StringIndexOutOfBoundsException:

I know this error has been asked a lot. I have looked at them but still don’t really understand how it applies to my problem. I am trying to write a program that creates a diamond pattern from a string.

public static String nameDiamond(String s) {
    int len = s.length();
    int i = 0;
    String out = "";
    while(i < len) { 
        out = out + s.substring(0, i + 1) + "n";
        i++;
    }
    while(i > 0) { 
        int numSpaces = len - i + 1;
        for(int j = 0; j < numSpaces; j++)
            out = out + " ";
        out = out + s.substring(len - i + 1, i - 1) + "n";
        i--;
    }
    return out;
}

Now my understanding of substring is, that it is structured like s.substring(startIndex, endIndex) and a cause of this error can be when the end index is less then the start but I don’t think this is the problem here. I also saw some causes of the error is when something does not exist but again I don’t think this is the problem. What am I missing?

When it is tested with "Marty" it says -1 and with "Bill Nye" it says -2. Would the space be causing this?

I looked at lots of other versions of the questions but the ones that seemed relevent were:

StringIndexOutOfBoundsException: string index out of range

StringIndexOutOfBoundsException: String index out of range: -1

But I don’t think they apply.

For reference, this is the problem:
https://www.codestepbystep.com/problem/view/java/strings/nameDiamond

public static void printDiamond(int len) {
    String str = "*";
    String space = "                              ";
    for (int i = 0; i < len/2 + 1; i++) {
        System.out.print(space.substring(0,len/2-i));
        System.out.println(str);
        str = str+"**";
    }
    str = str.substring(2);
    for(int i = 0; i < len/2; i++) {
        str = str.substring(2);
        System.out.print(space.substring(0,i+1));
        System.out.println(str);
    }
}