Finding the longest substring with unique characters.

Here’s a Java program that finds the longest substring with unique characters:

  import java.util.HashSet;

  public class LongestSubstring {

    public static String findLongestSubstring(String input) {

        if (input == null || input.isEmpty()) {

            return "";

        }

        int n = input.length();

        int maxLength = 0;

        int start = 0;

        int end = 0;

        HashSet<Character> set = new HashSet<>();

        while (end < n) {

            char c = input.charAt(end);

            if (!set.contains(c)) {

                set.add(c);

                if (end - start + 1 > maxLength) {

                    maxLength = end - start + 1;

                }

                end++;

            } else {

                set.remove(input.charAt(start));

                start++;

            }

        }

        return input.substring(start, start + maxLength);

    }

    public static void main(String[] args) {

        String input = "abcabcbb";

        String longestSubstring = findLongestSubstring(input);

        System.out.println("Longest substring with unique characters: " + longestSubstring);
   }

}

In this program, we use a sliding window approach to find the longest substring with unique characters. We maintain a set to keep track of the characters in the current window. If a character is not present in the set, we add it and update the maximum length. If a character is already in the set, we remove the character at the start of the window and move the start pointer forward.

The `findLongestSubstring` method takes an input string and returns the longest substring with unique characters. In the `main` method, we demonstrate its usage by finding the longest substring in the string “abcabcbb” and printing the result.

When you run the program, it will output:

Longest substring with unique characters: abc

This means the longest substring with unique characters in the input string “abcabcbb” is “abc”.