[java] list sum

### Integer

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);  
Integer sum1 = numbers.stream().reduce(0, Integer::sum);  
int sum2 = numbers.stream().mapToInt(i -> i).sum();

### BigInteger

List<BigInteger> numbers = Arrays.asList(
        BigInteger.valueOf(1),
        BigInteger.valueOf(2),
        BigInteger.valueOf(3),
        BigInteger.valueOf(4),
        BigInteger.valueOf(5));
BigInteger sum = numbers.stream().reduce(BigInteger.ZERO, BigInteger::add);

https://kapentaz.github.io/java/Java8-stream-sum-%EA%B5%AC%ED%95%98%EA%B8%B0/#

Java8 stream sum 구하기