Java8 Stream针对List先分组再求和、最大值、最小值、平均值等

解题思路：JAVA8使用stream()根据类型对List进行分组统计。

核心功能代码片段：

1.  
   //分组求和
2.  
   Map<String, LongSummaryStatistics> collect = list.stream().collect(
3.  
   Collectors.groupingBy(Fruit::getType,
4.  
   Collectors.summarizingLong(Fruit::getTotal)));
5.  
   for (Map.Entry<String, LongSummaryStatistics> entry : collect.entrySet()) {
6.  
   LongSummaryStatistics longSummaryStatistics = entry.getValue();
7.  
   System.out.println("----------------key----------------" + entry.getKey());
8.  
   System.out.println("求和:" + longSummaryStatistics.getSum());
9.  
   System.out.println("求平均" + longSummaryStatistics.getAverage());
10.  
    System.out.println("求最大:" + longSummaryStatistics.getMax());
11.  
    System.out.println("求最小:" + longSummaryStatistics.getMin());
12.  
    System.out.println("求总数:" + longSummaryStatistics.getCount());
13.  
    }

演示功能代码：

1.  
   package com.zzg.test;
2.  
    
3.  
   import java.math.BigDecimal;
4.  
   import java.util.ArrayList;
5.  
   import java.util.List;
6.  
   import java.util.LongSummaryStatistics;
7.  
   import java.util.Map;
8.  
   import java.util.stream.Collectors;
9.  
    
10.  
    import cn.hutool.json.JSONUtil;
11.  
     
12.  
    /**
13.  
    * 基于Java8 分组再统计
14.  
    * @author zzg
15.  
    *
16.  
    */
17.  
    public class GroupByStatissticsTest {
18.  
     
19.  
    static List<Fruit> initDate(){
20.  
    List<Fruit> list = new ArrayList<Fruit>();
21.  
     
22.  
    Fruit one = new Fruit();
23.  
    one.setName("苹果一级");
24.  
    one.setSid("1");
25.  
    one.setPrice(new BigDecimal("123456.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
26.  
    one.setTotal(1100L);
27.  
    one.setType("1");
28.  
     
29.  
     
30.  
    Fruit two = new Fruit();
31.  
    two.setName("苹果二级");
32.  
    two.setSid("2");
33.  
    two.setPrice(new BigDecimal("123546.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
34.  
    two.setTotal(89L);
35.  
    two.setType("1");
36.  
     
37.  
    Fruit three = new Fruit();
38.  
    three.setName("苹果三级");
39.  
    three.setSid("3");
40.  
    three.setPrice(new BigDecimal("987.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
41.  
    three.setTotal(1039L);
42.  
    three.setType("1");
43.  
     
44.  
    Fruit four = new Fruit();
45.  
    four.setName("梨子一级");
46.  
    four.setSid("4");
47.  
    four.setPrice(new BigDecimal("97.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
48.  
    four.setTotal(39L);
49.  
    four.setType("2");
50.  
     
51.  
    Fruit five = new Fruit();
52.  
    five.setName("梨子二级");
53.  
    five.setSid("5");
54.  
    five.setPrice(new BigDecimal("970.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
55.  
    five.setTotal(399L);
56.  
    five.setType("2");
57.  
     
58.  
    Fruit six = new Fruit();
59.  
    six.setName("西瓜一级");
60.  
    six.setSid("6");
61.  
    six.setPrice(new BigDecimal("1970.98").setScale(BigDecimal.ROUND_HALF_UP, 2) );
62.  
    six.setTotal(2399L);
63.  
    six.setType("3");
64.  
     
65.  
    list.add(one);
66.  
    list.add(two);
67.  
    list.add(three);
68.  
    list.add(four);
69.  
    list.add(five);
70.  
    list.add(six);
71.  
    return list;
72.  
    }
73.  
     
74.  
    public static void main(String[] args) {
75.  
    // TODO Auto-generated method stub
76.  
    List<Fruit> list = initDate();
77.  
     
78.  
    //分组
79.  
    Map<String,List<Fruit>> map = list.stream().collect(Collectors.groupingBy(Fruit::getType));
80.  
    for (Map.Entry<String, List<Fruit>> entry : map.entrySet()) {
81.  
    System.out.println("分组" + JSONUtil.toJsonStr(entry));
82.  
    }
83.  
     
84.  
    //分组求和
85.  
    Map<String, LongSummaryStatistics> collect = list.stream().collect(
86.  
    Collectors.groupingBy(Fruit::getType,
87.  
    Collectors.summarizingLong(Fruit::getTotal)));
88.  
    for (Map.Entry<String, LongSummaryStatistics> entry : collect.entrySet()) {
89.  
    LongSummaryStatistics longSummaryStatistics = entry.getValue();
90.  
    System.out.println("----------------key----------------" + entry.getKey());
91.  
    System.out.println("求和:" + longSummaryStatistics.getSum());
92.  
    System.out.println("求平均" + longSummaryStatistics.getAverage());
93.  
    System.out.println("求最大:" + longSummaryStatistics.getMax());
94.  
    System.out.println("求最小:" + longSummaryStatistics.getMin());
95.  
    System.out.println("求总数:" + longSummaryStatistics.getCount());
96.  
    }
97.  
     
98.  
     
99.  
    }
100.  
      
101.  
     static class Fruit{
102.  
     private String sid;
103.  
     private String name;
104.  
     private String type;
105.  
     private Long total;
106.  
     private BigDecimal price;
107.  
     public String getSid() {
108.  
     return sid;
109.  
     }
110.  
     public void setSid(String sid) {
111.  
     this.sid = sid;
112.  
     }
113.  
     public String getName() {
114.  
     return name;
115.  
     }
116.  
     public void setName(String name) {
117.  
     this.name = name;
118.  
     }
119.  
     public String getType() {
120.  
     return type;
121.  
     }
122.  
     public void setType(String type) {
123.  
     this.type = type;
124.  
     }
125.  
     public Long getTotal() {
126.  
     return total;
127.  
     }
128.  
     public void setTotal(Long total) {
129.  
     this.total = total;
130.  
     }
131.  
     public BigDecimal getPrice() {
132.  
     return price;
133.  
     }
134.  
     public void setPrice(BigDecimal price) {
135.  
     this.price = price;
136.  
     }
137.  
      
138.  
      
139.  
      
140.  
     }
141.  
      
142.  
     }

效果截图：

 

转自：https://blog.csdn.net/zhouzhiwengang/article/details/123558048