[REST API] 3. Stream 실습

package ex02;

import lombok.Data;

import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.Collectors;

public class StEx01 {
    public static void main(String[] args) {
        var list = Arrays.asList(1, 2, 3, 4, 5);
        Cart c1 = new Cart(1, "바나나", 1000);
        Cart c2 = new Cart(2, "바나나", 1000);
        Cart c3 = new Cart(3, "딸기", 2000);
        Cart c4 = new Cart(4, "사과", 3000);

        var cartList = Arrays.asList(c1, c2, c3, c4);

        // 1. map(가공)
        var new11 = list.stream().map(i -> i * 2).toList();
        System.out.println(new11);

        var new12 = list.stream()
                .map(i -> i * 2)
                .filter(i -> i != 10)
                .toList();
        System.out.println(new12);

        // 2. filter(검색, 삭제)
        var new21 = list.stream()
                .filter(i -> i < 3)
                .toList();
        System.out.println(new21);

        // 3. count(개수), sorted(정렬), distinct(중복 제거)
        var new31 = list.stream()
                .sorted(Comparator.reverseOrder())
                .map(i -> i / 3)
                .distinct()
                .count();
        System.out.println(new31);

        // 4. mapToInt, sum, min, max, average
        var new41 = cartList.stream() // 1. 물가에 던져진 카트 4개
                .mapToInt(cart -> cart.getPrice()) // 2. 물가에 던져진 int 4개
                .min();
        System.out.println(new41);

        // 5. group by
        // [key=[c1, c2], key=[c3], key=[c4]]
        var new51 = cartList.stream()
                .collect(Collectors.groupingBy(cart -> cart.getName()))
                .entrySet()
                .stream()
                .map(en -> en.getValue())
                .toList();

        System.out.println(new51);

    }

    @Data
    static class Cart {
        private int id;
        private String name;
        private int price;

        public Cart(int id, String name, int price) {
            this.id = id;
            this.name = name;
            this.price = price;
        }
    }
}