FibonacciNumber.java

import java.math.BigInteger;
import java.util.Arrays;
public class FibonacciNumber {
    static final BigInteger F0 = BigInteger.ZERO;
    static final BigInteger F1 = BigInteger.ONE;
    static BigInteger f(final int n) {
        if (n <= 0) return F0;
        if (n == 1) return F1;
        final BigInteger[] a = new BigInteger[] {F0, F1};
        for (int i = 2; i < n; i++) {
            final BigInteger b = a[1].add(a[0]);
            a[0] = a[1];
            a[1] = b;
        }
        return a[1].add(a[0]);
    }
    public static void main(final String... args) {
        Arrays.stream(args)
            .mapToInt(Integer::parseInt)
            .filter(n -> n >= 0)
            .forEach(n -> System.out.printf("f(%1$d): %2$d\n", n, f(n)));
    }
}