Recently I wrote a small library called StreamEx which extends Java streams and provides this exact method among many other features: StreamEx.of(-2,1,2,-5) .filter(n -> n > 0) .map(n -> n * n) .toList(); Also toSet(), toCollection(Supplier), joining(), groupingBy() and other shortcut methods are available there.
The JDK’s standard implementation of Stream is the internal class java.util.stream.ReferencePipeline, you cannot instantiate it directly. Instead you can use java.util.stream.Stream.builder(), java.util.stream.StreamSupport.stream(Spliterator<T>, boolean) and various1, 2 other static factory methods to create an instance of the default implementation. Using a spliterator is probably the most powerful approach as it allows you to provide objects lazily … Read more
You can use two flatMap then a filter then you can pick the first one or if no result return null : C c1 = listOfAObjects.stream() .flatMap(a -> a.getList().stream()) .flatMap(b -> b.getPr().stream()) .filter(c -> c.getName().equalsIgnoreCase(name)) .findFirst() .orElse(null);
static void assertStreamEquals(Stream<?> s1, Stream<?> s2) { Iterator<?> iter1 = s1.iterator(), iter2 = s2.iterator(); while(iter1.hasNext() && iter2.hasNext()) assertEquals(iter1.next(), iter2.next()); assert !iter1.hasNext() && !iter2.hasNext(); }
@Test public void test14() throws Exception { Map<String, Integer> m1 = ImmutableMap.of(“a”, 2, “b”, 3); Map<String, Integer> m2 = ImmutableMap.of(“a”, 3, “c”, 4); Map<String, Integer> mx = Stream.of(m1, m2) .map(Map::entrySet) // converts each map into an entry set .flatMap(Collection::stream) // converts each set into an entry stream, then // “concatenates” it in place of the … Read more
You say you want to sort by value, but you don’t have that in your code. Pass a lambda (or method reference) to sorted to tell it how you want to sort. And you want to get the keys; use map to transform entries to keys. List<Type> types = countByType.entrySet().stream() .sorted(Comparator.comparing(Map.Entry::getValue)) .map(Map.Entry::getKey) .collect(Collectors.toList());
You can use an IntStream to generate the indices followed by mapToObj: char[] arr = {‘a’,’c’,’e’}; Stream<Character> cStream = IntStream.range(0, arr.length).mapToObj(i -> arr[i]);
What I understood is that both will return the first matched element from the stream, for example, when used in conjunction with filter? That’s not true. According to the javadoc, Stream#findAny(): Returns an Optional<T> describing some element of the stream, or an empty Optional<T> if the stream is empty. The behavior of this operation is … Read more
I don’t think there is a way to do that out-of-the-box. A possibly cleaner solution would be: Stream.of(objects) .filter(c -> c instanceof Client) .map(c -> (Client) c) .map(Client::getID) .forEach(System.out::println); or, as suggested in the comments, you could use the cast method – the former may be easier to read though: Stream.of(objects) .filter(Client.class::isInstance) .map(Client.class::cast) .map(Client::getID) .forEach(System.out::println);
When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns … Read more