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Transcript
Tirgul no. 13
Topics covered:



String parsing.
Text file I/O.
Extending Filters.
1
Parsing Strings
 Many different tasks involve taking a string and parsing it into
Tokens.
 A Token is a part of the string.
 Tokens are separated by Delimeters.
 a Delimeter can be any char.
Examples: space, \n
 Tokens may be separated by many delimeters:
“I
have a
car”
2
Parsing Strings (contd.)
 We can define more than one Delimeter.
For example many times the Delimiters may be all White Space
characters: space \n \t \r
 The package java.util includes a class called StringTokenizer that
makes it easy to parse a string into tokens.
 The StringTokenizer’s default delimiter list is all whitespace
characters.
 The simple constructor receives the String to be parsed:
StringTokenizer tokenizer;
tokenizer = new StringTokenizer(sentence);
3
StringTokenizer
 The method nextToken reads the next Token from the String
and returns it as a String:
String word= tokenizer.nextToken();
 All delimeter characters (by default whitespace chars) are
ignored. The word returned is a sequence of all non delimiter
characters from the current position – up to the first delimiter
char.
4
StringTokenizer (contd.)
For example consider the sentence:
“This is a lovely
to go out
day
“
Assuming that the delimiters are whitespace chars this
sentence contains 8 Tokens: This,is,a,lovely,day,to,go,out.
All other chars in the sentence are whitespace and the
StringTokenizer will not include these in the tokens that it
returns.
5
StringTokenizer Example
import java.util.*; //so we can use the StringTokenizer class
public class SentenceParser{
public static void main(String[] args){
System.out.print(“Please enter a sentence: “);
String sentence= EasyInput.readString();
//init a StringTokenizer object that we will use
//to parse the sentence.
StringTokenizer tokenizer= new StringTokenizer(sentence);
while( tokenizer.hasMoreTokens() ){
String word= tokenizer.nextToken();
System.out.println(word);
}
}
}
6
Reading Text from Files
BufferedReader
Requires the following import:
import java.io.BufferedReader;
or
import java.io.*;
Construction:
public BufferedReader(Reader in) {…}
Create a buffering character-input stream that uses a
default-sized input buffer.
Line Reading:
public String readLine() throws IOException {…}
Read a line of text. A line is considered to be terminated by any one of a line feed
('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.
Returns a String containing the contents of the line, not including
any line-termination characters, or null if the end of the stream has been reached.
7
Reading Text from Files (contd.)
FileReader
Requires the following import:
import java.io.FileReader;
or
import java.io.*;
Construction:
public FileReader(String fileName) throws FileNotFoundException {…}
Creates a new FileReader, given the name of the file to read from.
8
Reading Text from Files Example
import java.io.*;
import java.util.StringTokenizer;
try {
int lineNum, wordNum;
String line;
BufferedReader inStream = new BufferedReader(new FileReader(args[0]));
lineNum = wordNum = 0;
do {
line = inStream.readLine();
if(line != null) {
lineNum++;
StringTokenizer st = new StringTokenizer(line,args[1]);
wordNum += st.countTokens();
}
}while(line != null);
System.out.println("There are " + lineNum + " lines.");
System.out.println("There are " + wordNum + " words.");
}catch(FileNotFoundException fnfe) {
System.out.println("File ("+ args[0] +") not found.");
}catch(IOException ioe) {
System.out.println("I/O error while reading file ("+ args[0] +")");
}
9
Extending Filter Classes



FilterReader and FilterWriter are filter streams that
can be extended to provide new formatting of
textual data.
Recall that a FilterStream is a stream that connects
to other Streams and therefore can be connected to
any Reader/Writer.
We will now design new Filters for reading and
writing which can Translate (or encode/decode)
strings.
10
TranslateWriter
import java.io.*;
public class TranslateWriter extends FilterWriter{
private String from;
private String to;
private static char[] oneChar;
private static char[] charArray;
static{
oneChar= new char[1];
}
public TranslateWriter(Writer w, String from, String to){
super(w);
this.from= from;
this.to= to;
}
public TranslateWriter(Writer w){
super(w);
String upper= "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String lower= "abcdefghijklmnopqrstuvwxyz";
this.from= upper + lower;
this.to= lower + upper;
}
11
TranslateWriter (cont.)
public void write(int c) throws IOException {
oneChar[0]= (char)c;
write(oneChar,0,1);
}
public void write(char[] cbuf, int off, int len) throws IOException{
if(cbuf == null)
throw new IOException();
int i;
for(i=0; i<cbuf.length; i++){
int mapIndex= from.indexOf(cbuf[i]);
if(mapIndex >=0)
cbuf[i]= to.charAt(mapIndex);
}
super.write(cbuf,off,len);
}
public void write(String s, int off, int len) throws IOException{
if(s == null)
throw new IOException();
charArray= s.toCharArray();
write(charArray,off,len);
}
12
TranslateWriter (main)
public static void main(String[] args){
OutputStreamWriter osw;
TranslateWriter translateWriter;
osw= new OutputStreamWriter(System.out);
translateWriter= new TranslateWriter(osw);
try{
translateWriter.write("Java is Great");
translateWriter.flush();
}catch(IOException ioe){
System.err.println(ioe);
System.exit(1);
}
}
}
13
TranslateReader
import java.io.*;
public class TranslateReader extends FilterReader{
private String from;
private String to;
private static char[] oneChar;
static{
oneChar= new char[1];
}
public TranslateReader(Reader r, String from, String to){
super(r);
this.from= from;
this.to= to;
}
public TranslateReader(Reader r){
super(r);
String upper= "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String lower= "abcdefghijklmnopqrstuvwxyz";
this.from= upper + lower;
this.to= lower + upper;
}
14
TranslateReader cont.
public int read() throws IOException {
int result= read(oneChar,0,1);
if(result < 0)
return(result);
else
return(oneChar[0]);
}
public int read(char[] cbuf, int off, int len) throws IOException{
int n= super.read(cbuf,off,len);
int i;
for(i=0; i<n; i++){
int mapIndex= from.indexOf(cbuf[i]);
if(mapIndex >=0)
cbuf[i]= to.charAt(mapIndex);
}
return(n);
}
15
TranslateReader (main)
public static void main(String[] args){
StringReader sr= new StringReader("Java is Great");
TranslateReader translateReader= new TranslateReader(sr);
LineNumberReader lnr= new LineNumberReader(translateReader);
try{
String line= lnr.readLine();
System.out.println(line);
}catch(IOException ioe){
}
}
}//end of TranslateReader class.
16