Java - StreamTokenizer slashSlashComments(boolean flag) method

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Description

The Java StreamTokenizer slashSlashComments(boolean flag) method determines whether or not the tokenizer recognizes C++-style comments. If the flag argument is true, this stream tokenizer recognizes C++-style comments. Any occurrence of two consecutive slash characters ('/') is treated as the beginning of a comment that extends to the end of the line. If the flag argument is false, then C++-style comments are not treated specially.

Declaration

Following is the declaration for java.io.StreamTokenizer.slashSlashComments(boolean flag) method.

public void slashSlashComments(boolean flag)

Parameters

flag − true indicates to recognize and ignore C++-style comments.

Return Value

This method does not return a value.

Exception

NA

Example - Usage of StreamTokenizer slashSlashComments(boolean flag) method

The following example shows the usage of StreamTokenizer slashSlashComments(boolean flag) method.

StreamTokenizerDemo.java

package com.tutorialspoint;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Reader;
import java.io.StreamTokenizer;

public class StreamTokenizerDemo {
   public static void main(String[] args) {      
      String text = "Hello. This is a text \n that //will be split "
         + "into tokens. 1 + 1 = 2";
         
      try {
         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write something in the file
         oout.writeUTF(text);
         oout.flush();

         // create an ObjectInputStream for the file we created before
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));

         // create a new tokenizer
         Reader r = new BufferedReader(new InputStreamReader(ois));
         StreamTokenizer st = new StreamTokenizer(r);

         // set slash-slash comments as recognizable
         st.slashSlashComments(true);

         // print the stream tokens
         boolean eof = false;
         
         do {
            int token = st.nextToken();
            
            switch (token) {
               case StreamTokenizer.TT_EOF:
                  System.out.println("End of File encountered.");
                  eof = true;
                  break;
                  
               case StreamTokenizer.TT_EOL:
                  System.out.println("End of Line encountered.");
                  break;
                  
               case StreamTokenizer.TT_WORD:
                  System.out.println("Word: " + st.sval);
                  break;
                  
               case StreamTokenizer.TT_NUMBER:
                  System.out.println("Number: " + st.nval);
                  break;
                  
               default:
                  System.out.println((char) token + " encountered.");
                  
                  if (token == '!') {
                     eof = true;
                  }
            }
         } while (!eof);
         
      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Word: CHello.
Word: This
Word: is
Word: a
Word: text
Word: that
End of File encountered.

Example - Ignoring comments from //

The following example shows the usage of StreamTokenizer slashSlashComments(boolean flag) method.

StreamTokenizerDemo.java

package com.tutorialspoint;

import java.io.IOException;
import java.io.Reader;
import java.io.StreamTokenizer;
import java.io.StringReader;

public class StreamTokenizerDemo {
   public static void main(String[] args) throws IOException {
      String input = "value1 // this is a comment\nvalue2";

      Reader reader = new StringReader(input);
      StreamTokenizer tokenizer = new StreamTokenizer(reader);

      tokenizer.slashSlashComments(true); // Enable // comments
      tokenizer.wordChars('a', 'z');
      tokenizer.wordChars('A', 'Z');
      tokenizer.wordChars('0', '9');

      System.out.println("Tokens:");
      while (tokenizer.nextToken() != StreamTokenizer.TT_EOF) {
         if (tokenizer.ttype == StreamTokenizer.TT_WORD) {
            System.out.println("Word: " + tokenizer.sval);
         }
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Tokens:
Word: value1
Word: value2

Explanation

• The tokenizer skips everything after // on that line.

• Only value1 and value2 are processed.

Example - Mix of data and comments

The following example shows the usage of StreamTokenizer slashSlashComments(boolean flag) method.

StreamTokenizerDemo.java

package com.tutorialspoint;

import java.io.IOException;
import java.io.Reader;
import java.io.StreamTokenizer;
import java.io.StringReader;

public class StreamTokenizerDemo {
   public static void main(String[] args) throws IOException {
      String input = "100 // skip this\n200 // skip that too\n300";

      Reader reader = new StringReader(input);
      StreamTokenizer tokenizer = new StreamTokenizer(reader);

      tokenizer.slashSlashComments(true); // Treat // as comment
      tokenizer.parseNumbers(); // Enable number recognition

      System.out.println("Numbers:");
      while (tokenizer.nextToken() != StreamTokenizer.TT_EOF) {
         if (tokenizer.ttype == StreamTokenizer.TT_NUMBER) {
            System.out.println("Number: " + tokenizer.nval);
         }
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Numbers:
Number: 100.0
Number: 200.0
Number: 300.0

Explanation

• The tokenizer ignores all comments and processes only numeric tokens.