libs4RandomAndUuid

1package pkg13libs;2 3import java.security.SecureRandom;4import java.util.Random;5import java.util.UUID;6import java.util.concurrent.ThreadLocalRandom;7import java.util.stream.Collectors;8 9/*10 * libs4RandomAndUuid.java11 * -----------------------12 * Randomness done right: Random, ThreadLocalRandom, SecureRandom, and UUIDs.13 *14 * DEFINITION:15 *   Random is a fast pseudo-random generator (predictable from its seed).16 *   SecureRandom is cryptographically strong (for tokens/keys). UUID is a17 *   128-bit unique identifier.18 *19 * KEY POINTS:20 *   - Seed a Random to get reproducible sequences (great for tests).21 *   - Use ThreadLocalRandom in concurrent code (no contention).22 *   - Use SecureRandom for anything security-sensitive — never plain Random.23 *   - UUID.randomUUID() (v4) is the go-to for distributed unique ids.24 */25public class libs4RandomAndUuid {26 27    public static void main(String[] args) {28        // Seeded Random is reproducible29        Random seeded = new Random(42);30        System.out.print("Seeded(42) ints : ");31        for (int i = 0; i < 5; i++) System.out.print(seeded.nextInt(100) + " ");32        System.out.println("(same every run)");33 34        // Ranges and other types35        Random r = new Random();36        System.out.println("\nrandom double   : " + r.nextDouble());37        System.out.println("random boolean  : " + r.nextBoolean());38        System.out.println("dice (1..6)     : " + (r.nextInt(6) + 1));39 40        // Streams of random numbers (Java 8+)41        String nums = r.ints(5, 0, 10).mapToObj(Integer::toString).collect(Collectors.joining(", "));42        System.out.println("5 ints [0,10)   : " + nums);43 44        // ThreadLocalRandom — preferred in multithreaded code45        System.out.println("\nThreadLocalRandom: " + ThreadLocalRandom.current().nextInt(1000));46 47        // SecureRandom — for tokens, salts, keys48        SecureRandom sr = new SecureRandom();49        byte[] token = new byte[8];50        sr.nextBytes(token);51        System.out.println("Secure token    : " + java.util.HexFormat.of().formatHex(token));52 53        // UUIDs54        UUID id = UUID.randomUUID();55        System.out.println("\nUUID v4         : " + id);56        System.out.println("version         : " + id.version() + ", variant: " + id.variant());57    }58}