Generating very large random numbers java

Question

How can we generate very large random number in java? I am talking something like 10000 digits? I know we have to use BigInteger but how can we do this? What is the most efficent way of doing something like this? Please provide a small example. Thank you.

Mcgarry

Answer 1

Well, one way is to go to Random.org and download a one of the binary random files. The files are generated from atmospheric noise, so it's very random. I used it for Zobrist keys in my chess engine.

Alternatively you could go

BigInteger b = new BigInteger(256, new Random());

which will give you what you want. In this example, a BigInteger consisting of 256 bits.

Ajaajaccio

+1. It was four constructors down in the documentation, I didn't see it :D. – Subminiature

Answer 2

Combine Random.nextBytes(byte[]) with BigInteger(byte[]).

import java.util.*;
import java.math.*;
class Test{
    public static void main(String[]_){

        int n = 16;

        Random r = new Random();
        byte[] b = new byte[n];
        r.nextBytes(b);
        BigInteger i = new BigInteger(b);

        System.out.println(i);
    }
}

Subminiature

can you please explain how this works? i'm more or less confused on the bytes part.. thanks – Mcgarry

You allocate an array of bytes, nextBytes fills it with random values, and then the BigInteger is initialized as the binary number defined by that sequence of bytes. But I think Jaco's solution is more concise. – Subminiature

Answer 3

You can just type:

int number = (int)(Math.random() * 100);

Also, you can generate even bigger numbers if you change the multiplier:

int number = (int)(Math.random() * 1000);

P.S. You don't need to import a class.

Progressist

The question asks for numbers with 10000 digits. This answer does not even come close. – Whitacre

1000 digits won't fit into variable of int type. Multiplying random value by some constant makes it not random. – Sur

Although this answer might work for small numbers, the question is clear that it is for very large (much bigger than an int) numbers. So this is not a valid answer. – Gahnite