Java8利用stream对list集合中的bigdecimal进行分组求和，均值，最大值，最小值
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);
}
}
新建实体类Person
@Data
class Person{
private String sex;
private Integer age;
private BigDecimal score;
public Person(String sex, Integer age, BigDecimal score) {
this.sex = sex;
this.age = age;
this.score = score;
}
}
测试用例
List<Person> list = new ArrayList<>();
list.add(new Person("男",18,new BigDecimal(100)));
list.add(new Person("男",19,new BigDecimal(90)));
list.add(new Person("女",20,new BigDecimal(80)));
list.add(new Person("女",20,new BigDecimal(70)));
list.add(new Person("女",20,null));
开始测试
//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));
输出结果为
----按照性别分组求分数总和----
key: 女 , value: 150
key: 男 , value: 190
----按照性别求分数平均值----
key: 女 , value: 75.00
key: 男 , value: 95.00
----按照性别年龄分组求分数总和----
key: 20#女 , value: 150
key: 19#男 , value: 90
key: 18#男 , value: 100
----按照性别年龄分组求分数平均值----
key: 20#女 , value: 75.00
key: 19#男 , value: 90.00
key: 18#男 , value: 100.00
参考自：
https://blog.csdn.net/wohennis1/article/details/103834296
https://mp.weixin.qq.com/s/bd5tuRaF4dCnHHNKBCcAkQ