Error with my filter program

My program is doing filtering. The user input a file and the file
contains a java code with comment //, /*, */. My goal is output to a
file that the code should be without comment line;
e.g.
class ALIST { /* The given // class (the user-defined */
    String name, address; // type )"
}

public class Question1{ /*The parts
 in white will // be same */
    public static void main( String args[ ] ) { // for all Java programs
        ALIST list [ ] = new ALIST[3]; // Define an array
        //for (int j=0; j<list.length; j++) {
            //list[ j] = new ALIST(); // Create ALIST object
         //list[ j].name = "name" + j; // Initialization
         //list[ j].address = "address" + j;
                list[0] = new ALIST();
                list[1] = new ALIST();
                list[0].name = "name0";
                list[1].name = "name1";
-------------so on-------------------------

The output.txt should be:
class ALIST {
    String name, address;
}

public class Question1 {
    public static void main( String args[ ] ) {
        ALIST list [ ] = new ALIST[2];

                list[0] = new ALIST();
                list[1] = new ALIST();
                list[0].name = "name0";
                list[1].name = "name1";
------------so on---------------------------

The // may come before /* and vice verse, so I have to check whether
ignore the whole line or only after the /* (upto */ part), /* and */
may not be in the same line

-----------------------------------------Here is my
code------------------------------------
import java.util.Scanner;
import java.io.*;
public class FilterProgram {
    public static void main(String[] args){
        try{
            String read;
            Scanner readfile = new Scanner(System.in);
            read = readfile.nextLine();
            File openfile = new File(read);
            PrintWriter pw = new PrintWriter(new BufferedWriter(new
FileWriter("output.txt")));

            if(!openfile.exists()){
                throw new FileNotFoundException("File not found" +
read);
            }
            Scanner scanfile = new Scanner(openfile);
            String tryread;
            String before = null;
            while((tryread = scanfile.nextLine())!=null){
                int doubleslash = tryread.indexOf("//");
                int block1 = tryread.indexOf("/*");
                int block2 = -1;
                if((doubleslash >= 0) || block1>=0){
                    if(doubleslash<block1){
                        tryread = tryread.substring(0,doubleslash);
                        System.out.println("doubleslash");
                    }
                    else
                    {
                        block2 = tryread.indexOf("/*");
                        before = tryread.substring(0,block1);
                        if(block2 < 0 || block2<=block1 + 1){
                            do{
                                tryread = scanfile.nextLine();
                                block2 = tryread.indexOf("*/");

                            }while(block2<0);
                        }
                         tryread = before + tryread.substring(block2 +
2);
                    }
                    System.out.println("block2 can't be reached");
                }
                pw.println(tryread);

            }
            System.out.println("here3");
            readfile.close();
            pw.close();
        }
        catch(FileNotFoundException ex){
            System.err.println(ex.getMessage());
            System.exit(1);
        }
        catch(IOException ex){
            System.err.println(ex.getMessage());
            System.exit(2);
        }

    }

}

when I type input.txt, the compiler complains:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException:
String index out of range: -1
        at java.lang.String.substring(String.java:1938)
        at FilterProgram.main(FilterProgram.java:35)
Java Result: 1
How can I fix it and can anyone tell the problem?