Using Processing Stream

Predefined Streams

• System.in

InputStream used to read bytes from the keyboard

• System.out

PrintStream used to write bytes to the screen

• System.err

PrintStream used to report errors.

import java.io.*;

public class StdinRead {

public static void main(String args[]) {

int i=0; char c;

try {

do {

i = System.in.read();

c = (char)i;

System.out.println(c +”(“+i+”)”);

}while(i != -1);

} catch (IOException ioe) {

System.err.println(“IO Error”);

}

}

hello year 2000h(104)e(101)l(108)l(108)o(111) (32)y(121)e(101)a(97)r(114) (32)2(50)0(48)0(48)0(48)

(10)?(-1)

Reading int, float, double,...• System.in is an InputStream, with which we can only read

bytes.

• To read in int, float, …, you may want to put a processing stream at the end of stream pipe line.

• DataInput is an interface that defines all the methods for this purpose. – readBoolean(), readInt(), readDouble(), ….

• DataInputStream is a stream class that implements DataInput interface.

• Therefore, to read a java primitive data type from a InputStream, adding DataInputStream as a processing stream is one solution.

• However, does this work with System.in ?

import java.io.*;

public class StdinRead {

public static void main(String args[]) {

try {

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

do {

int value = din.readInt();

System.out.println(value);

} while ( value != -1 )

} catch (IOException ioe) {

System.err.println(“IO Error”);

}

}

Wrong solution, Not what we expected!!!

2000 2001 2002 2003 2004 2005218772016808460338808464672842018866540160048857748016808722482808465677

• To read a java primitive data type from a InputStream, the stream should provide exact data type in its binary form.

• However, a keyboard input is not for typing in a java primitive data type in binary form.

• Then how do we read various primitive data types from the keyboard?

Read everyting as a String

• What we expect from keyboard inputs is a sequence of characters.

• However System.in is not a Reader stream, but an InputStream.

• Therefore, to read in various types of data from the keyboard, we need to take three steps.– Convert InputStream to Reader

– Read in each line as a String.

– Parse the String and convert it to appropriate type.

How to read a line from InputStream

• InputStreamReader is the bridge between InputStream and Reader.– InputStreamReader isr = new InputStreamReader(System.in);

• With a BufferedReader, we can read a line (as a String) from Reader stream.– BufferedReader breader = new BufferedReader(isr);

• Now we can read a String line from the keyboard.– String line = breader.readLine();

import java.io.*;

class LineReader{ BufferedReader breader;

LineReader(Reader input) throws IOException {breader = new BufferedReader(input);

} public String getLine() throws IOException {

return breader.readLine(); }

public static void main(String args[]) {try { String line=null; LineReader lreader =

new LineReader(new InputStreamReader(System.in));

while ( (line=lreader.getLine()) != null )System.out.println(line);

}catch (IOException ioe) {}

}}

StringTokenizer

• StringTokenizer class allows us to break a String into tokens.

• Without any given delimeter, the space will be used as a default delimeter.

• “Hello year 2000”, will be broken into

“Hello”, “year”, “2000”

• StringTokenizer class provides methods for retrieving and checking availability of tokens.– boolean hasMoreTokens();

– String nextToken();

import java.io.*;

public class InputParser {

public void static main(String args[]) {

try {

LineReader reader = new LineReader(new FileReader(args[0]));

String line = null;

while ( (line=reader.getLine()) != null ) {

StringTokenizer st = new StringTokenizer(line);

while ( st.hasMoreTokens() ) {

String token = st.nextToken();

// Parse a token

// int year = Integer.parseInt(token);

}

}

}catch (IOException e) { }

}

}