A simple implementation using Arrays instead of hashmap. It uses a fixed size array of 255 limit because the characters in unicode or ascii are 255.

public int longestSubString(String s) {

int longest =0; int current=0; int[] charCount = new int[255]; int endPtr = 0; int strPtr = 0;

while(endPtr 1) {

if(current > longest) { longest = current; }

current = 0; charCount = new int[255]; strPtr++; endPtr = strPtr; // slide the window continue; }

current++;

if(current > longest) { longest = current; }

endPtr++;

}

return longest;

}