Java Stream：在列表中聚合所有在map()中收到的非空集合

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我有包含字符串ID的集合idList.
函数getCollection为单个ID返回一个项集合(类型为MyType).此外,它可能会返回null.

所以对于来自idList的许多ID,我会得到一些空值和一些集合.

目标是将一组ID的getCollection的所有回复收集到最终List中.

我想象过像

 List<MyType> reply = idList.stream().map(id -> getCollection(id))
       .filter(p -> p != null).collect(Collectors.toList());

但它似乎不是一个有效的表达.如何使其有效？

此外,这个实现的性能如何？

解决方法:

你需要使用flatMap –

List<MyType> reply = idList.stream()
    .map(id -> getCollection(id))
    .filter(collection -> collection != null)
    .flatMap(Collection::stream)
    .collect(Collectors.toList());

如果您正在考虑地图和flatMap操作之间的差异,那么您可以咨询this excellent answer –

Both map and flatMap can be applied to a Stream and they both
return a Stream. The difference is that the map operation produces
one output value for each input value, whereas the flatMap operation
produces an arbitrary number (zero or more) values for each input
value.

This is reflected in the arguments to each operation.

The map operation takes a Function, which is called for each value in
the input stream and produces one result value, which is sent to the
output stream.

The flatMap operation takes a function that conceptually wants to
consume one value and produce an arbitrary number of values. However,
in Java, it’s cumbersome for a method to return an arbitrary number of
values, since methods can return only zero or one value. One could
imagine an API where the mapper function for flatMap takes a value and
returns an array or a List of values, which are then sent to the
output. Given that this is the streams library, a particularly apt way
to represent an arbitrary number of return values is for the mapper
function itself to return a stream! The values from the stream
returned by the mapper are drained from the stream and are passed to
the output stream. The “clumps” of values returned by each call to the
mapper function are not distinguished at all in the output stream,
thus the output is said to have been “flattened.”

Typical use is for the mapper function of flatMap to return
Stream.empty() if it wants to send zero values, or something like
Stream.of(a, b, c) if it wants to return several values. But of course
any stream can be returned.

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