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。
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
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;
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
}
]
}
];
讲了那么多废话，这个才是最直接的
对 List<Foo> 按照name分组统计得到 List<Bar>
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
*/
}
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;
}
}
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();
}
}
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;
}
}
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~~~