Beginner programming- strange problem

Hi everyone,
Sorry if this is blatently obvious but i am new to programming.
My problem is with this piece of code:

currEx = in.charAt(currSubStr);

Java says the following:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(String.java:686)
at AssigOne2011.main(AssigOne2011.java:38)

I assume this means that the int currSubString is not valid, however i know foor sure that this is 0 seeing as i assign that value one line before the one i am having a problem with.

Any help would be greatly appreciated, as i am stumped!

Are you validating the length of the String before you use charAt()? Maybe the String is not as long as you think it is. If a String's length is 0, it has *no* valid indexes.