Identify the Length of the Longest Substring Without Repeating Characters
LinkedBlockingQueue is not the best choice. It's based on a linked list, which lacks spacial locality. An continuous array-backed queue would be more efficient (there're a few options in the standard library).
 if (substringCharacters.size() &gt; longestSubstringLength) {
     longestSubstringLength = substringCharacters.size();
 }


I'd use the Math.max method here.
You can avoid using the queue altogether. You can keep the leftmost index of the current "window" and the set instead.