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         


        

Answer 1

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.

Answer 2

You've overflowed long.
Use BigInteger instead.

Answer 3

Check out http://en.wikipedia.org/wiki/Integer_overflow, I know the title refers to an integer but the principle stands for int, long , double etc.

In short, a primitive datatype has a max value, when you go over that, it wraps aroun and starts again. if you really want to get geeky about it, learn about binary addition to fully understand it.

Answer 4

Here's what you're missing: when a signed integer primitive type (such as short, int, long) gets incremented above a signed value that it can represent, it tries to flip its sign bit, which is the leftmost bit, which is only supposed to be used to indicate the sign of the number. 1 in the sign bit indicates a negative value. This phenomenon is called integer overflow.

Consider a fictional 3-bit signed primitive data type (for comparison, a Java long is 64 bits). It can represent numbers between -4 and 3.

3, the biggest positive value a 3-bit number can represent, looks like this: 011

add 1 to 011 and you get: 100 (the number part overflows into the sign part)

the decimal version of 100 is -4

However, when you get to dealing with the capacity of a long, there are a lot of digits to count, so here's a quick way to determine the biggest number defined by a given nondecreasing sequence (in this case, factorial):

long n = 1;
while (factorial(n) > 0) {
    System.out.println("factorial of " + n++ + " can fit in a long!");
}

This looks like it ought to be an infinite loop, but it isn't; eventually, factorial(n) will return negative due to integer overflow. This will give you the following output:

factorial of 1 can fit in a long!
factorial of 2 can fit in a long!
factorial of 3 can fit in a long!
factorial of 4 can fit in a long!
factorial of 5 can fit in a long!
factorial of 6 can fit in a long!
factorial of 7 can fit in a long!
factorial of 8 can fit in a long!
factorial of 9 can fit in a long!
factorial of 10 can fit in a long!
factorial of 11 can fit in a long!
factorial of 12 can fit in a long!
factorial of 13 can fit in a long!
factorial of 14 can fit in a long!
factorial of 15 can fit in a long!
factorial of 16 can fit in a long!
factorial of 17 can fit in a long!
factorial of 18 can fit in a long!
factorial of 19 can fit in a long!
factorial of 20 can fit in a long!

Answer 5

Overflow (and underflow) is silent according to the JLS, which is why your result was a "surprise".

You have two choices:

• if an exact answer is required, use BigInteger
• if exactness is not required, use double (although even that will overflow past 170!)

Answer 6

25! is bigger than Long.MAX_VALUE...