package test;import java.util.ArrayList;import java.util.List;import java.util.stream.Collectors;public class ListAttrTest {  public static void main(String[] args) {    List<Coupon> couponList = new ArrayList<>();    Coupon coupon1 = new Coupon(1,100,"優(yōu)惠券1");    Coupon coupon2 = new Coupon(2,200,"優(yōu)惠券2");    Coupon coupon3 = new Coupon(3,300,"優(yōu)惠券3");    couponList.add(coupon1);    couponList.add(coupon2);    couponList.add(coupon3);    List<Integer> resultList = couponList.stream().map(Coupon::getCouponId).collect(Collectors.toList());    System.out.println(resultList);  }}

public class Coupon {  private Integer couponId;  private Integer price;  private String name;  public Coupon(Integer couponId, Integer price, String name) {    this.couponId = couponId;    this.price = price;    this.name = name;  }  public Integer getCouponId() {    return couponId;  }  public void setCouponId(Integer couponId) {    this.couponId = couponId;  }  public Integer getPrice() {    return price;  }  public void setPrice(Integer price) {    this.price = price;  }  public String getName() {    return name;  }  public void setName(String name) {    this.name = name;  }}