Session 05 Java Strings and Files

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Session 05Java Strings and Files
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Exercise

• Complete the quick-and-dirty class
  CharacterCounter containing only a main() method
  that displays the number of non-space characters
  on the command line after the command. For
  example
• java CharacterCounter
• 0
• java CharacterCounter a
• 1
• java CharacterCounter a bc def ghij
• 10

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CharacterCount template

• public class CharacterCounter
• public static void main( String args )
• int characterCount 0
• // end main
• // end class CharacterCounter

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StringTokenizer

• Useful tool for processing a String object
• Allows you to sequentially walk down a String and
  extract words/tokens that are delimited by
  specified characters
• What delimiter normally aids us in parsing a long
  string into words?

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StringTokenizer

• General usage of a StringTokenizer
• create one using a constructor that takes a
  string argument to process
• send one of two messages hasMoreTokens() and
  nextToken
• use a stereotypical loop to process a sequence
  of strings
• A default StringTokenizer uses spaces as
  delimiters.

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StringTokenizer Example

• import java.util.StringTokenizer
• public class EchoWordsInArgumentV1
• public static void main( String args )
• StringTokenizer words new
  StringTokenizer(args0)
• while( words.hasMoreElements() )
• String word words.nextToken()
• System.out.println( word )
• // end while
• // end main
• // end class EchoWordsInArgumentV1

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StringTokenizer Example

• java EchoWordsInArgumentV1 "StringTokenizer,
  please process me."
• StringTokenizer,
• please
• process
• me.
• Notice the quotes () in the command line so the
  whole string is read as args0.
• The comma (,) and period (.)are part of the
  words and not delimiters by default.

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StringTokenizer Example 2

• Fortunately, we can construct a StringTokenizer
  that uses specified characters for delimiters.
• The designer of the StringTokenizer was planning
  ahead for future usage!!!
• java EchoWordsInArgumentV2 "StringTokenizer,
  please process me."
• StringTokenizer
• please
• process
• me

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StringTokenizer Example 2

• import java.util.StringTokenizer
• public class EchoWordsInArgumentV2
• public static void main( String args )
• String delimiters " .?!()?/\\,-\'\
  "\t\n\r"
• StringTokenizer words new
  StringTokenizer( args0, delimiters
  )
• while( words.hasMoreElements() )
• String word words.nextToken()
• System.out.println( word )
• // end while
• // end main
• // end class EchoWordsInArgumentV2

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UNIX/Linux pipe

• character on the command line
• Allows the output of one program to be sent as
  input to another program, like the UNIX sort
  utility.
• java EchoWordsInArgumentV2 "StringTokenizer,
  please process me. sort
• StringTokenizer
• me
• please
• process
• Is this sorted? How can we fix this?

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StringTokenizer Example 3

• import java.util.StringTokenizer
• public class EchoWordsInArgumentV3
• public static void main( String args )
• String delimiters " .?!()?/\\,-\'\
  "\t\n\r"
• StringTokenizer words new
  StringTokenizer( args0,
• delimiters )
• while( words.hasMoreElements() )
• String word words.nextToken()
• word word.toLowerCase()
• System.out.println( word )
• // end while
• // end main
• // end class EchoWordsInArgumentV3

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StringTokenizer Example 3

• java EchoWordsInArgumentV3 "StringTokenizer,
  please process me." sort
• me
• please
• process
• stringtokenizer

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Java File I/O

• Allows us to write and read permanent
  information to and from disk
• How would file I/O help improve the capabilities
  of the MemoPadApp?

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Java File I/O Example Echo.java

• echoes all the words in one file to an output
  file, one per line.
• java Echo hamlet.txt hamlet.out
• less hamlet.out
• 1604
• the
• tragedy
• of
• hamlet
• prince
• of
• denmark
• by
• william
• shakespeare ...

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Study Echo.javas File I/O

• have constructors that allow convenient and
  flexible processing
• send input message readLine()
• send output messages print() and println()
• use a stereotypical loop to process a file of
  lines
• use of the stereotypical StringTokenizer loop as
  inner loop

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import java.io. import java.util.StringTokenizer
public class Echo public static void
main( String args ) throws IOException
String delimiters " .?!()?/\\,-\'\"\t\n
\r" BufferedReader inputFile new
BufferedReader(new FileReader(args0) )
PrintWriter outputFile new PrintWriter( new
FileWriter( args1 ) ) String buffer
null while( true ) buffer
inputFile.readLine() if ( buffer
null ) break buffer
buffer.toLowerCase() StringTokenizer
tokens new StringTokenizer( buffer, delimiters
) while( tokens.hasMoreElements() )
String word
tokens.nextToken()
outputFile.println( word ) // end
while // end while(true)... //
end main // end class Echo
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wc - UNIX/Linux utility

• wc prints the number of lines, words, and
  characters in a file to standard output.
• For example
• wc hamlet.txt
• 4792 31957 196505 hamlet.txt

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Exercise

• Using Echo.java as your starting point, create a
  WordCount.java program that does the same thing
  as wc, i.e., prints the number of lines, words,
  and characters in a file to standard output. For
  example
• java WordCount hamlet.txt
• 4792 32889 130156

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import java.io. import java.util.StringTokenizer
public class WordCount public static
void main( String args ) throws IOException
String delimiters " .?!()?/\\,-\'\
"\t\n\r" BufferedReader inputFile new
BufferedReader( new FileReader( args0 ) )
String buffer null int chars
0 int words 0 int
lines 0 while( true )
buffer inputFile.readLine() if (
buffer null ) break lines
buffer buffer.toLowerCase()
StringTokenizer tokens new StringTokenizer(
buffer, delimiters ) while(
tokens.hasMoreElements() )
String word tokens.nextToken()
words chars word.length()
// end while // end while(
true )... System.out.println( "" lines
" " words " " chars ) // end
main // end class WordCount
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Why the difference in the number of words and
number of characters?

• wc hamlet.txt
• 4792 31957 196505 hamlet.txt
• java WordCount hamlet.txt
• 4792 32889 130156