摘要：背景平時(shí)在編寫前端代碼時(shí)，習(xí)慣使用來編寫野生的提供來一套完整的對(duì)對(duì)象等進(jìn)行操作，這其中就包括和，即分組和聚合不知道該怎么翻譯合適。使用這些野生的能夠極大的提高我本人編寫代碼的效率。
背景
平時(shí)在編寫前端代碼時(shí)，習(xí)慣使用lodash來編寫‘野生’的JavaScript;
lodash提供來一套完整的API對(duì)JS對(duì)象（Array,Object,Collection等）進(jìn)行操作，這其中就包括_.groupBy 和 _.reduce，即分組和"聚合"（reduce不知道該怎么翻譯合適）。
使用這些‘野生’的API能夠極大的提高我本人編寫JS代碼的效率。而JAVA8開始支持stream和lambda表達(dá)式，這些和lodash的API有很多類似的功能。因此我在熟悉lodash的前提下嘗試使用JAVA8的新特性減少冗余代碼的編寫。
需求
在開發(fā)后端某功能接口的過程中，需要對(duì)一個(gè)從數(shù)據(jù)庫中取出的數(shù)據(jù)List進(jìn)行按照ID進(jìn)行聚合統(tǒng)計(jì)
JAVA8 reduce API
API個(gè)人理解
U reduce(U u,BiFunction accumulator,BinaryOperator combiner)
#第一個(gè)參數(shù)返回實(shí)例u，傳遞你要返回的U類型對(duì)象的初始化實(shí)例u
#第二個(gè)參數(shù)累加器accumulator，可以使用二元?表達(dá)式（即二元lambda表達(dá)式），聲明你在u上累加你的數(shù)據(jù)來源t的邏輯
#例如(u,t)->u.sum(t),此時(shí)lambda表達(dá)式的行參列表是返回實(shí)例u和遍歷的集合元素t，函數(shù)體是在u上累加t
#第三個(gè)參數(shù)組合器combiner，同樣是二元?表達(dá)式，(u,t)->u
#lambda表達(dá)式行參列表同樣是(u,t)，函數(shù)體返回的類型則要和第一個(gè)參數(shù)的類型保持一致
偽代碼
#1.聲明一個(gè)返回結(jié)果U
#2.對(duì)List進(jìn)行遍歷，在U和每個(gè)T實(shí)例上應(yīng)用一個(gè)累加器進(jìn)行累加操作
#3.返回最終結(jié)果U
U result = identity;
for (T element : this stream)
result = accumulator.apply(result, element)
return result;
數(shù)據(jù)準(zhǔn)備
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
講了那么多廢話，這個(gè)才是最直接的
代碼執(zhí)行大意
對(duì) List 按照name分組統(tǒng)計(jì)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);
});
}
/*
輸出結(jié)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(" ").append("type:").append(baz.getType()).append(System.lineSeparator());
sb.append(" ").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
等下次有空補(bǔ)上不使用stream().reduce 實(shí)現(xiàn)同樣操作的比較繁瑣的代碼，啦啦啦啦啦~~~
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