JAVA JDK8 List分组获取第一个元素的方法

package test;
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.serializer.SerializerFeature;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class ListGroupFindFirstTest3 {
  public static void main(String[] args) {
    List<Coupon> couponList = new ArrayList<>();
    Coupon coupon1 = new Coupon(1,100,"优惠券1");
    Coupon coupon2 = new Coupon(2,200,"优惠券2");
    Coupon coupon3 = new Coupon(3,300,"优惠券3");
    Coupon coupon4 = new Coupon(3,400,"优惠券4");
    couponList.add(coupon1);
    couponList.add(coupon2);
    couponList.add(coupon3);
    couponList.add(coupon4);
    Map<Integer, Coupon> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId,Collectors.collectingAndThen(Collectors.toList(),value->value.get(0))));
    System.out.println(JSON.toJSONString(resultList, SerializerFeature.PrettyFormat));
  }
}

package test;
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;
  }
}