java string StringTokenizer 无法识别“//”之后的标记？

Question

i am writing a code where i want to print only comments in a java file , it worked when i have a comments like this

// a comment

but when i have a comment like this :

// /* cdcdf

it will not print "/* cdcdf" , it only prints a blank line anyone know why this happens ?

here is my code :

package printC;

import java.io.*; 
import java.util.StringTokenizer;
import java.lang.String ;


public class PrintComments {
    

    public static void main(String[] args) {
        try {
            String line;
            BufferedReader br = new BufferedReader(new FileReader(args[0]));
            while ((line = br.readLine()) != null) {
                if  (line.contains("//") ) {
                     StringTokenizer st1 =  new StringTokenizer(line, "//"); 
                     if(!(line.startsWith("//"))) {
                         st1.nextToken();
                     }
                     System.out.println(st1.nextToken()); 
                }
            }   
        }catch (Exception e) {
            System.out.println(e);
        }       
    }
}

Answer 1

StringTokenizer takes a collection of delimiters, not a single string delimiter. so it is splitting on the '/' char. the "second" token is the empty token between the two initial "//".

If you just want the rest of the line after the "//", you could use:

if(line.startsWith("//")) {
  line = line.substring(2);
}

Answer 2

You can simplify the code by just looking for the first position of the // . indexOf works fine for this. You don't need to tokenize as you really just want everything after a certain position (or text), you don't need to split the line into multiple pieces.

If you find the // ( indexOf doesn't return -1 for "not found"), you use substring to only print the characters starting at that position.

This minimal example should do what you want:

import java.io.*;
import java.util.StringTokenizer;


public class PrintComments {
    public static void main(String[] args) throws IOException {
        String line;    // comment
        BufferedReader br = new BufferedReader(new FileReader(args[0]));
        while ((line = br.readLine()) != null) {
            int commentStart = line.indexOf("//");
            if (commentStart != -1) {
                System.out.println(line.substring(commentStart));
            }
        } // /* that's it
    }
}

If you don't want to print the // , just add 2 to commentStart .

Note that this primitive approach to parsing for comments is very brittle. If you run the program on its own source, it will happily report //"); as well, for the line of the indexOf . Any serious attempt to find comments need to properly parse the source code.

Edit: If you want to look for other comments marked by /* and */ as well, do the same thing for the opening comment, then look for the closing comment at the end of the line. This will find a /* comment */ when all of the comment is on a single line. When it sees the opening /* it looks whether the line ends with a closing */ and if so, uses substring again to only pick the parts between the comment markers.

import java.io.*;
import java.util.StringTokenizer;


public class PrintComments {
    public static void main(String[] args) throws IOException {
        String line;    // comment
        BufferedReader br = new BufferedReader(new FileReader(args[0]));
        while ((line = br.readLine()) != null) {
            int commentStart;
            String comment = null;

            commentStart = line.indexOf("//");
            if (commentStart != -1) {
                comment = line.substring(commentStart + 2);
            }

            commentStart = line.indexOf("/*");
            if (commentStart != -1) {
                comment = line.substring(commentStart + 2);
                if (comment.endsWith("*/")) {
                    comment = comment.substring(0, comment.length() - 2);
                }
            }

            if (comment != null) {
                System.out.println(comment);
            }
        } // /* that's it
        /* test */
    }
}

To extend this for comments that span multiple lines, you need to remember whether you're in a multi-line comment, and if you are keep printing line and checking for the closing */ .

Answer 3

Additional to @jtahlborn answer. You can check all of the token by iterating token: eg:

...
 StringTokenizer st1 =  new StringTokenizer(line, "//"); 
 while (st1.hasMoreTokens()){
        System.out.println("token found:" + st1.nextToken());
}
...

Answer 4

If you are reading per line, the StringTokenizer don't do much in your code. Try this, change the content of if like this:

if(line.trim().startWith("//")){//true only if líne start with //,aka: comment line
//Do stuff with líne
String cleanLine = line.trim().replace("//"," ");//to remove all // in line
String cleanLine = línea.trim().substring(2,línea.trim().lenght());//to remove only the first //
}

Note: try to always use the trim() to remove all Blanc spaces at begin and end of string.

To split the líne per // use: líne.split("//")

For more general purpose,check out :

Java - regular expression finding comments in code