To go beyond the primitive types, we need to use a new class. Fortunately, the BigInteger

class is one of the standard Java classes, so to get extremely large integers, we can use this

class. Copy the program factorial.java to bifactorial.java using the cp

command. Now, go in and change the class name to bifactorial. Think about and modify

all of the places where you will need to change int to BigInteger $($result variable

declaration at the end of main, method return type, method local variable$).$

$2.$ Changing to a non$-$primitive value takes a bit of doing. First of all, you need to include the

library at the top of the program with

import java.math.BigInteger;

$3.$ Second, in the fact class method, there is no direct conversion between integers and

BigIntegers, so you'll need to replace product $=1$; with

BigInteger product $=$ BigInteger.ONE;

BigInteger.ONE is a public class variable in the BigInteger class. Your parameter will

still be a regular, simple int and your for loop index will be an int too. If you are using an

integer value other than $0$ or $1,$ you can create a new BigInteger with its value using

BigInteger.valueOf $($the integer$).$ So$,$ you could have initialized the product

using

product $=$ BigInteger.valueOf$(1)$;

$4.$ Finally, you can not use the standard mathematical symbols $+,-,*,$ etc on new types. $($Some

programming languages do allow you to redefine these, but Java is not one of those languages.$)$

So$,$ it should look like this:

private static BigInteger fact $($int n$)$

$\{$

BigInteger product $=$ BigInteger.ONE;

for $($int i $=2$; i $<=$ n; i$++)$

$\{$

product $=$ product.multiply $($BigInteger$.$valueOf$($i$))$;

$\}$

return product;

$\}$

Get your program to compile and try a run that computes the factorial of a large number

$($somewhere between $200$ and $1000).$ Notice that we are returning a BigInteger value

now, not a simple integer.