Java8 Collectors求和功能的自定义扩展操作
业务中需要将一组数据分类后收集总和，原本可以使用Collectors.summingInt()，但是我们的数据源是BigDecimal类型的，而Java8原生只提供了summingInt、summingLong、summingDouble三种基础类型的方法。
于是就自己动手丰衣足食吧。。
自定义工具类
publicclassMyCollectors{
privateMyCollectors(){
}
//publicstaticCollectorsummingBigDecimal(Functionmapper){}
//BigDecimal类型的集合求和
publicstaticCollectorsummingBigDecimal(ToBigDecimalFunctionmapper){
returnnewCollectorImpl<>(
()->newBigDecimal[]{BigDecimal.ZERO},
(a,t)->a[0]=a[0].add(mapper.applyAsInt(t)),
(a,b)->{
a[0]=a[0].add(b[0]);
returna;
},
a->a[0],
Collections.emptySet()
);
}
staticclassCollectorImplimplementsCollector{
//创建一个计算用的容器
privatefinalSuppliersupplier;
//计算逻辑
privatefinalBiConsumeraccumulator;
//合并逻辑
privatefinalBinaryOperatorcombiner;
//返回最终计算值
privatefinalFunctionfinisher;
//空Set
privatefinalSetcharacteristics;
CollectorImpl(Suppliersupplier,BiConsumeraccumulator,BinaryOperatorcombiner,
Functionfinisher,Setcharacteristics){
this.supplier=supplier;
this.accumulator=accumulator;
this.combiner=combiner;
this.finisher=finisher;
this.characteristics=characteristics;
}
CollectorImpl(Suppliersupplier,BiConsumeraccumulator,BinaryOperatorcombiner,
Setcharacteristics){
this(supplier,accumulator,combiner,castingIdentity(),characteristics);
}
@Override
publicBiConsumeraccumulator(){
returnaccumulator;
}
@Override
publicSuppliersupplier(){
returnsupplier;
}
@Override
publicBinaryOperatorcombiner(){
returncombiner;
}
@Override
publicFunctionfinisher(){
returnfinisher;
}
@Override
publicSetcharacteristics(){
returncharacteristics;
}
}
@SuppressWarnings("unchecked")
privatestaticFunctioncastingIdentity(){
returni->(R)i;
}
}
自定义函数式接口
@FunctionalInterface publicinterfaceToBigDecimalFunction{ BigDecimalapplyAsInt(Tvalue); }
测试入口
publicclassAnswerApp{
publicstaticvoidmain(String[]args){
Listlist=Lists.newArrayList();
for(inti=0;i<24;i++){
list.add(BigDecimal.valueOf(i+10.2121543));
}
//方式1
BigDecimalsum=list.stream().collect(MyCollectors.summingBigDecimal(e->e));
System.out.println(sum.doubleValue());
//方式2
Optionalreduce=list.stream().reduce(BigDecimal::add);
System.out.println(reduce.orElse(BigDecimal.valueOf(0)));
}
}
//OUTPUT:521.0917032
补充：Collectors扩展接口实现BigDecimal的相加
第一步
创建ToBigDecimalFunction接口
importjava.math.BigDecimal; @FunctionalInterface publicinterfaceToBigDecimalFunction{ BigDecimalapplyAsBigDecimal(Tvalue); }
第二步
创建工具类实现接口
importjava.math.BigDecimal;
importjava.util.Collections;
importjava.util.Set;
importjava.util.function.BiConsumer;
importjava.util.function.BinaryOperator;
importjava.util.function.Function;
importjava.util.function.Supplier;
importjava.util.stream.Collector;
publicclassCollectorsUtil{
staticfinalSetCH_NOID=Collections.emptySet();
privateCollectorsUtil(){
}
@SuppressWarnings("unchecked")
privatestaticFunctioncastingIdentity(){
returni->(R)i;
}
/**
*Simpleimplementationclassfor{@codeCollector}.
*
*@param
*thetypeofelementstobecollected
*@param
*thetypeoftheresult
*/
staticclassCollectorImplimplementsCollector{
privatefinalSuppliersupplier;
privatefinalBiConsumeraccumulator;
privatefinalBinaryOperatorcombiner;
privatefinalFunctionfinisher;
privatefinalSetcharacteristics;
CollectorImpl(Suppliersupplier,BiConsumeraccumulator,BinaryOperatorcombiner,
Functionfinisher,Setcharacteristics){
this.supplier=supplier;
this.accumulator=accumulator;
this.combiner=combiner;
this.finisher=finisher;
this.characteristics=characteristics;
}
CollectorImpl(Suppliersupplier,BiConsumeraccumulator,BinaryOperatorcombiner,
Setcharacteristics){
this(supplier,accumulator,combiner,castingIdentity(),characteristics);
}
@Override
publicBiConsumeraccumulator(){
returnaccumulator;
}
@Override
publicSuppliersupplier(){
returnsupplier;
}
@Override
publicBinaryOperatorcombiner(){
returncombiner;
}
@Override
publicFunctionfinisher(){
returnfinisher;
}
@Override
publicSetcharacteristics(){
returncharacteristics;
}
}
publicstaticCollectorsummingBigDecimal(ToBigDecimalFunctionmapper){
returnnewCollectorImpl<>(()->newBigDecimal[1],(a,t)->{
if(a[0]==null){
a[0]=BigDecimal.ZERO;
}
a[0]=a[0].add(mapper.applyAsBigDecimal(t));
},(a,b)->{
a[0]=a[0].add(b[0]);
returna;
},a->a[0],CH_NOID);
}
}
使用测试
importcom.example.javademo.JavaDemoApplicationTests;
importcom.example.javademo.pojo.Student;
importcom.example.javademo.utils.DataUtils;
importorg.junit.Test;
importjava.math.BigDecimal;
importjava.util.stream.Collectors;
publicclassTestBigDecimalextendsJavaDemoApplicationTests{
@Test
publicvoidtestGroupByAfterBigdecimal(){
/*
自定义实现对分组后的集合，属性为bigdecmal进行相加
*/
System.out.println(DataUtils.getData().stream().collect(Collectors.groupingBy(Student::getSchool,CollectorsUtil.summingBigDecimal(Student::getMoney))));
//归约造作
BigDecimalreduce=DataUtils.getData().stream().map(Student::getMoney).reduce(BigDecimal.ZERO,BigDecimal::add);
System.out.println(reduce);
intsum=DataUtils.getData().stream().mapToInt(Student::getAge).sum();
System.out.println(sum);
}
}
以上为个人经验，希望能给大家一个参考，也希望大家多多支持毛票票。如有错误或未考虑完全的地方，望不吝赐教。
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