Is there a Java Stream method equivalent to Scala's collections "collect"?

Question

Scala's collection provides a method called collect that merges filter and map into a single method. It's particularly useful when filtering an Object collection to produce a subset of that collection containing only a particular type.

Is there any such thing with Java 8's Stream?

Answer 1

Having a combination of filter and map in one step is not that useful in Java given its type system. E.g. there is no such PartialFunction type.

The closest thing is flatMap:

List<Object> list=Arrays.asList(0, "foo", 0L, 42.0, true, "false");
List<String> sList = list.stream()
  .flatMap(o-> o instanceof String? Stream.of((String)o): Stream.empty())
  .collect(Collectors.toList());
System.out.println(sList);

but the expressiveness of

.flatMap(o-> o instanceof String? Stream.of((String)o): Stream.empty())

is not that bigger compared to

.filter(o -> o instanceof String).map(o -> (String)o)

Of course, you may create a helper method encapsulating these steps in a single, undissolvable operation:

static <T> Stream<T> onlyType(Stream<?> s, Class<T> type) {
    return s.filter(type::isInstance).map(type::cast);
}

then you can use it like

List<String> sList=onlyType(list.stream(), String.class).collect(Collectors.toList());