[프로그래머스/Java] 없는 숫자 더하기

-Code

import java.util.*;
import java.util.stream.*;

class Solution {
    public int solution(int[] numbers) {
        List<Integer> numbers_list = Arrays.stream(numbers)
                .boxed().collect(Collectors.toList());        
        return IntStream.rangeClosed(1, 9)
                .filter(i -> !numbers_list.contains(i))
                .sum();
    }
}