JDK8 List分组

对List进行分组是日常开发中，经常遇到的，在JDK 8中对List按照某个属性分组的代码，超级简单。

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 ListGroupTest {
    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, List<Coupon>> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId));
        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;
    }
}

上面的例子是对List按照couponId分组，couponId一样的，归为一组。打印结果如下：

{
    1:[
            {
                "couponId":1,
                "name":"优惠券1",
                "price":100
            }
      ],
    2:[
            {
                "couponId":2,
                "name":"优惠券2",
                "price":200
            }
      ],
    3:[
            {
                "couponId":3,
                "name":"优惠券3",
                "price":300
            },
            {
                "couponId":3,
                "name":"优惠券4",
                "price":400
            }
      ]
}

如果分组后，分组内并不想是对象，而是对象的属性，也可以做到的。

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 ListGroupTest2 {
    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, List<String>> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId,Collectors.mapping(Coupon::getName,Collectors.toList())));
        System.out.println(JSON.toJSONString(resultList, SerializerFeature.PrettyFormat));
    }
}

这样分组内就是name属性了。打印结果如下：

{
    1:[
        "优惠券1"
      ],
    2:[
        "优惠券2"
      ],
    3:[
        "优惠券3",
        "优惠券4"
      ]
}