Stream流

对BigDecimal类型进行分组聚合

参考:
https://blog.csdn.net/weixin_44905182/article/details/105792291
https://www.jianshu.com/p/b906355858f3

Java8原生只提供了summingInt、summingLong、summingDouble三种基础类型的方法，想要对BigDecimal类型的数据操作需要自己新建工具类如下：

新建接口ToBigDecimalFunction

@FunctionalInterface
public interface ToBigDecimalFunction<T> {
    BigDecimal applyAsBigDecimal(T value);
}

新建工具类CollectorsUtil

public class CollectorsUtil {
    static final Set<Collector.Characteristics> CH_NOID = Collections.emptySet();
    private CollectorsUtil() {
    }
    @SuppressWarnings("unchecked")
    private static <I, R> Function<I, R> castingIdentity() {
        return i -> (R) i;
    }
    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, castingIdentity(), characteristics);
        }
        @Override
        public BiConsumer<A, T> accumulator() {
            return accumulator;
        }
        @Override
        public Supplier<A> supplier() {
            return supplier;
        }
        @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
    public static <T> Collector<T, ?, BigDecimal> summingBigDecimal(ToBigDecimalFunction<? super T> mapper) {
        return new CollectorImpl<>(
                () -> new BigDecimal[]{BigDecimal.ZERO},
                (a, t) -> { a[0] = a[0].add(mapper.applyAsBigDecimal(t)); },
                (a, b) -> { a[0] = a[0].add(b[0]) ; return a; },
                a -> a[0], CH_NOID);
    }
    //求最大值
    public static <T> Collector<T, ?, BigDecimal> maxBy(ToBigDecimalFunction<? super T> mapper) {
        return new CollectorImpl<>(
                () -> new BigDecimal[]{new BigDecimal(Long.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], CH_NOID);
    }
    //求最小值
    public static <T> Collector<T, ?, BigDecimal> minBy(ToBigDecimalFunction<? super T> mapper) {
        return new CollectorImpl<>(
                () -> new BigDecimal[]{new BigDecimal(Long.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], CH_NOID);
    }
    //求平均值
    public static <T> Collector<T, ?, BigDecimal> averagingBigDecimal(ToBigDecimalFunction<? super T> mapper, int newScale, int roundingMode) {
        return new CollectorImpl<>(
                () -> new BigDecimal[]{BigDecimal.ZERO,BigDecimal.ZERO},
                (a, t) -> { a[0] = a[0].add(mapper.applyAsBigDecimal(t)); a[1] = a[1].add(BigDecimal.ONE); },
                (a, b) -> { a[0] = a[0].add(b[0]) ; return a; },
                a -> a[0].divide(a[1],BigDecimal.ROUND_HALF_UP).setScale(newScale, roundingMode), CH_NOID);
    }
}

使用示例:

//单条件筛选
//按照性别分组求分数总和
Map<String, BigDecimal> scoreCount = list.stream()
        .filter(t -> t.getScore() != null)
        .collect(Collectors.groupingBy(Person::getSex, CollectorsUtil.summingBigDecimal(Person::getScore)));
System.out.println("----按照性别分组求分数总和----");
scoreCount.forEach((k,v) -> System.out.println("key: " + k + " , " + "value: " + v));
//按照性别求分数平均值
Map<String, BigDecimal> scoreAvg = list.stream()
        .filter(t -> t.getScore() != null)
        .collect(Collectors.groupingBy(Person::getSex, CollectorsUtil.averagingBigDecimal(Person::getScore,2)));
System.out.println("----按照性别求分数平均值----");
scoreAvg.forEach((k,v) -> System.out.println("key: " + k + " , " + "value: " + v));
//多条件筛选
//多条件筛选分组属性
private static String fetchGroupKey(Person p) {
    return p.getAge() + "#" + p.getSex();
}
//按照性别年龄分组求分数总和
Map<String, BigDecimal> ageScoreCount = list.stream()
        .filter(t -> t.getScore() != null)
        .collect(Collectors.groupingBy(p -> fetchGroupKey(p), CollectorsUtil.summingBigDecimal(Person::getScore)));
System.out.println("----按照性别年龄分组求分数总和----");
ageScoreCount.forEach((k,v) -> System.out.println("key: " + k + " , " + "value: " + v));
//按照性别年龄分组求分数平均值
Map<String, BigDecimal> ageScoreAvg = list.stream()
        .filter(t -> t.getScore() != null)
        .collect(Collectors.groupingBy(p -> fetchGroupKey(p), CollectorsUtil.averagingBigDecimal(Person::getScore, 2)));
System.out.println("----按照性别年龄分组求分数平均值----");
ageScoreAvg.forEach((k,v) -> System.out.println("key: " + k + " , " + "value: " + v));