Example: Find First Non-Repeating Character in Java

// Find First Non-Repeating Character
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

// Main class containing the entry point of the program
public class Main {

    public static char findFirstNonRepeatingChar(String str) {
        // Step 1: Initialize a HashMap to store character counts.
        // Key: Character, Value: Count of occurrences
        Map<Character, Integer> charCounts = new HashMap<>();

        // Step 2: First pass - Populate the HashMap with character frequencies.
        for (char c : str.toCharArray()) {
            charCounts.put(c, charCounts.getOrDefault(c, 0) + 1);
        }

        // Step 3: Second pass - Iterate through the string again to find the first character with a count of 1.
        for (char c : str.toCharArray()) {
            if (charCounts.get(c) == 1) {
                return c; // Found the first non-repeating character
            }
        }

        // Step 4: If no non-repeating character is found, return a sentinel value (e.g., '\0').
        // Or, one could throw an exception or return a special character/string.
        return '\0';
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a string: ");
        String inputString = scanner.nextLine();

        char result = findFirstNonRepeatingChar(inputString);

        if (result != '\0') {
            System.out.println("The first non-repeating character is: '" + result + "'");
        } else {
            System.out.println("No non-repeating character found in the string.");
        }

        scanner.close();
    }
}