문제 1

@GetMapping("/api/v1/calc")
public Calculator calculate(@RequestParam int num1, @RequestParam int num2) {
    Calculator calculator = new Calculator(num1, num2);

    return calculator;
}

public class Calculator {

    private int add;
    private int minus;
    private int multiply;

    public Calculator(int num1, int num2) {
        this.add = num1 + num2;
        this.minus = num1 - num2;
        this.multiply = num1 * num2;
    }

    public int getAdd() {
        return add;
    }

    public int getMinus() {
        return minus;
    }

    public int getMultiply() {
        return multiply;
    }
}

문제 2

@GetMapping("/api/v1/day-of-week")
public DayOfTheWeek dayOfTheWeek(@RequestParam String date) {
    DayOfTheWeek dayOfTheWeek = new DayOfTheWeek(date);

    return dayOfTheWeek;
}

import java.time.DayOfWeek;
import java.time.LocalDate;

public class DayOfTheWeek {

    private String dayOfTheWeek;

    public DayOfTheWeek(String date) {
        this.dayOfTheWeek = getDayOfTheWeek(date);
    }

    public String getDayOfTheWeek(String date) {
        String[] dateArr = date.split("-");
        int year = Integer.parseInt(dateArr[0]);
        int month = Integer.parseInt(dateArr[1]);
        int day = Integer.parseInt(dateArr[2]);

        LocalDate localDate = LocalDate.of(year, month, day);

        DayOfWeek dayOfWeek = localDate.getDayOfWeek();

        return calDayOfTheWeek(dayOfWeek.getValue());
    }

    public String calDayOfTheWeek(int dayOfWeekNum) {

        String result="";

        switch (dayOfWeekNum) {
            case 1:
                result = "MON";
                break;
            case 2:
                result = "TUS";
                break;
            case 3:
                result = "WEN";
                break;
            case 4:
                result = "THR";
                break;
            case 5:
                result = "FRI";
                break;
            case 6:
                result = "SAT";
                break;
            case 7:
                result = "SUN";
                break;
        }

        return result;
    }
}

문제 3

@PostMapping
public int sum(@RequestBody NumRequest numrequest) {
    return request.getNumbers().stream()
            .mapToInt(Integer::intValue)
            .sum();
}

public class NumRequest {

    private List<Integer> numbers = new ArrayList<>();

    public List<Integer> getNumbers() {
        return numbers;
    }
}