Does anyone know how to generate a number within specific bits in Java?

This is the code to generate a random number from 1 to 10:

int a = (int)(Math.random() * 10) + 1;

But I have been thinking, what if I want to generate a number that is 100 bits randomly? how can I do this with BigInteger datatype?

New answer:

The BigInteger class has a constructor that you can use to generate a random number with the given number of bits. This is how you could use it:

BigInteger random(int bits) {
    Random random = new Random();
    return new BigInteger(bits, random);
}

Old answer with a long-winded way of doing the same:

The java.util.Random class has a nextBytes method to fill an array of bytes with random values. You can use that to generate a BigInteger with an arbitrary number of bytes.

The complication is, what to do when number of bits isn't a multiple of 8? My suggestion is to generate one byte too much and get rid of the extra bits with a shift operation.

BigInteger random(int bits) {
    Random random = new Random();
    // One byte too many if not multiple of 8
    byte[] bytes = new byte[(bits + 7) / 8];
    random.nextBytes(bytes);
    BigInteger randomNumber = new BigInteger(1, bytes);
    // Get rid of extra bits if not multiple of 8
    return randomNumber.shiftRight(bytes.length*8 - bits);
}