????????List集合中的某个对象值是BigDecimal类型的，且需要对所有对象中相同的BigDecimal类型的属性进行求和，找不到现成的工具类可以使用，可以自己来定义一个。
1.定义工具类
package com.kakarote.core.utils;
import lombok.Data;
import lombok.experimental.Accessors;
import java.math.BigDecimal;
import java.math.MathContext;
import java.util.*;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Collectors;
/**
* JAVA8 分组求和使用方法
*/
public class CollectorsUtils {
private static final Set<Collector.Characteristics> CHARACTERISTICS = Collections.emptySet();
private CollectorsUtils() {
}
@SuppressWarnings("unchecked")
private static <I, R> Function<I, R> check() {
return i -> (R) i;
}
@SuppressWarnings("hiding")
static class CollectorImpl<T, A, R> implements Collector<T, A, R> {
private final Supplier<A> supplier;
private final BiConsumer<A, T> accumulator;
private final BinaryOperator<A> combiner;
private final Function<A, R> finisher;
private final Set<Characteristics> characteristics;
CollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Set<Characteristics> characteristics) {
this.supplier = supplier;
this.accumulator = accumulator;
this.combiner = combiner;
this.finisher = finisher;
this.characteristics = characteristics;
}
CollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Set<Characteristics> characteristics) {
this(supplier, accumulator, combiner, check(), characteristics);
}
@Override
public Supplier<A> supplier() {
return supplier;
}
@Override
public BiConsumer<A, T> accumulator() {
return accumulator;
}
@Override
public BinaryOperator<A> combiner() {
return combiner;
}
@Override
public Function<A, R> finisher() {
return finisher;
}
@Override
public Set<Characteristics> characteristics() {
return characteristics;
}
}
/**
* 求和方法
* @param mapper
* @param <T>
* @return
*/
public static <T> Collector<T, ?, BigDecimal> summingBigDecimal(ToBigDecimalFunction<? super T> mapper) {
return new CollectorImpl<>(
() -> new BigDecimal[]{new BigDecimal(0)},
(a, t) -> {
a[0] = a[0].add(mapper.applyAsBigDecimal(t), MathContext.DECIMAL32);
},
(a, b) -> {
a[0] = a[0].add(b[0], MathContext.DECIMAL32);
return a;
},
a -> a[0], CHARACTERISTICS
);
}
/**
* 求最大值，这里的最小MIN值，作为初始条件判断值，如果某些数据范围超过百亿，可以根据需求换成 Long.MIN_VALUE 或者 Double.MIN_VALUE
* @param mapper
* @param <T>
* @return
*/
public static <T> Collector<T, ?, BigDecimal> maxBy(ToBigDecimalFunction<? super T> mapper) {
return new CollectorImpl<>(
() -> new BigDecimal[]{new BigDecimal(Integer.MIN_VALUE)},
(a, t) -> {
a[0] = a[0].max(mapper.applyAsBigDecimal(t));
},
(a, b) -> {
a[0] = a[0].max(b[0]);
return a;
},
a -> a[0], CHARACTERISTICS
);
}
/**
* 求最小值，这里的最大MAX值，作为初始条件判断值，如果某些数据范围超过百亿，可以根据需求换成 Long.MAX_VALUE 或者 Double.MAX_VALUE
* @param mapper
* @param <T>
* @return
*/
public static <T> Collector<T, ?, BigDecimal> minBy(ToBigDecimalFunction<? super T> mapper) {
return new CollectorImpl<>(
() -> new BigDecimal[]{new BigDecimal(Integer.MAX_VALUE)},
(a, t) -> {
a[0] = a[0].min(mapper.applyAsBigDecimal(t));
},
(a, b) -> {
a[0] = a[0].min(b[0]);
return a;
},
a -> a[0], CHARACTERISTICS
);
}
/**
* 求平均，并且保留小数，返回一个平均值
* @param mapper
* @param newScale
* @param roundingMode
* @param <T>
* @return
*/
public static <T> Collector<T, ?, BigDecimal> averagingBigDecimal(ToBigDecimalFunction<? super T> mapper,
int newScale, int roundingMode) {
return new CollectorImpl<>(
() -> new BigDecimal[]{new BigDecimal(0), new BigDecimal(0)},
(a, t) -> {
a[0] = a[0].add(mapper.applyAsBigDecimal(t));
a[1] = a[1].add(BigDecimal.ONE);
},
(a, b) -> {
a[0] = a[0].min(b[0]);
return a;
},
a -> a[0].divide(a[1], MathContext.DECIMAL32).setScale(newScale, roundingMode), CHARACTERISTICS
);
}
// 自定义函数
@FunctionalInterface
interface ToBigDecimalFunction<T> {
BigDecimal applyAsBigDecimal(T t);
}
}
2.定义实体类
@Data
@Accessors(chain = true) // 开启链式编程
static class People{
Long id;
Integer sex;
BigDecimal money;
}
3.测试
public static void main(String[] args) {
List<People> list = new ArrayList<People>();
list.add(new People().setId(1l).setSex(1).setMoney(BigDecimal.ONE));
list.add(new People().setId(2l).setSex(1).setMoney(BigDecimal.TEN));
list.add(new People().setId(3l).setSex(0).setMoney(BigDecimal.ZERO));
list.add(new People().setId(4l).setSex(0).setMoney(BigDecimal.TEN));
list.add(new People().setId(5l).setSex(1).setMoney(BigDecimal.valueOf(5)));
BigDecimal sum = list.stream().collect(CollectorsUtils.summingBigDecimal(People::getMoney));
System.out.println("求和：" + sum);
Map<Integer, BigDecimal> collect = list.stream().collect(Collectors.groupingBy(People::getSex, CollectorsUtils.summingBigDecimal(People::getMoney)));
System.out.println("分组求和：" + collect);
}
4.结果
求和：26
分组求和：{0=10, 1=16}
