JAVA JDK8 List分组的实现和用法

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。

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"
   ]
}

总结

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