Java Strings don't work like that. They don't have a char '\0' in their last index to denote the end. Also unless you are doing this just to train your java skills all of this is unnecessary complicated. You can just call length() on your Strings to get their length. More on stackoverflow.com

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Are you using a hacked client? Because if you use one and use an username longer then 16 characters the game will crash for all the other players. More on reddit.com

This java error occurs when you are trying to do a String operation (in this case, checking the length) while the value is not set (is null). More on reddit.com