Generate random number of n digits or more

Question

I saw many examples to generate random numbers in a specific range [min-max], but i need java code that generates random numbers of n digits or more, so in this case min= 10000000 and no max.

Note - I am using BigInteger

"no max" assuming you are using `int` or `long`, there is a max: `Integer.MAX_VALUE` or `Long.MAX_VALUE`. — Andy Turner, Jul 05 '16 at 14:52
org.apache.commons.lang3.RandomStringUtils may help you here, you can create String that contains only digits with length you need. — dty, Jul 05 '16 at 14:53
Even if you're using `BigInteger`, you must have a practical limitation on the number you want to generate. I mean, you could generate a number with as many digits as your heap will allow... but what are you going to do with that? — Andy Turner, Jul 05 '16 at 14:55
Would suggest using strings in case the use case is well served as pointer by Andy in comments, there has to be a purpose. — Naman, Jul 05 '16 at 14:56
As Andy has pointed out, however, you're going to have to cap it somehow or you'll reach system limitations. — Dando18, Jul 05 '16 at 15:00
I know it’s not what you asked, but I want to mention anyway: for cryptography you will probably want to look into SecureRandom (used the right way it can generate digit strings long enough for your cryptographical strength requirements). — Ole V.V., Jul 05 '16 at 15:04

score 3 · Accepted Answer · answered Jul 05 '16 at 15:42

You can use the constructor BigInteger(int numBits, Random rnd) to generate positive random numbers with N bits.

As you want to have a minimum, you can add that as an offset to the generated numbers:

Random random = ThreadLocalRandom.current();
BigInteger base = BigInteger.valueOf(10000000); // min
int randomBits = 50; // set as many bits as you fancy

BigInteger rnd = base.add(new BigInteger(randomBits, random));

score 1 · Answer 2 · answered Jul 05 '16 at 15:25

BigInteger accepts a decimal String in one of its constructors. Generate individual digits and append them to a String. When you have enough digits in your String, create your BigInteger from the String. You may want to constrain the first digit to be in [1 .. 9] to avoid leading zeros, depending on your exact requirement.

Generate random number of n digits or more

2 Answers2