概述
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。
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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));
}
}
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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;
}
}
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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。
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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));
}
}
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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"
]
}
总结
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原文链接：https://blog.csdn.net/linsongbin1/article/details/83933703