simple base conversion task

import java.math.BigInteger;

/**
 * @author Daniil Shreyder
 * Date: 16.06.2021
 */

public class Test {
    public static void main(String[] args) {
        BigInteger inputBase10 = BigInteger.valueOf(2)
                .multiply(BigInteger.valueOf(7).pow(2))
                .add(BigInteger.valueOf(42)
                        .multiply(BigInteger.valueOf(49).pow(49)))
                .add(BigInteger.valueOf(-357));

        System.out.println(inputBase10);
        final BigInteger base = BigInteger.valueOf(7);
        BigInteger inputBase7 = convertToBase(inputBase10, base);
        System.out.println(inputBase7);

        System.out.println("Number of 6: " +
                inputBase7.toString()
                        .chars()
                        .filter(character -> character == '6')
                        .count());
    }

    public static BigInteger convertToBase(BigInteger inputBase10, BigInteger base) {
        int counter = 0;

        BigInteger inputBaseX = BigInteger.ZERO;

        while (!inputBase10.equals(BigInteger.ZERO)) {
            inputBaseX = inputBaseX.add(inputBase10.mod(base).multiply(BigInteger.valueOf(10).pow(counter)));
            inputBase10 = inputBase10.divide(base);
            counter++;
        }

        return inputBaseX;
    }
}