JAVA JDK8 List分組的實現和用法

2024-07-14 08:43:01

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概述

對List進行分組是日常開發中，經常遇到的，在JDK 8中對List按照某個屬性分組的代碼，超級簡單。

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,"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, 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;  }}

上面的例子是對List按照couponId分組，couponId一樣的，歸為一組。打印結果如下：

{
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
   },
   {
    "couponId":3,
    "name":"優惠券4",
    "price":400
   }
   ]
}

如果分組后，分組內并不想是對象，而是對象的屬性，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,"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, List<String>> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId,Collectors.mapping(Coupon::getName,Collectors.toList())));    System.out.println(JSON.toJSONString(resultList, SerializerFeature.PrettyFormat));  }}

這樣分組內就是name屬性了。打印結果如下：