自定义工具类
public class MyCollectors {
private MyCollectors() {
}
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.applyAsInt(t)),
(a, b) -> {
a[0] = a[0].add(b[0]);
return a;
},
a -> a[0],
Collections.emptySet()
);
}
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;
}
}
@SuppressWarnings("unchecked")
private static <I, R> Function<I, R> castingIdentity() {
return i -> (R) i;
}
}
自定义函数式接口
@FunctionalInterface
public interface ToBigDecimalFunction<T> {
BigDecimal applyAsInt(T value);
}
测试入口
public class AnswerApp {
public static void main(String[] args) {
List<BigDecimal> list = Lists.newArrayList();
for (int i = 0; i < 24; i++) {
list.add(BigDecimal.valueOf(i + 10.2121543));
}
BigDecimal sum = list.stream().collect(MyCollectors.summingBigDecimal(e -> e));
System.out.println(sum.doubleValue());
Optional<BigDecimal> reduce = list.stream().reduce(BigDecimal::add);
System.out.println(reduce.orElse(BigDecimal.valueOf(0)));
}
}
参考地址