12 Digit unique random number generation in Java

Improved checked solution using StringBuilder():

public static long generateRandom() {
    Random random = new Random();
    StringBuilder sb = new StringBuilder();

    // first not 0 digit
    sb.append(random.nextInt(9) + 1);

    // rest of 11 digits
    for (int i = 0; i < 11; i++) {
        sb.append(random.nextInt(10));
    }

    return Long.valueOf(sb.toString()).longValue();
}

(long)Math.random()*1000000000000L

But there are chances of collision

Why not use sequence? Starting from 100,000,000,000 to 999,999,999,999? keep a record of last generated number.

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Edit: thanks to bence olah, I fixed a creepy mistake

Generate each digit by calling random.nextInt. For uniqueness, you can keep track of the random numbers you have used so far by keeping them in a set and checking if the set contains the number you generate each time.

public static long generateRandom(int length) {
    Random random = new Random();
    char[] digits = new char[length];
    digits[0] = (char) (random.nextInt(9) + '1');
    for (int i = 1; i < length; i++) {
        digits[i] = (char) (random.nextInt(10) + '0');
    }
    return Long.parseLong(new String(digits));
}