Slide 15.a: A word counter

Slide 15.16: Method overriding (cont.): Multi-level overriding
Slide 16.1: Generics
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A Word Counter

The program below demonstrates use of a map to count the frequency of words in a file, where a word is delimited by the regular expression [^a-zA-Z']+. The following classes and interface are used in the program:

The class Scanner can parse primitive types and strings using regular expressions. A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace. The resulting tokens may then be converted into values of different types using the various next methods.

The interface Map<K,V> maps keys to values where K is the type of keys maintained by this map and V is the type of mapped values.

The class TreeMap<K,V> is sorted according to the natural ordering of its keys.

WordCounter.java (a simple word counter)

 import java.io.File;
 import java.io.IOException;
 import java.util.Map;
 import java.util.Scanner;
 import java.util.TreeMap;

 public class WordCounter {
   public static void main( String[] args ) {
     try {
       File f = new File( args[0] );
       Scanner sc;
       sc = new Scanner( f );
       // sc.useDelimiter( "[^a-zA-Z']+" );
       Map<String, Integer> wordCount = new TreeMap<String, Integer>( );
       while ( sc.hasNext( ) ) {
         String word = sc.next( );
         if( !wordCount.containsKey( word ) )
           wordCount.put( word, 1 );
         else
           wordCount.put( word, .get( word ) + 1 );
       }
            
       // Show results.
       for ( String word : wordCount.keySet( ) )
         System.out.println( word + " " + wordCount.get( word ) );
       System.out.println( wordCount.size( ) );
     }
     catch( IOException e ) {
       System.out.println( "Unable to read from file." );
     }
   }
 }

shell> java WordCounter

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Slide 15.16: Method overriding (cont.): Multi-level overriding
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