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进行按照ID进行聚合统计

JAVA8 reduce API

API个人理解

U reduce(U u,BiFunction accumulator,BinaryOperator 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 按照name分组统计得到 List

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 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 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 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;

}

}

PS

等下次有空补上不使用stream().reduce 实现同样操作的比较繁琐的代码，啦啦啦啦啦~~~