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”
]
}
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版权声明：本文为CSDN博主「Sam_Deep_Thinking」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/linsongbin1/article/details/83933703