我只需要回溯实现
flatMapping
。它仅需要2个方法和1个类,没有其他依赖项。
此外,当升级到Java 9时,您可以将其版本标记为弃用,并使用正确的版本替换任何使用它的地方。
下面的代码来自JDK。我没有编写它。我已经使用您的示例测试过它并返回了相同的结果。
class Nikollectors
{
public static <T, U, A, R> Collector<T, ?, R> flatMapping(Function<? super T, ? extends Stream<? extends U>> mapper, Collector<? super U, A, R> downstream) {
BiConsumer<A, ? super U> downstreamAccumulator = downstream.accumulator();
return new CollectorImpl<>(downstream.supplier(),
(r, t) -> {
try (Stream<? extends U> result = mapper.apply(t)) {
if (result != null)
result.sequential().forEach(u -> downstreamAccumulator.accept(r, u));
}
},
downstream.combiner(), downstream.finisher(),
downstream.characteristics());
}
private 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;
}
}
private static <I, R> Function<I, R> castingIdentity() {
return i -> (R) i;
}
}
使用示例:
Map<Integer, List<Integer>> map =list.stream()
.collect(Collectors.groupingBy(
Collection::size,
Nikollectors.flatMapping( // <<<
l -> l.stream().filter(i -> i % 2 == 0),
Collectors.toList()
)
)
);