[TOC]
声明
这一系列文章旨在帮助大家理解 Collector 的执行流程，至于实现的是否高效、是否优雅、是否合理等暂且不论。
现在来实现一个和 java.util.stream.Collectors#toList 一样效果的 ToListCollector。
若对 Collector 的各个流程有疑问请移驾此处：http://blog.csdn.net/hylexus/article/details/78941843
简单实现
static class ToListCollector<T> implements Collector<T, List<T>, List<T>> {
@Override
public Supplier<List<T>> supplier() {
return () -> Lists.newArrayList();
}
@Override
public BiConsumer<List<T>, T> accumulator() {
return (list, e) -> list.add(e);
}
@Override
public BinaryOperator<List<T>> combiner() {
return (acc1, acc2) -> {
acc1.addAll(acc2);
return acc1;
};
}
@Override
public Function<List<T>, List<T>> finisher() {
return (acc) -> acc;
}
@Override
public Set<Characteristics> characteristics() {
Set<Characteristics> ret = new HashSet<>();
ret.add(Characteristics.CONCURRENT);
ret.add(Characteristics.IDENTITY_FINISH);
return Collections.unmodifiableSet(ret);
}
}
简化代码
static class ToListCollector2<T> implements Collector<T, List<T>, List<T>> {
@Override
public Supplier<List<T>> supplier() {
return ArrayList::new;
}
@Override
public BiConsumer<List<T>, T> accumulator() {
return List::add;
}
@Override
public BinaryOperator<List<T>> combiner() {
return (acc1, acc2) -> {
acc1.addAll(acc2);
return acc1;
};
}
@Override
public Function<List<T>, List<T>> finisher() {
return Function.identity();
}
@Override
public Set<Characteristics> characteristics() {
return Collections.unmodifiableSet(
EnumSet.of(
Characteristics.CONCURRENT,
Characteristics.IDENTITY_FINISH
)
);
}
}
使用自定义Collector
@Test
public void testListCollector() {
List<Integer> list = IntStream.rangeClosed(1, 5).boxed()
.collect(new ToListCollector<>());
System.out.println(list);
List<Integer> list2 = Stream.iterate(1, i -> i + 1).limit(5)
.collect(new ToListCollector2<>());
System.out.println(list2);
}
