I don't understand why this is marked duplicate when clearly the "duplicate" question referred here doesn't ask the same question - though an answer down below contains this information. In any case, the answer I was looking for is below, incase if it helps anyone else.

private String generateSafeToken() {
    SecureRandom random = new SecureRandom();
    byte bytes[] = new byte[20];
    random.nextBytes(bytes);
    Encoder encoder = Base64.getUrlEncoder().withoutPadding();
    String token = encoder.encodeToString(bytes);
    return token;
}

Initialize an array containing all the accepted chars (CHARS_ARRAY), then instantiate a SecureRandom instance, and call nextInt(CHARS_ARRAY.length) repeatedly to get a random index in your char array. Append each char to a StringBuilder until you get the expected number of chars.