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使用JAVA8 stream中三个参数的reduce方法对List进行分组统计

背景

平时在编写前端代码时，习惯使用lodash来编写‘野生’的JavaScript;

lodash提供来一套完整的API对JS对象（Array,Object,Collection等）进行操作，这其中就包括_.groupBy 和 _.reduce，即分组和'聚合'（reduce不知道该怎么翻译合适）。

使用这些‘野生’的API能够极大的提高我本人编写JS代码的效率。而JAVA8开始支持stream和lambda表达式，这些和lodash的API有很多类似的功能。因此我在熟悉lodash的前提下尝试使用JAVA8的新特性减少冗余代码的编写。

需求

在开发后端某功能接口的过程中，需要对一个从数据库中取出的数据List<T>进行按照ID进行聚合统计

JAVA8 reduce API

API个人理解

<U> U reduce(U u,BiFunction<U,? super T,U> accumulator,BinaryOperator<U> combiner)
  #第一个参数返回实例u，传递你要返回的U类型对象的初始化实例u

  #第二个参数累加器accumulator，可以使用二元ℷ表达式（即二元lambda表达式），声明你在u上累加你的数据来源t的逻辑
  #例如(u,t)->u.sum(t),此时lambda表达式的行参列表是返回实例u和遍历的集合元素t，函数体是在u上累加t

  #第三个参数组合器combiner，同样是二元ℷ表达式，(u,t)->u
  #lambda表达式行参列表同样是(u,t)，函数体返回的类型则要和第一个参数的类型保持一致

伪代码

#1.声明一个返回结果U
  #2.对List<T>进行遍历，在U和每个T实例上应用一个累加器进行累加操作
  #3.返回最终结果U
  U result = identity;
  for (T element : this stream)
      result = accumulator.apply(result, element)
  return result;

数据准备

var source =
[
    {"name": "A","type": "san","typeValue": 1.0,"count": 2},
    {"name": "A","type": "nas","typeValue": 13.0,"count": 1},
    {"name": "B","type": "san","typeValue": 112.0,"count": 3},
    {"name": "C","type": "san","typeValue": 43.0,"count": 5},
    {"name": "B","type": "nas","typeValue": 77.0,"count": 7}
];
var target =
[
    {
        "name": "A",
        "count": 3,
        "totalTypeValue": 14.0,
        "bazList": [
            {
                "type": "san",
                "typeValue": 1.0
            },
            {
                "type": "nas"
                "typeValue": 13.0
            }
        ]
    }, 
    {
        "name": "B",
        "count": 10,
        "totalTypeValue": 189.0,
        "bazList": [
            {
                "type": "san",
                "typeValue": 112.0
            }, {
                "type": "nas"
                "typeValue": 77.0
            }
        ]
    }, 
    {
        "name": "C",
        "count": 5,
        "totalTypeValue": 43.0,
        "bazList": [
            {
                "type": "san",
                "typeValue": 43.0
            }
        ]
    }
];

Code

讲了那么多废话，这个才是最直接的

代码执行大意

对 List<Foo> 按照name分组统计得到 List<Bar>

ReduceTest.java

import com.google.common.collect.Lists;
import Bar;
import Foo;

import java.util.List;
import java.util.stream.Collectors;


public class ReduceTest {
    
    public static void main(String[] args) throws Exception{
        List<Foo> fooList = Lists.newArrayList(
            new Foo("A","san",1.0,2),
            new Foo("A","nas",13.0,1),
            new Foo("B","san",112.0,3),
            new Foo("C","san",43.0,5),
            new Foo("B","nas",77.0,7)
        );
        List<Bar> barList = Lists.newArrayList();
        fooList
            .stream()
            .collect(Collectors.groupingBy(Foo::getName,Collectors.toList()))
            .forEach((name,fooListByName)->{
                Bar bar = new Bar();
                bar = fooListByName
                        .stream()
                        .reduce(bar,(u,t)->u.sum(t),(u,t)->u);
                System.out.println(bar.toString());
                barList.add(bar);
            });
    }
    /*
    输出结果
    name:A
    count:3
    totalTypeValue:14.0
    bazList:
        type:san
        typeValue:1.0
        type:nas
        typeValue:13.0
    
    name:B
    count:10
    totalTypeValue:189.0
    bazList:
        type:san
        typeValue:112.0
        type:nas
        typeValue:77.0
    
    name:C
    count:5
    totalTypeValue:43.0
    bazList:
        type:san
        typeValue:43.0
    */
}

Foo.java

public class Foo{
    private String name;
    private String type;
    private Double typeValue;
    private Integer count;

    public Foo(String name, String type, Double typeValue, Integer count) {
        this.name = name;
        this.type = type;
        this.typeValue = typeValue;
        this.count = count;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public Double getTypeValue() {
        return typeValue;
    }

    public void setTypeValue(Double typeValue) {
        this.typeValue = typeValue;
    }

    public Integer getCount() {
        return count;
    }

    public void setCount(Integer count) {
        this.count = count;
    }
}

Bar.java

import com.google.common.collect.Lists;

import java.util.List;

public class Bar{
    private String name;
    private Integer count;
    private Double totalTypeValue;
    private List<Baz> bazList;

    public Bar() {
        this.name = null;
        this.count = 0;
        this.totalTypeValue = 0.0;
        this.bazList = Lists.newArrayList();
    }

    public Bar sum(Foo foo){
        if(name == null){
            this.name = foo.getName();
        }
        this.count += foo.getCount();
        this.totalTypeValue += foo.getTypeValue();
        this.bazList.add(new Baz(foo.getType(),foo.getTypeValue()));
        return this;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getCount() {
        return count;
    }

    public void setCount(Integer count) {
        this.count = count;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("name:").append(this.name).append(System.lineSeparator());
        sb.append("count:").append(this.count).append(System.lineSeparator());
        sb.append("totalTypeValue:").append(this.totalTypeValue).append(System.lineSeparator());
        sb.append("bazList:").append(System.lineSeparator());
        this.bazList.forEach(baz->{
            sb.append("/t").append("type:").append(baz.getType()).append(System.lineSeparator());
            sb.append("/t").append("typeValue:").append(baz.getTypeValue()).append(System.lineSeparator());
        });
        return sb.toString();
    }
}

Baz.java

public class Baz{
    private String type;
    private Double typeValue;

    public Baz(String type, Double typeValue) {
        this.type = type;
        this.typeValue = typeValue;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public Double getTypeValue() {
        return typeValue;
    }

    public void setTypeValue(Double typeValue) {
        this.typeValue = typeValue;
    }
}