How to generate a random string

Short answer

Pick random characters from an alphabet string. For passwords and tokens use SecureRandom — its output cannot be predicted:

static final String ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
static final SecureRandom RANDOM = new SecureRandom();
 
String randomString(int length) {
    var result = new StringBuilder(length);
    for (int i = 0; i < length; i++) {
        result.append(ALPHABET.charAt(RANDOM.nextInt(ALPHABET.length())));
    }
    return result.toString();
}
 
void main() {
    IO.println(randomString(12));
}

Output:

G0fHdMG1t29T

The result is different every time.

A token from bytes

For long tokens it is easier to take random bytes and encode them with URL-safe Base64:

void main() {
    byte[] bytes = new byte[24];
    new SecureRandom().nextBytes(bytes);
 
    IO.println(Base64.getUrlEncoder().withoutPadding().encodeToString(bytes));
}

Output:

odBXRC78P41xba6fnFUZf53ovW79tmul

A unique identifier

If you just need a unique ID, use UUID:

void main() {
    IO.println(UUID.randomUUID());
}

Output:

26d2e800-7d9e-48cd-b6bf-6645d141f886

A repeatable result

Tests sometimes need the same "random" string. Random with a fixed seed produces the same sequence every time:

void main() {
    var random = new Random(42);
    var result = new StringBuilder();
    for (int i = 0; i < 6; i++) {
        result.append((char) ('a' + random.nextInt(26)));
    }
    IO.println(result);
}

Output:

ahwmar

Common mistake

Generating passwords and tokens with Random or Math.random(). Their sequence can be reconstructed from a few values. Anything security-related needs SecureRandom.

Thanks for reading!

Short answer

A token from bytes

A unique identifier

A repeatable result

Common mistake

Functions used in this recipe

Thanks for reading!