概述

在JDK8 List分組一文中介紹了JDK 8如何對list進行分組，但是沒有提到如何在分組後，獲取每個分組的第一個元素。其實這個也很簡單，程式碼如下：

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,"優惠券1");
        Coupon coupon2 = new Coupon(2,200,"優惠券2");
        Coupon coupon3 = new Coupon(3,300,"優惠券3");
        Coupon coupon4 =
 
 new Coupon(3,400,"優惠券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;
    }
}

需要藉助Collectors.collectingAndThen方法，對組內的元素進行處理，這裡是獲取第一個元素。

程式碼輸出結果如下：

{   1:{
		"couponId":1,
		"name":"優惠券1",
		"price":100
	},
	2:{
		"couponId":2,
		"name":"優惠券2",
		"price":200
	},
	3:{
		"couponId":3,
		"name":"優惠券3",
		"price":300
	}
}