Factorial in Java

Question

I've been using this factorial program for Java:

public static long factorial(int a) {

    if(a<1) {
        return 1;
    }
    long result=1;
    long x=a;
    while(x>1) {
        result*=x;                     
        x--;
    }
    return result;
}

However, it seems to "break" and return a negative number after the factorial of 25. It returns a negative number for a while then just returns "0."

Am I doing anything wrong that is causing this?

Answer 1

You've overflowed long.
Use BigInteger instead.

Answer 2

25! = 15511210043330985984000000

The maximum value of a long in Java is 2^63-1 = 9223372036854775807 (source).

25! is about 1.7*10^6 the size of the largest value a long can store in Java. Use a BigInteger instead.