Factorial in Java using recursion

Carvia Tech | July 24, 2020 | 2 min read | 229 views | Java Coding Challenges

Factorial of a number is calculated using below formula:

factorial (n) = n x (n-1) x (n-2) ... 1

the factorial of a number is calculated by formula number * (number -1) till zero and since the value of factorial zero is 1, it acts as a base case in the recursive version of the factorial method.

Recursive approach

Recursive function to calculate factorial

public class Factorial {
    public long factorial(long n) {
        if (n == 1)
            return 1;
        else {
            return n * factorial(n - 1);
        }
    }
}

Iterative approach

Iterative approach for factorial

class Scratch {
    public long fact(long number) {
        long result = 1L;
        while (number != 0) {
            result = result * number;
            number--;
        }
        return result;
    }
}

Factorial using Java 8 Streams

Java 8 streams with reduction method can be used to calculate factorial of a number.

java 8 Streams for factorial calculation

import java.util.stream.LongStream;

class Scratch {
    public static long factorialStreams(long n) {
        return LongStream.rangeClosed(2, n)
                .reduce(1, (long a, long b) -> a * b);
    }
}

LongStream.rangeClosed(2, n) method creates a Stream of longs from [2 to n].
Lambda function supplied for reduce (a,b) → a * b will multiply each pair of a and b and return the result. The result then carries over to a for the next round. The value of 1 is used in the reduction method as the starting value for variable a for the first iteration.

Handling very large numbers

Here we are using long to hold the result of multiplication, which can quickly overflow due to maximum limit an long can hold. We can improve the above two programs to use BigInteger as discussed in the below article:

How will you calculate factorial of a large number in Java

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Factorial in Java using recursion

Recursive approach

Iterative approach

Factorial using Java 8 Streams

Handling very large numbers

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