UUID Generator — Java source

// RFC 4122 v4 UUID generator (cryptographically secure).
//
// Language: Java (17+)
// CosmoDev polyglot showcase port of the `uuid` tool.
// Ported from src/lib/uuid.ts — display source, part of CosmoDev's
// polyglot tool pages.
//
// Produces a random (v4) UUID: 36 characters, lowercase, hyphenated as
// 8-4-4-4-12 hex digits, with the RFC version (4) and variant (10xx)
// bits set. SecureRandom is Java's bridge to the host CSPRNG, so
// output is unpredictable and collision-safe.

import java.security.SecureRandom;

public final class UuidTool {

    private static final SecureRandom CSPRNG = new SecureRandom();

    /** Return a freshly generated RFC 4122 version-4 UUID string. */
    static String uuidV4() {
        byte[] b = new byte[16];
        CSPRNG.nextBytes(b);

        b[6] = (byte) ((b[6] & 0x0f) | 0x40); // version 4: high nibble of byte 6
        b[8] = (byte) ((b[8] & 0x3f) | 0x80); // variant  : high bits of byte 8

        StringBuilder out = new StringBuilder(36);
        for (int i = 0; i < 16; i++) {
            if (i == 4 || i == 6 || i == 8 || i == 10)
                out.append('-'); // 8-4-4-4-12 group boundaries
            out.append(String.format("%02x", b[i]));
        }
        return out.toString();
    }

    public static void main(String[] args) {
        System.out.println(uuidV4());
    }
}