是否存在用于将超类映射到子类的特定聚合操作？

Question

Is there a specific aggregate operation to map superclasses to subclasses?

I frequently find myself doing operations such as:

list
.stream()
.map((A element) -> (SubClassA) element);

Is there an operation that does the equivalent of the map function above, that is, takes a Stream of values and casts them to a subclass?

Answer 1

The easiest I can think of is:

Stream<X> stream1 = ...
Stream<SubClassA> stream2 = stream1.map(SubClassA.class::cast);

Answer 2

Use generic wirldcard.

List<A> list = new ArrayList<>();

Stream<A> streamA1 = list.stream();
Stream<B> streamB1 = (Stream<B>)streamA1;   // ERROR! Cannot cast from Stream<A> to Stream<B>

Stream<? extends A> streamA2 = list.stream();
Stream<B> streamB2 = (Stream<B>)streamA2;   // OK