public class Recursion {
    static UnaryOperator<Integer> factorial;

    public static void main(String[] args) {
        factorial = x -> (x == 0) ? 1 : (x * factorial.apply(i - 1));
        System.out.println(factorial.apply(10));
    }
}

package testintfn;

import java.util.function.IntUnaryOperator;

import static java.lang.System.out;

public class Recursion {

    static IntUnaryOperator fact() {
        return x -> x == 0 ? 1 : x * fact().applyAsInt(x - 1);
    }

    public static void main(String[] args) {
        out.println(
                fact().applyAsInt(10)
        );

        out.println();
        for (int i = 0; i < 5; ++i) {
            out.println(
                    "fact hashCode: " + fact().hashCode() + " ;)"
            );
        }
    }
}

package streamfactorial;

import java.math.BigInteger;
import java.util.stream.LongStream;

import static java.lang.System.out;
import static java.math.BigInteger.ONE;

public class StreamFactorial {

    public static void main(String[] args) {

        printFactorialOf(100_000);
    }

    static void printFactorialOf(long num) {
        out.println(
                "factorial"                 + "\n" +
                "  of " + num               + "\n" +
                "  is " + factorial(num)
        );
    }

    static BigInteger factorial(long num) {
        return LongStream
                .rangeClosed(1, num)
                .mapToObj(BigInteger::valueOf)
                .parallel()
                .reduce(ONE, BigInteger::multiply);
    }
}