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Last modified: December 19, 2018

Given a non-negative integer *n*, factorial is the product of all positive integers less than or equal to *n*.

In this quick tutorial, we’ll explore **different ways to calculate factorial for a given number in Java**.

Let’s see a basic factorial algorithm using a *for* loop:

public long factorialUsingForLoop(int n) { long fact = 1; for (int i = 2; i <= n; i++) { fact = fact * i; } return fact; }

The above solution will work **fine for numbers up to 20**. But, if we try something bigger than 20, then it will fail because **results would be too large to be fit into a long**, causing an overflow.

Let’s see a few more, noting that each of **these will only work for small numbers.**

We can also use the Java 8 *Stream* API to calculate factorials quite easily:

public long factorialUsingStreams(int n) { return LongStream.rangeClosed(1, n) .reduce(1, (long x, long y) -> x * y); }

In this program, we first use *LongStream* to iterate through the numbers between 1 and *n*. We then used *reduce()*, which uses an identity value and accumulator function for the reduction step.

And let’s see another example of a factorial program, this time using recursion:

public long factorialUsingRecursion(int n) { if (n <= 2) { return n; } return n * factorialUsingRecursion(n - 1); }

Apache Commons Math has a *CombinatoricsUtils* class with a static *factorial* method that we can use to calculate the factorial.

To include Apache Commons Math, we’ll add the *commons-math3* dependency into our *pom*:

<dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-math3</artifactId> <version>3.6.1</version> </dependency>

Let’s see an example using the *CombinatoricsUtils *class:

public long factorialUsingApacheCommons(int n) { return CombinatoricsUtils.factorial(n); }

Notice that its return type is *long*, just like our home-grown solutions.

That means here that if the computed value exceeds *Long.MAX_VALUE*, a *MathArithmeticException* is thrown.

To get any bigger,** we are going to need a different return type.**

As discussed before, the *long* datatype can be used for factorials only for *n <= 20*.

For larger values of *n*, **we can use the BigInteger class** from the

public BigInteger factorialHavingLargeResult(int n) { BigInteger result = BigInteger.ONE; for (int i = 2; i <= n; i++) result = result.multiply(BigInteger.valueOf(i)); return result; }

Google’s Guava library also provides a utility method for calculating factorials for larger numbers.

To include the library, we can add its the *guava *dependency to our *pom*:

<dependency> <groupId>com.google.guava</groupId> <artifactId>guava</artifactId> <version>25.1-jre</version> </dependency>

Now, we can use the static *factorial* method from the *BigIntegerMath* class to calculate the factorial of a given number:

public BigInteger factorialUsingGuava(int n) { return BigIntegerMath.factorial(n); }

In this article, we saw a few ways of calculating factorials using core Java as well as a couple of external libraries.

We first saw solutions using the *long* data type for calculating factorials of numbers up to *20*. Then, we saw a couple of ways to use *BigInteger *for numbers greater than 20.

The code presented in this article is available over on Github.

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