Java - StreamTokenizer slashStarComments(boolean flag) method

Description

The Java StreamTokenizer slashStarComments(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. All text between successive occurrences of /* and */ are discarded.

If the flag argument is false, then C-style comments are not treated specially.

Declaration

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

public void slashStarComments(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 slashStarComments(boolean flag) method

The following example shows the usage of StreamTokenizer slashStarComments(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-star comments as recognizable
         st.slashStarComments(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: EHello.
Word: This
Word: will
Word: be
Word: split
Word: into
Word: tokens.
Number: 1.0
+ encountered.
Number: 1.0
= encountered.
Number: 2.0
End of File encountered.

Example - Skip block comment between tokens

The following example shows the usage of StreamTokenizer slashStarComments(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 = "start /* this is a comment */ end";

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

      tokenizer.slashStarComments(true); // Enable /* ... */ comments
      tokenizer.wordChars('a', 'z');
      tokenizer.wordChars('A', 'Z');

      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: start
Word: end

Explanation

• The comment /* this is a comment */ is completely skipped by the tokenizer.

• Only the actual words start and end are processed.

Example - Block comment across multiple lines

The following example shows the usage of StreamTokenizer slashStarComments(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\nmulti-line comment */ 200";

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

      tokenizer.slashStarComments(true); // Enable block comments
      tokenizer.parseNumbers(); // Recognize numbers

      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

Explanation

• The multi-line comment /* skip this\nmulti-line comment */ is ignored.

• Only the numeric tokens 100 and 200 are processed.