Tokenizing

[Mohit G Gupta]

searching using the scanner Class
K&B book pg no-501

  

import java.util.*;
class ScanIn {
  public static void main(String[] args) {
    System.out.print("input: ");
    System.out.flush();
    try {
      Scanner s = new Scanner(System.in);
      String token;
      do {
        token = s.findInLine(args[0]);
        System.out.println("found " + token);
      } while (token != null);    
    } catch (Exception e) { System.out.println("scan exc"); }
  }
}



  

The invocation and input
java ScanIn "\d\d"
input: 1b2c335f456
produce the following:
found 33
found 45
found null




why is null in the output ?
i am not able to understand it.

[Ankit Garg]

Since we have a do-while loop, so after 45 is matched, we look for another match, no match is found so s.findInLine returns null. We display null and then check if nothing was found and then end the loop...

[Unmesh Chowdhury]

If you do the program as follows then you will not get null as output:

     
import java.util.*;  
 public class ScanIn {  
   public static void main(String[] args) {  
     System.out.print("input: ");  
     System.out.flush();  
     try {  
       Scanner s = new Scanner(System.in);  
       String token;  
       while((token = s.findInLine(args[0])) != null) {  
         System.out.println("found " + token);  
       }
     } catch (Exception e) { System.out.println("scan exc"); }  
   }  
 }

In this case, your while loop body will not be executed if the token is null. Actually, here the loop condition is test first and then print the output (execute the loop body) when condition is true, but, in your case the output is printed first (execute the loop body) then test the loop condition for the next iteration.