## 1. Overview

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**.

## 2. Factorial for Numbers up to 20

### 2.1. Factorial Using a *for* Loop

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.**

### 2.2. Factorial Using Java 8 Streams

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.

### 2.3. Factorial Using Recursion

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);
}
```

### 2.4. Factorial Using Apache Commons Math

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.**

## 3. Factorial for Numbers Greater Than 20

### 3.1. Factorial Using *BigInteger*

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 *java.math* package, which can hold **values up to ***2^Integer.MAX_VALUE*:

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

### 3.2. Factorial Using Guava

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>31.0.1-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);
}
```

## 4. Conclusion

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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