StringIndexOutOfBoundsException

Question

This program I'm making isn't compiling right, I keep getting the error: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -9 at java.lang.String.charAt(String.java:687) at pro1.main(pro1.java:161)

Here's my code:

import java.io.*;
import java.util.*;
public class pro1 {
    static String str="";
    static String str1="";
    static int range=250;
    static int length;
    static String key="";
    static String ep="";
       static String pt="";
    static char temp;
    static int t,p,h;
    static int r,x,y,z,w;
    static Random generator = new Random();
 static public String getContents(File aFile) 
   {
   StringBuilder contents = new StringBuilder();
   try {
      BufferedReader input =  new BufferedReader(new FileReader(aFile));
      try {
        String line = null; 
        while (( line = input.readLine()) != null){
          contents.append(line);
          contents.append(System.getProperty("line.separator"));
        }
      }
      finally {
        input.close();
      }
    }
    catch (IOException ex){
      ex.printStackTrace();
    }
    str1=contents.toString();
    return str1;
    }



public static void main (String args[]) throws IOException {
      File testFile = new File("/home/amritha/Desktop/sam.txt");
      System.out.println("Original file contents: " + getContents(testFile));
      System.out.println("message:"+str1);
       String sbox="abcdefghijklmnopqrstuvwxyz";
       length=str1.length()-1;
        for(int i=0;i<length;i++)
        {
           t=(int)str1.charAt(i);
             if(t==32)
              {
               int t1=32;
               temp=(char)t;
              }
            else
              {
               range=generator.nextInt(26)+1;
               temp=sbox.charAt(range);
              }
              key+=""+temp;
        }
       System.out.println("Key:"+key);
       for(int i=0;i<length;i++)
       {
         t=(int)str1.charAt(i);
          {
            if(t==32)
             {
               t=32;
               temp=(char)t;
             }
            else
             {
               t-=97;
             }
          }
        p=(int)key.charAt(i);
          {
           if(p==32)
            {
               p=32;
               temp=(char)p;
            }
           else
            {
              p-=97;
            }
          }
        if((t==32)&&(p==32))
           {
                int v=32;
                temp=(char)v;
           }
           else
           {
            r=(t+p)%26;
           temp=sbox.charAt(r);
           }
           ep+=""+temp;
       }
         System.out.println("Encrypted Text:"+ep);

   for(int i=0;i<length;i++)
       {
         y=(int)ep.charAt(i);
          {
            if(y==32)
             {
               y=32;
               temp=(char)y;
             }
            else
             {
               y-=97;
             }
          }
        x=(int)key.charAt(i);
          {
           if(x==32)
            {
               x=32;
               temp=(char)x;
            }
           else
            {
              x-=97;
            }
          }
        if((x==32)&&(y==32))
           {
                int w=32;
                temp=(char)w;
           }
           else
           {
            z=(y-x)%26;
           temp=sbox.charAt(z);
           }
           pt+=""+temp;
       }
         System.out.println("deccrypted Text:"+pt);
  }
}

Answer 1

Your code looks fishy in every way, and I cannot imagine anyone wanting to read 170 lines of this code.

Look at the exception: It tells you exactly what's going wrong: You pass -9 to charAt() as an index, which is - obviously - out of range, as you should only pass 0 ... (length-1) in there.

And it gives you the line number, too... so go to line number 161 and look what's in there and how it got there.

Answer 2

My guess would be it has something to do with this line:

z=(y-x)%26;

If x is larger than y the result of the % operation may be negative (or 0). And charAt (which is what z is given as parameter to) expects a positive value. You should try:

z=Math.abs(y-x)%26;

Edit: As a side note, this shouldn't be to hard to figure out on your own, by looking at the exception as was pointed out by others, or in the worst case using a debugger and seeing exactly what values the different variables have and why when the exception occurs.