Tirgul no. 13

1
Tirgul no. 13

• Topics covered
• String parsing.
• Text file I/O.
• Extending Filters.

2
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

3
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 StringTokenizers default delimiter list is
  all whitespace characters.
• The simple constructor receives the String to be
  parsed
• StringTokenizer tokenizer
• tokenizer new StringTokenizer(sentence)

4
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.

5
StringTokenizer (contd.)

• For example consider the sentence
• This is a lovely day
• to go out
• 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.

6
StringTokenizer Example
import java.util. //so we can use the
StringTokenizer class public class
SentenceParser public static void
main(String args) System.out.print(Plea
se 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(w
ord)
7
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.
8
Reading Text from Files (contd.)
FileReader Requires the following import
import java.io.FileReader
or
import java.io. Constructi
on public FileReader(String fileName)
throws FileNotFoundException
Creates a new FileReader, given the name of the
file to read from.
9
Reading Text from Files Example
import java.io. import java.util.StringTokenizer
try int lineNum, wordNum String
line BufferedReader inStream new
BufferedReader(new FileReader(args0))
lineNum wordNum 0 do line
inStream.readLine() if(line ! null)
lineNum StringTokenizer
st new StringTokenizer(line,args1)
wordNum st.countTokens()
while(line ! null) System.out.println("Th
ere are " lineNum " lines.")
System.out.println("There are " wordNum "
words.") catch(FileNotFoundException fnfe)
System.out.println("File (" args0 ") not
found.") catch(IOException ioe)
System.out.println("I/O error while reading file
(" args0 ")")
10
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.

11
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 char1
• public TranslateWriter(Writer w, String from,
  String to)
• super(w)
• this.from from
• this.to to

12
TranslateWriter (cont.)

• public void write(int c) throws IOException
• oneChar0 (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(i0 iltcbuf.length i)
• int mapIndex from.indexOf(cbufi)
• if(mapIndex gt0)
• cbufi to.charAt(mapIndex)
• super.write(cbuf,off,len)

13
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)

14
TranslateReader

• import java.io.
• public class TranslateReader extends
  FilterReader
• private String from
• private String to
• private static char oneChar
• static
• oneChar new char1
• public TranslateReader(Reader r, String from,
  String to)
• super(r)
• this.from from
• this.to to
• public TranslateReader(Reader r)

15
TranslateReader cont.

• public int read() throws IOException
• int result read(oneChar,0,1)
• if(result lt 0)
• return(result)
• else
• return(oneChar0)
• public int read(char cbuf, int off, int len)
  throws IOException
• int n super.read(cbuf,off,len)
• int i
• for(i0 iltn i)
• int mapIndex from.indexOf(cbufi)
• if(mapIndex gt0)
• cbufi to.charAt(mapIndex)
• return(n)

16
TranslateReader (main)

• public static void main(String args)
• StringReader sr new StringReader("Java is
  Great")
• TranslateReader translateReader new
  TranslateReader(sr)
• LineNumberReader lnr new LineNumberReader(trans
  lateReader)
• try
• String line lnr.readLine()
• System.out.println(line)
• catch(IOException ioe)
• //end of TranslateReader class.