Java8 List对象中 BigDecimal类型求和

List集合中的对象值是BigDecimal对所有对象都需要相同的类型BigDecimal求和类型的属性，找不到现成的工具类，可以自己定义一个。

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 CHARACTERISTICS = Collections.emptySet();      private CollectorsUtils() {     }      @SuppressWarnings("unchecked")     private static  Function check() {         return i -> (R) i;     }      @SuppressWarnings("hiding")     static class CollectorImpl implements Collector {          private final Supplier supplier;         private final BiConsumer accumulator;         private final BinaryOperator combiner;         private final Function finisher;         private final Set characteristics;          CollectorImpl(Supplier supplier, BiConsumer accumulator, BinaryOperator combiner, Function finisher, Set characteristics) {             this.supplier = supplier;             this.accumulator = accumulator;             this.combiner = combiner;             this.finisher = finisher;             this.characteristics = characteristics;         }          CollectorImpl(Supplier supplier, BiConsumer accumulator, BinaryOperator combiner, Set characteristics) {             this(supplier, accumulator, combiner, check(), characteristics);         }          @Override         public Supplier supplier() {             return supplier;         }          @Override         public BiConsumer accumulator() {             return accumulator;         }          @Override         public BinaryOperator combiner() {             return combiner;         }          @Override         public Function finisher() {             return finisher;         }          @Override         public Set characteristics() {             return characteristics;         }     }      /**      * 求和方法      * @param mapper      * @param       * @return      */     public static  Collector summingBigDecimal(ToBigDecimalFunction 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         );     }      /**      * 寻求最大值，这里最小值MIN如果某些数据范围超过100亿，作为初始条件判断值，可根据需要替换 Long.MIN_VALUE 或者 Double.MIN_VALUE      * @param mapper      * @param       * @return      */     public static  Collector maxBy(ToBigDecimalFunction 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，这里最大值MAX如果某些数据范围超过100亿，作为初始条件判断值，可根据需要替换 Long.MAX_VALUE 或者 Double.MAX_VALUE      * @param mapper      * @param       * @return      */     public static  Collector minBy(ToBigDecimalFunction 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，返回平均值      * @param mapper      * @param newScale      * @param roundingMode      * @param       * @return      */     public static  Collector averagingBigDecimal(ToBiDecimalFunction 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 {

        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 list = new ArrayList();
        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 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}

java8listBigDecimal求和

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