Java random number generator

Question

Is it possible to have a user choose the number of digits of a random number, specifically a random Big Integer? For example if the user wants it to be 15 digits long the random number generator would only produce 15 digit long Big Integers.

Answer 1

You can use the constructor of BigInteger where you specify the number of binary digits: BigInteger(int numBits, Random rnd). You need roughly ten binary digits for each three decimal digits that the user wants. For example, if you need a 30-digit random BigInt, use 100 binary digits.

You can cut off the unnecessary digits by using remainder(10^30), and do it in a loop to ensure that the initial digit is not zero, ensuring the correct number of digits, like this:

Random rnd = new Random(123);
BigInteger tenPow30 = new BigInteger("10").pow(30);
BigInteger min = new BigInteger("10").pow(29);
BigInteger r;
do {
        r = new BigInteger(100, rnd).remainder(tenPow30);
} while (r.compareTo(min) < 0);
System.out.println(r);

Link to a demo.

Answer 2

You can always generate individual digits of the number randomly. This way for a 15 digit number you can generate 15 digits randomly and then form the number.

Another way:

Lets change the problem to generate random 5 digit number.

Min = 10000
Max = 99999

Now generate a random number between 0 and Max - Min which is 0 and 89999 and add it to Min.

Random = Min + Math.random() * (Max - Min)