csc 1214: object-oriented programming - serval.ugserval.ug/~jona/materials/csc1214/09...

CSC 1214: Object-Oriented Programming

J. Kizito

Makerere University

e-mail: [email protected]: http://serval.ug/~jona

materials: http://serval.ug/~jona/materials/CSC1214

e-learning environment: http://muele.mak.ac.ug

office: block A, level 3, department of computer sciencealt. office: institute of open, distance, and eLearning, room 2

Java Input/Output (I/O)

Kizito (Makerere University) CSC 1214 April, 2018 1 / 22

http://serval.ug/~jona

http://serval.ug/~jona/materials/CSC1214

http://muele.mak.ac.ug

Overview

1 Java I/OI/O StreamsPredefined StreamsConsole I/OFile I/O


Java I/O

Java I/OIntroduction

Apart from print() and println(), none of the I/O methods havebeen used significantly because most real applications of Java are nottext-based, console programs

Java provides strong, flexible support for I/O as it relates to files andnetworks

Java programs perform I/O through streams


Java I/O I/O Streams

Java I/OI/O Streams

A stream is an abstraction that either produces or consumesinformation

A stream is linked to a physical device by the Java I/O system

An input stream can abstract many different kinds of input: disk file,keyboard, network socket

An output stream may refer to the console, disk file, or networkconnection

Java implements streams within class hierarchies defined in thejava.io package



Java I/OByte streams and Character streams

Java 2 defines two types of classes: byte and character

Byte streams provide a convenient means for handling input and output of bytes

Character streams provide a convenient means for handling input and output ofcharacters

In some cases, character streams are more efficient than byte streams

The Byte Stream

Defined by using two classhierarchies

At the top are two abstract classes:InputStream and OutputStream

These classes define several keymethods including read() andwrite(), which, respectively, readand write bytes of data

The Character Stream

Defined by using two classhierarchies

At the top are two abstract classes:Reader and Writer

These classes define several keymethods including read() andwrite(), which read and writecharacters of data, respectively



java.io



The Byte Stream Classes

BufferedInputStream

BufferedOutputStream

ByteArrayInputStream Reads from a byte arrayByteArrayOutputStream Writes to a byte arrayDataInputStream Reading Java standard data typesDataOutputStream Writing Java standard data typesFileInputStream Reads from a fileFileOutputStream Writes to a fileFilterInputStream Implements InputStream

FilterOutputStream Implements OutputStream

InputStream Abstract classOutputStream Abstract classPipedInputStream Input pipePipedOutputStream Ouyput pipePrintStream Contains print() and println()

PushBackInputStream Supports one-byte “unget” – returns a byte to the inputstream

RandomAccessFile Random access file I/OSequenceInputStream Combination of two or more input streams that will be

read sequentiallyKizito (Makerere University) CSC 1214 April, 2018 7 / 22


java.io.InputStream



The Character Stream Classes

BufferedReader Buffered input character streamBufferedWriter Buffered output character streamCharArrayReader Reads from a character arrayCharArrayWriter Writes to a character arrayFileReader Reads from a fileFileWriter Writes to a fileFilterReader Filtered readerFilterWriter Filtered writerInputStreamReader Translates bytes to charactersLineNumberReader Counts linesOutputStreamWriter Translates characters to bytesPipedReader Input pipePipedWriter Output pipePrintWriter Contains print() and println()

PushBackReader Allows characters to be returned to the input streamReader Abstract classStringReader Reads from a stringStringWriter Writes to a stringWriter Abstract class



java.io.Reader


Java I/O Predefined Streams

Java I/OPredefined Streams

The java.lang package defines a class called System which containsthree predefined stream variables: in, out, and err

They are defined as public and static so they can be used by anyother part of your program without reference to a specific Systemobject

System.out refers to the standard output stream (default: console)

System.err refers to the standard error stream (default: console)

These streams may be redirected to any compatible I/O devices

System.in is an object of type InputStream; System.out andSystem.err are objects of type PrintStream

We have seen sample uses of System.out in previous examples

We have also seen a Console class that makes use of System.in



Predefined Streamsjava.util.Scanner

Declaration

public final class Scanner

extends Object implements Iterator<String>, Closeable

Constructors

1 Scanner(File source)

2 Scanner(File source, String charsetName)

3 Scanner(InputStream source)

4 Scanner(InputStream source, String charsetName)

5 Scanner(Readable source)

6 Scanner(ReadableByteChannel source)

7 Scanner(ReadableByteChannel source, String charsetName)

8 Scanner(String source)



Predefined Streamsjava.util.Scanner methods

Defines over 50 methods

nextX() methods:

1 String next()

2 String next(Pattern pattern)

3 String next(String pattern)

4 BigDecimal nextBigDecimal()

5 BigInteger nextBigInteger()

6 BigInteger

nextBigInteger(int radix)

7 boolean nextBoolean()

8 byte nextByte()

9 byte nextByte(int radix)

10 double nextDouble()

11 float nextFloat()

12 int nextInt()

13 int nextInt(int radix)

14 String nextLine()

15 long nextLong()

16 long nextLong(int rad)

17 short nextShort()

18 short

nextShort(int radix)Kizito (Makerere University) CSC 1214 April, 2018 13 / 22

Java I/O Console I/O

Java I/OReading Console Input

To obtain a stream that is attached to the console, we use thefollowing constructor:BufferedReader(Reader iReader)

For exampleBufferedReader br = new BufferedReader(

new InputStreamReader(System.in)

);

In this case, br is a character-based stream that is linked to theconsole through System.in



Reading Console InputBufferedReader Example

1. import java.io.*;

2.3. class BRRead {4. public static void main(String args[])

5. throws IOException {6. char c;

7. InputStreamReader sr = new InputStreamReader(System.in);

8. BufferedReader br = new BufferedReader(sr);

9. System.out.println("Enter characters, ’q’ to quit.");

10. // read characters11. do {12. c = (char) br.read();

13. System.out.println(c);

14. } while(c != ’q’);

15. }16. }

Sample output

Enter characters, ’q’ to quit.

12abcq

1

2

a

b

c

q



Reading Console InputReading Characters and Strings

Reading characters

To read a character from BufferedReader, use read():int read() throws IOException

read() reads a character from the input stream and returns it as aninteger value. -1 when the end of stream is encountered

Reading strings

To read a string, use the version of readLine() that is a member ofthe BufferedReader class:String readLine() throws IOException

For exampleBufferedReader br = new BufferedReder(

new InputStreamReader(System.in));

String str = br.readLine();



Java I/OWriting Console Output

Use methods print(), println(), and write() defined byPrintStream (a type of object referenced by System.out)

Example:class WriteDemo {

public static void main(String args[]) {int b = ’A’;

System.out.write(b);

System.out.write(’\n’);System.out.println("A string");

}}



Java I/OThe PrintWriter class

PrintWriter defines several constructors. E.g.,PrintWriter(OutputStream os, boolean flushOnNewline)

flushOnNewline controls whether Java flushes the output stream every time anew line (’\n’) character is output

PrintWriter supports the print() and println() methods

PrintWriter Example

import java.io.*;

public class PrintWriterDemo {public static void main(String args[]) {

PrintWriter pw = new PrintWriter(System.out, true);

pw.println("This is a string");

int i = -7;

pw.println(i);

double d = 4.5e-7;

pw.println(d);

}}

Output

This is a string

-7

4.5E-7


Java I/O File I/O

Java I/OReading and Writing Files

Two of the most often-used streams are:1 FileInputStream(String filename) throws

FileNotFoundException2 FileOutputStream(String filename) throws

FileNotFoundException

When done with a file, close it using close() defined by bothFileInputStream and FileOutputStream

To read, use read() defined by FileInputStream

To write, use write() defined by FileOutputStream


Java I/O File I/O

Reading Example


2.3. class ShowFile {4. public static void main(String args[]) throws IOException {5. int i;

6. FileInputStream fin;

7.8. try {9. fin = new FileInputStream(args[0]);

10. } catch(FileNotFoundException e) {11. System.out.println("File Not Found");

12. return;

13. } catch(ArrayIndexOutOfBoundsException e) {14. System.out.println("Usage: ShowFile File");

15. return;

16. }17. do { // read characters until EOF is encountered18. i = fin.read();

19. if(i != -1) System.out.print((char) i);

20. } while(i != -1);

21. fin.close();

22. }23. }


Java I/O File I/O

Reading and Writing (Copy) Example


2. class CopyFile {3. public static void main(String args[]) throws IOException {4. int i;

5. FileInputStream fin;

6. FileOutputStream fout;

7. try {8. try { fin = new FileInputStream(args[0]); // open input file9. } catch(FileNotFoundException e) {10. System.out.println("Input File Not Found");

11. return;

12. }13. try { fout = new FileOutputStream(args[1]); } // open output file14. catch(FileNotFoundException e) {15. System.out.println("Error Opening Output File");

16. return;

17. }18. } catch(ArrayIndexOutOfBoundsException e) {19. System.out.println("Usage: CopyFile From To");

20. return;

21. }22. try { // Copy File23. do {24. i = fin.read();

25. if(i != -1) fout.write(i);

26. } while(i != -1);

27. } catch(IOException e) { System.out.println("File Error"); }28. fin.close();

29. fout.close();

30. }31. }


Java I/O File I/O

Modify File Contents

1. import java.util.*;


3.4. class FileReplace {5. public static void main(String args[]) {6. ArrayList<String> lines = new ArrayList<String>();

7. try {8. File f = new File(args[0]);

9. BufferedReader br = new BufferedReader(new FileReader(f));

10. for (String line; (line = br.readLine()) != null; ) {11. if (line.contains("Java")) // line to modify12. line = line.replace("Java", "C");

13. lines.add(line);

14. }15. br.close();

16.17. PrintStream ps = new PrintStream(f); // open for writing18. for (Iterator i = lines.iterator(); i.hasNext(); )

19. ps.println(i.next());

20. ps.close();

21. } catch (Exception ex) { ex.printStackTrace(); }22. }23. }


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