random 16-digit number function in Java

Question

I need to do a random with a 16-digit number. And the random function I got doesn't work with big numbers. I got this function:

 int pin = new Random().nextInt(10000);

But if I put a big number like 1000000000000, it won't work because it's not int, and the nextLong is not working to me like that. Thanks.

Why can you not use `nextLong()` and do divisions and `mod`s? Also, is this math or Java homework? Because if you are doing math homework, there is subtleties involved in getting a uniformly distributed pseudo-random sequence. — Dilum Ranatunga, Mar 24 '12 at 04:23
`nextLong` and `mod` is a great way to get _bitten_ by the subtleties involved in a uniformly distributed pseudo-random sequence. — Louis Wasserman, Mar 24 '12 at 10:55

dcow · Accepted Answer · 2012-03-24T09:58:11.517

3

You could concatenate two random numbers to create a longer random number.

So:

Get an 8 digit random number using nextInt(100000000).
Bit shift it 8 tens digits, or about 10 bits to the left.
Get a second 8 digit random number.
Bitwise OR the two numbers to get your final Long random number.

Does that make sense?

edited Mar 24 '12 at 09:58

answered Mar 24 '12 at 04:04

dcow

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Yes that makes sense. I was thinking about that but I was looking if it was another shortest way, but thank you very much. – Johnny Dahdah Mar 24 '12 at 04:12
Left shifting 8 bits does not do what you want. The closest approximation is 10 bits for every x 1000. – Dilum Ranatunga Mar 24 '12 at 04:20
Yes, sorry. You need to shift 8 tens digits to the left so that about 10 binary bits. – dcow Mar 24 '12 at 09:57

score 1 · Answer 2 · edited May 23 '17 at 10:08

1

You could try using a bigInteger, see this post for creating a random big int How to generate a random BigInteger value in Java?

edited May 23 '17 at 10:08

Community

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answered Mar 24 '12 at 15:10

George Phillips

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score 0 · Answer 3 · answered Mar 24 '12 at 23:14

You might want to have a look the documentation for Random.nextInt(int n), which discusses the algorithm used, and adapt it to work with longs.

From the documentation:

public int nextInt(int n) {
  if (n <= 0)
    throw new IllegalArgumentException("n must be positive");

  if ((n & -n) == n)  // i.e., n is a power of 2
    return (int)((n * (long)next(31)) >> 31);

  int bits, val;
  do {
    bits = next(31);
    val = bits % n;
  } while (bits - val + (n-1) < 0);
  return val;
}

Follow the link and read the docs for more info.

score -1 · Answer 4 · answered Mar 24 '12 at 11:38

-1

If you wish to generate arbitrarily large random numbers with N digits, try

StringBuffer buf=new StringBuffer();
rng=new Random();
for(int a=0;a<N;a++)
    buffer.append((char) ('0'+rng.nextInt(10)));
return buffer.toString();

answered Mar 24 '12 at 11:38

o_o

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You should not post answers in Homework questions. He's learning, and instead of telling him the answer, we should help him to get to the answer with his own legs. – SHiRKiT Mar 24 '12 at 15:40

random 16-digit number function in Java

4 Answers4