I keep getting a "java.lang.StringIndexOutOfBoundsException" even though my for loop counter adjusts for length by subtracting 1

Question

So my main goal is to create a program that can decrypt a caesar cipher. I have everything set up so that I have the regular alphabet set up in one array and then a decoder array where I have shifted the alphabet based on a shift/key the user inputs. (ie. if the user inputs a shift of 6, the regular alphabet array starts a, b, c... and then the decoder array starts g, h, i...).

I am currently stuck on going through the encrypted message (user inputted) and decoding each letter one by one by matching it up to the decoder array. It keeps giving me an index out of bounds error even though in the for loop that I have set up, I have subtracted one from the length of the message to compensate for it starting at 0.

The error only happens at the last character too (that's what the 35 is. I have a 35 character string that I'm using to test). But when I try to print the decoded message up until the error, I'm just getting nulls (I'm storing each decoded char in an array and print each character stored as the for loop is running).

Here is the piece of code that I'm currently stuck on:

//Decoding message by looping through each letter in String message
for (int x = 0; x < (message.length() - 1); x ++) //Gets next letter in message
{
    for (int y = 0; y < (decodedArray.length - 1); x++) //Goes through decodedArray array
    {
        if (String.valueOf(message.charAt(x)) == (decodedArray[y])) //Comparing character to each position in decodedArray
        {
            decodedMessage[x] = alphabetArray[y];
        }
        System.out.print(decodedMessage[x]); //Testing only. Prints each character stored in array but only printing nulls.
    }
}

I haven't added any checks for spaces yet because I'm currently stuck on this error but if any of you can add that, that would be greatly appreciated. I'm not sure if comparing a char to a space would work.

For starters, you're using `==` to compare strings, which is incorrect; use `==` to compare the `char` values directly. — chrylis -cautiouslyoptimistic-, Feb 25 '20 at 06:26
could you add decodedMessage as well, I would guess it's a string? — Sabina Orazem, Feb 25 '20 at 06:28
Im not sure about your question very much, but cant you get the shift value and then try to backtrack. — , Feb 25 '20 at 06:30
@chrylis-onstrike- He is using `==` to compare `chars`, not `Strings`. — user207421, Feb 25 '20 at 06:32
Either use a debugger or put in a lot of print statements. And the for statement probably should be for (int i; i < message.length; ... If you want to use the entire string. My thought is that one of the strings is shorter than you think it is. — NomadMaker, Feb 25 '20 at 06:49
I se the problem. Look at the second for statement. You're incrementing x again. — NomadMaker, Feb 25 '20 at 06:50

Prakhar Londhe · Accepted Answer · 2020-02-25T06:40:07.070

4

According to the current code in the question, I can say there are 2 problems with your code,

If you need to compare char, you can directly use to == operator without needing it to convert it to string by using String.ValueOf().
You are incrementing x in the 2nd loop, instead you need to increment y

edited Feb 25 '20 at 06:40

answered Feb 25 '20 at 06:28

Prakhar Londhe

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He is comparing `chars` with `==`, not `Strings. Your second point is correct. – user207421 Feb 25 '20 at 06:33
2

Omg. I've been staring at this code for over an hour trying to figure out why it's not working. You were right... I was incrementing x in the second loop and that was what was causing the problem. I've also fixed the comparisons. I've just converted everything to char. – Nelly_Boi18 Feb 25 '20 at 06:35
@user207421 Sorry, he was using String.valueOf() hence I thought he was comparing the strings. – Prakhar Londhe Feb 25 '20 at 06:39
@Nelly_Boi18 You can accept the answer if it answered your question ;) – Prakhar Londhe Feb 25 '20 at 06:40
@user207421 - The left char is wrapped inside `String.valueOf`, so I believe he's checking value equality of a String (albeit a 1-character String) and a char, which will always be false. – charles-allen Feb 25 '20 at 06:40

I keep getting a "java.lang.StringIndexOutOfBoundsException" even though my for loop counter adjusts for length by subtracting 1

1 Answers1