面试题 以下程序的输出结果是什么

import java.math.BigInteger;

public class Test {

public static void main(String[] args) throws NumberFormatException{

BigInteger one = new BigInteger("1");

BigInteger two = new BigInteger("2");

BigInteger three = new BigInteger("3");

BigInteger sum = new BigInteger("0");

sum.add(one);

sum.add(two);

sum.add(three);

System.out.println(sum.toString());

}

}

输出结果是：0

要想输出结果为6，改成：

import java.math.BigInteger;

public class Test {

public static void main(String[] args) throws NumberFormatException{

BigInteger one = new BigInteger("1");

BigInteger two = new BigInteger("2");

BigInteger three = new BigInteger("3");

BigInteger sum = new BigInteger("0");

sum=sum.add(one);

sum=sum.add(two);

sum=sum.add(three);

System.out.println(sum.toString());

}

}