Unable to find out error

Question

hi all
need help to solve this problem
In a file called AllBalancedOfSize.java write a method with the signature:
public void printBalanced(int len)
that uses recursion (either directly or indirectly) and prints all balanced strings, composed solely of the characters a and b of length len in lexical (dictionary) order. A balanced string is a string with an equal number of a and b characters. So, for example, abab is a balanced string and bbab is not a balanced string.
As an example, the call printBalanced(4) must print:
aabb
abab
abba
baab
baba
bbaa
A call printBalanced(5) must print nothing because there are no balanced strings of length 5 (same for any odd parameter value). A call printBalanced(0) must print an empty line because there is one balance string of length zero, the empty string! You may assume that len is always greater than or equal to zero.
i have done this
public class AllBalancedOfSize {
     public static void main(String[] args) {
          AllBalancedOfSize q4 = new AllBalancedOfSize();
          q4.printBalanced(4);
     public void printBalanced(int len) {
          if (len == 0) {
               System.out.println();
          } else {
               if (len % 2 == 0)
                    printBalanced(len, "");
     public void printBalanced(int len, String prefix) {
          if (len == 0) {
               int numofa = 0, numofb = 0;
               for (int i = 0; i <= prefix.length(); i++) {
                    char tmp = prefix.charAt(i);
                    if (tmp == 'a') {
                         numofa++;
                    } else {
                         numofb++;
               if (numofa == numofb) {
                    System.out.println(prefix);
          } else {
               printBalanced(len - 1, prefix + "a");
               printBalanced(len - 1, prefix + "b");
}but i got this from compiler
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4
     at java.lang.String.charAt(String.java:687)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:20)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:31)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:31)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:31)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:31)
     at AllBalancedOfSize.printBalanced(AllBalancedOfSize.java:12)
     at AllBalancedOfSize.main(AllBalancedOfSize.java:4)
need help to find out what is the error.
Edited by: dttl on May 10, 2009 11:44 AM

Answer 1

dttl wrote:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4
     at java.lang.String.charAt(String.java:687)
need help to find out what is the error.When a String has a length of 4 characters, you can access them though indexes 0 through 3.
When a String has a length of N characters, you can access them though indexes 0 through N-1.