Longest substring without repeating characters

import java.util.Set;
import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        /**
         * Problem:
         * Given a string, find the length of the longest substring without repeating characters.
         * 
         * Example:
         * Input: "abcabcbb"
         * Output: 3 (because "abc" is the longest substring without repeats)
         */
         
         final String input = "abcabcbb";
         
         // We need to find the longest substring without repeating characters
         // I think we can use the two pointers approach with the same direction
         // to traverse the string once
         
         // We may have an index which is moving character by character (fast)
         // Another index which is moving only when there is a repeated character.
         // We can store the characters checked in a List to check if it was already validated
         
         // We can also use a String to concatenate the characters and search for occurrences
         // using the indexOf method of the String class
         int slow = 0;
         
         // The maxSubstringLength variable will contain the maximum substring length found
         int maxSubstringLength = 0;
         
         // Let's use the List to store the characters processed
         Set<Character> charactersProcessed = new HashSet<>();
         for (int fast = 0; fast < input.length(); fast++) {
             // If the character in processing is present, we interrupt the substring and 
             // set the max length based on the charactersProcessed size only if the 
             // size is greater than maxSubstringLength
             char characterInProcessing = input.charAt(fast);
             if (charactersProcessed.contains(characterInProcessing)) {
                 if (charactersProcessed.size() > maxSubstringLength) {
                     maxSubstringLength = charactersProcessed.size();
                 }
                 charactersProcessed.clear();
             }
             charactersProcessed.add(characterInProcessing);
         }
         
         // I just noticed that I do not need two pointers given the charactersProcessed List 
         // The slow index could be useful if we check if the char is in the substring(slow, fast)
         // I am not sure about the time complexity of searching a char over a String vs searching an
         // element in a List
         System.out.println(maxSubstringLength);
         
         // I think it's better using a HashSet to avoid the time complexity of looking up
         // elements in a List or the String with indexOf
    }
}