Generate Random Strings in Java

import java.security.SecureRandom;

public class RandomStringGenerator {
    private static final String ALPHANUMERIC =
        "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    private static final String HEX_CHARS = "0123456789abcdef";

    public static String generateRandomString(int length, String customChars) {
        SecureRandom random = new SecureRandom();
        String chars = (customChars != null) ? customChars : ALPHANUMERIC;

        StringBuilder result = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            int index = random.nextInt(chars.length());
            result.append(chars.charAt(index));
        }

        return result.toString();
    }

    public static void main(String[] args) {
        // Generate a 16-character alphanumeric string
        String randomString = generateRandomString(16, null);
        System.out.println(randomString);  // Example: aB3xY7mN2pQ9zR5t

        // Generate a 12-character hex string
        String hexString = generateRandomString(12, HEX_CHARS);
        System.out.println(hexString);  // Example: a3f7b2e9c4d1

        // Generate a 10-character lowercase string
        String lowercaseString = generateRandomString(10, "abcdefghijklmnopqrstuvwxyz");
        System.out.println(lowercaseString);  // Example: xmkpqrstuv
    }
}