Some Java code to calculate large fibonacci numbers.

LargeFibonacci.java

import org.jetbrains.annotations.Contract;
import org.jetbrains.annotations.NotNull;

import java.math.BigInteger;

public class LargeFibonacci {
    
    public static void main(String[] args) {
        BigInteger x = new BigInteger("0");
        BigInteger y = new BigInteger("1");
        for (int i = 0; i < 1000; i++) {
            System.out.println(i + 1 + " => " + formatNumber(y));
            BigInteger b = new BigInteger(String.valueOf(y));
            y = nextFibonacci(x, y);
            x = new BigInteger(String.valueOf(b));
        }
    }

    private static @NotNull String formatNumber(@NotNull BigInteger number) {
        String numberString = number.toString();
        StringBuilder sb = new StringBuilder();
        int i = numberString.length() - 1;
        int passed = 0;
        while (i >= 0) {
            if (passed == 3) {
                passed = 0;
                sb.append(",");
                continue;
            }

            sb.append(numberString.charAt(i));

            i--;
            passed++;
        }
        return sb.reverse().toString();
    }

    @Contract(pure = true)
    private static @NotNull BigInteger nextFibonacci(@NotNull BigInteger one, BigInteger two) {
        return one.add(two);
    }

}