Output Streams
• The fundamental method in OutputStream is write()
• public abstract void write(byte b)
• This method writes a single unsigned byte of data that should be between 0 and 255.
• Larger numbers are reduced modulo 256 before writing.
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Jan. 2004 26
Ascii Chart Example
import java.io.*;public class AsciiChart{
public static void main(String args[]) {for (int i=32; i<127; i++)
System.out.write(i); //break line after every 8 characters
if (i%8 == 7) System.out.write(‘\n’);else System.out.write(‘\t’); }//forSystem.out.write(‘\n’);
}//main }//class
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Jan. 2004 27
Writing Arrays of Bytes
• The two remaining write methods write multiple bytes of data. - Public void write(byte b[])- Public void write(byte b[], int offset, int length)
• The first writes an entire byte array of data, while the second writes a sub-array of data starting at offset and continuing for length bytes.
• Remember that these methods write bytes, so data must be converted into bytes.
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Jan. 2004 28
AsciiArray Example
import java.io.*;public class AsciiArray{
public static void main(String args[]) {int index=O;byte[] b = new byte[(127-31)*2]; for (int i=32; i<127; i++) {
b[index++] = (byte)i;//break line after every 8 characters
if (i%8==7) b[index++] = (byte)‘\n’;else b[index++] = (byte) ‘\t’;
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Jan. 2004 29
AsciiArray Example
}//forb[index++] = (byte) ‘\n’;try {
System.out.write(b);}catch(IOException e) {}
}//main}//class
• The output is the same as AsciiChart.
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Jan. 2004 30
Writing to File Output Streams
import java.io.*;class FileOutputStreamDemo {
public static void main(String args[]) {try {//Create a file output stream
FileOutputStream fos = new FileOutputStream(args[0]);//Write 12 byte to the filefor (int i = 0; i < 12; i++) {
fos.write(i);}//Close file output streamfos.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 31
Flushing and Closing Output Streams
• As mentioned, many operating systems buffer output data to improve performance.
• Rather than sending a bytes at a time, bytes are accumulated until the buffer is full, and one write occurs.
• The flush() method forces the data to be written even if the buffer is not full.
- public void flush( ) throws IOException• Like input streams, output streams should be closed. For
output streams, closing them will also flush the contents of the buffer.
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Jan. 2004 32
Filter Streams
• java. io.FilterInputStream and java. io.FilterOutputStreamare subclasses of InputStream and OutputStream, respectively.
• These classes are rarely used, but their subclasses are extremely important.
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Jan. 2004 33
Filter Streams Classes
• Buffered Streams- These classes will buffer reads and writes by first reading the data into
a buffer(array of bytes)• Data Streams
- These classes read and write primitive data types and Strings.• Print Stream
- referenced by System.out and System.err.- It uses the platforms default character encoding to convert characters
into bytes.
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Jan. 2004 34
Buffered Streams
• Buffered input stream read more data than initially needed and store them in a buffer.
• So when the buffered stream's read() method is called, the data is removed from the buffer rather than from the underlying system.
• When the buffer is empty, the buffered stream refills the buffer.
• Buffered output stream store data in an internal byte array until the buffer is full or the stream is flushed. The data is then written out once.
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Jan. 2004 35
Buffered Streams
• Constructors- BufferedInputStream(InputStream in)- BufferedInputStream(Inputftream in, int size) - BufferedOutputStream(OutputStream out) - BufferedOutputStream(CrutputStream out, int size)
• The size argument is the size of the buffer.• If not specified, a default of 512 bytes is used.
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Jan. 2004 36
Buffered Streams
• Example:
URL u=new URL(“httP://java.sun.Com”);BufferedInputStream bis;bis= new BufferedlnputStream(u.openStream( ), 256)
• BufferedInputStreamand and BufferedOutputStream do not declare any new methods but rather override methods from Inputstream and outputstream, respectively.
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Jan. 2004 37
Writing to Buffered Output Streams
import java.io.*;class BufferedOutputStreamDemo {
public static void main(String args[]) {try {//Create a file output stream
FileOutputStream fos = new FileOutputStream(args[0]);//Create a buffered output straemBufferedOutputStream bos = new BufferedOutputStream(fos);//Write 12 byte to the filefor (int i = 0; i < 12; i++) {
bos.write(i);}//Close file output streambos.close(); fos.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 38
Data Streams
• java.io.DataInputStream and java.io.DataOutputStream read and write primitive data types and strings using the java.io.DataInputand java.io.DataOutput interfaces, respectively.
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Jan. 2004 39
Data Streams
• Generally you use DataInputStream to read data written by DataOutputStream
• public DataInputStrem(InputStream in)• public DataOutputStream(OutputStream out)• The usual methods associated with input and output streams
are present in data stream as well.• However, data streams have other methods that allow them to
read and write primitive type.
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Jan. 2004 40
Writing to Data Output Streams
import java.io.*;class DataOutputStreamDemo {
public static void main(String args[]) {try {//Create a file output stream
FileOutputStream fos = new FileOutputStream(args[0]);//Create a data output straemDataOutputStream dos = new DataOutputStream(fos);//Write various types of data to the filedos.writeBoolean(false);dos.writeByte(Byte.MAX_VALUE);
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Jan. 2004 41
Writing to Data Output Streams
dos.writeChar(‘A’);dos.writeDouble(Double.MAX_VALUE);dos.writeFloat(Float. MAX_VALUE);dos.writeInt(int. MAX_VALUE);dos.writeLong(Long. MAX_VALUE);dos.writeShort(Short. MAX_VALUE);//Close file output streamfos.close();
}catch(Exception e) {System.out.println(“Exception: ” + e);}}}
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Jan. 2004 42
Print Streams
• Allows very simple printing of both primitive values, objects, string literals.
• There are many overloaded print( ) and println( ) methods.
• This method is deprecated in Java 1.1.
• The biggest problem with this class is that it does not properly handle international character sets.
• Use the PrintWriter class instead.
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Jan. 2004 43
Readers and Writers
• Classes that read and write character based data.• These characters can have varying widths depending on the
character set being used.• Readers and writers know how to handle many different
character sets.
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Jan. 2004 44
Reader Class
• java.io.Reader
• This class is deliberately similar to the java.io.InputStream class.
• Methods in the Reader class are similar to the InputStream class except that the methods work on characters not bytes.
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Jan. 2004 45
Writer Class
• Java.io.Writer
• This class is similar to thejava.io.OutputStream class.
• Methods in the Writer class now work on characters and not bytes.
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Jan. 2004 46
InputStreamReader
• java. io.InputStreamReader acts as a translater between byte streams and character streams.
• It reads bytes from the input stream and translates them into characters according to a specified character encoding.
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Jan. 2004 47
InputStreamReader Class
• You can set the encoding scheme or you can use the platforms default setting.
• public InputstreamReader(Inputstream in)
• public InputStreamReader(InputStream in, String enc)throws UnsupportedEncoding Exception
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Jan. 2004 48
OutputStreamWriter
• java. io.OutputStreamWriter will write bytes of data to the output stream after translating the characters according to the specified encoding.
• public OutputStreamWriter(OutputStream out)• public OutputStreamWriter(OutputStream out, String enc)
throws UnsupportedEncodingException
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Jan. 2004 49
Buffered Reads/Writes
• There are classes that allow for a more efficient reading and writing of characters by buffering.
• java.io.BufferedReader• java.io.BufferedWriter• These classes are similar to the Buffered Stream classes.• Most notable for the readLine() Method. This allows data to be
read a line at a time. • public String readLine() throws IOException
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Buffered Reads/Writes
import java.io.*;
public class StringInputFile {public static void main(String[] arg) throws Exception {
PrintStream backup;FileOutputStream backupFileStream;File backupFile;backupFile = new File(“backup”);backupFileStream = new FileOutputStream(backupFile);backup = new PrintStream(backupFileStream);
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System.out.println(“This is my first data file”);backup.println(“This is my first data file”);System.out.println(“... but it won't be my last”);backup.println(“... but it won’t be my last”);}
}
Buffered Reads/Writes
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Writing output to a file involves three steps as follows:• Create an File object• Create an FileOutputStream object• Create a PrintStream object
Buffered Reads/Writes
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Buffered Reads/Writes
import java.io.*;
public class StringInputFile {public static void main(String[] arg) throws Exception {
InputStreamReader backup;BufferedReader br;FileInputStream backupFileStream;File backupFile;
String inputline;
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backupFile = new File(“backup”);backupFileStream = new FileInputStream(backupFile);backup = new InputStreamReader(backupFileStream);br = new BufferedReader(backup);inputline = br.readLine();System.out.println(inputline);inputline = br.readLine();System.out.println(inputline);}
}
Buffered Reads/Writes
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Reading data from a file involves three steps as follows:• Create an FileInputStream or BufferedInputStream object• Create an InputStreamReader object which we use to• Create a BufferedReader object
Buffered Reads/Writes
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Example: Send Data(l)
import java.net.*; import java.io.*;
public class SendData extends Thread {Socket sock;
public SendData (Socket sock) {
this.sock = sock;}//SendData constructor
public void run() {string line;
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Example: Send Data(2)
try {OutputStreamWriter outw=newoutputstreamwriter(sock.getOutputStream());BufferedWriter sockout=newBufferedWriter(outw);InputStreamReader inr = new InputStreamReader(System.in);BufferedReader in = new BufferedReader(inr);
while ((line = in.readLine()) != null) {
sockout.write(line+ “\n”);
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Example: Send Data(3)
sockout.flush(); yield( ); }//while
} //trycatch (java.io.IoException e) {
System.out.println(e);System.exit(0);
}//catch} //run
}//SendData
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Example: Receive Data(l)
import java.net.*;import java.io.*;
public class RcveData extends Thread {Socket sock;
public RcveData(Socket sock) {this.sock = sock;
}
public void run() {
String line;
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Example: Receive Data(2)
try {InputStreamReader inr = newInputStreamReader(sock.getlnputStream());
BufferedReader in = new BufferedReader(inr);
while ((line = in.readLine()) != null) {
System.out.print(mReceiving:System.out.println(line);yield();
}//while)//try
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Example: Receive Data(3)
catch (java.io.IOException e) {
System.out.println(e);
System.exit(0);
I }//catch}//run
}//RCVeData