java-stream

问题 This question already has answers here : Zipping streams using JDK8 with lambda (java.util.stream.Streams.zip) (14 answers) Closed 5 years ago . I'd like to iterate two Java-8-Streams together, so that I have in each iteration-step two arguments. Something like that, where somefunction produces something like Stream<Pair<A,B>> . Stream<A> as; Stream<B> bs; somefunction (as, bs) .forEach ((a, b) -> foo (a, b)); // or something like somefunction (as, bs) .forEach ((Pair<A, B> abs) -> foo (abs

问题 My colleague and I had a bug that was due to our assumption that an empty stream calling allMatch() would return false . if (myItems.allMatch(i -> i.isValid()) { //do something } Of course, it is kind of our fault for assuming and not reading documentation. But what I don't understand is why the default allMatch() behavior for an empty stream returns true . What was the reasoning for this? Like the anyMatch() (which contrarily returns false), this operation is used in an imperative way that

问题 I've already read this and this questions, but still doubt whether the observed behavior of Stream.skip was intended by JDK authors. Let's have simple input of numbers 1..20: List<Integer> input = IntStream.rangeClosed(1, 20).boxed().collect(Collectors.toList()); Now let's create a parallel stream, combine the unordered() with skip() in different ways and collect the result: System.out.println("skip-skip-unordered-toList: " + input.parallelStream().filter(x -> x > 0) .skip(1) .skip(1)

问题 I just started learning Java streams and faced a problem. Please take a look at a the following example. This is part of a Node class: private Map<String, Node> nodes; public Optional<Node> child(String name) { return Optional.<Node>ofNullable(nodes.get(name)); } private void findChildren(String name, List<Node> result) { child(name).ifPresent(result::add); nodes.values().stream() // .map(Node::findChildren(name, result)) // .forEach(Node::findChildren(name, result)) .forEach(node -> node

问题 I have a data set represented by a Java 8 stream: Stream<T> stream = ...; I can see how to filter it to get a random subset - for example Random r = new Random(); PrimitiveIterator.OfInt coin = r.ints(0, 2).iterator(); Stream<T> heads = stream.filter((x) -> (coin.nextInt() == 0)); I can also see how I could reduce this stream to get, for example, two lists representing two random halves of the data set, and then turn those back into streams. But, is there a direct way to generate two streams

问题 I have managed to write a solution using Java 8 Streams API that first groups a list of object Route by its value and then counts the number of objects in each group. It returns a mapping Route -> Long. Here is the code: Map<Route, Long> routesCounted = routes.stream() .collect(Collectors.groupingBy(gr -> gr, Collectors.counting())); And the Route class: public class Route implements Comparable<Route> { private long lastUpdated; private Cell startCell; private Cell endCell; private int

问题 Why does this throw a java.lang.NullPointerException ? List<String> strings = new ArrayList<>(); strings.add(null); strings.add("test"); String firstString = strings.stream() .findFirst() // Exception thrown here .orElse("StringWhenListIsEmpty"); //.orElse(null); // Changing the `orElse()` to avoid ambiguity The first item in strings is null , which is a perfectly acceptable value. Furthermore, findFirst() returns an Optional, which makes even more sense for findFirst() to be able to handle

问题 I am creating a Map from a List as follows: List<String> strings = Arrays.asList("a", "bb", "ccc"); Map<String, Integer> map = strings.stream() .collect(Collectors.toMap(Function.identity(), String::length)); I want to keep the same iteration order as was in the List . How can I create a LinkedHashMap using the Collectors.toMap() methods? 回答1: The 2-parameter version of Collectors.toMap() uses a HashMap : public static <T, K, U> Collector<T, ?, Map<K,U>> toMap( Function<? super T, ? extends K

问题 I understand that these methods differ the order of execution but in all my test I cannot achieve different order execution. Example: System.out.println("forEach Demo"); Stream.of("AAA","BBB","CCC").forEach(s->System.out.println("Output:"+s)); System.out.println("forEachOrdered Demo"); Stream.of("AAA","BBB","CCC").forEachOrdered(s->System.out.println("Output:"+s)); Output: forEach Demo Output:AAA Output:BBB Output:CCC forEachOrdered Demo Output:AAA Output:BBB Output:CCC Please provide

问题 General question: What's the proper way to reverse a stream? Assuming that we don't know what type of elements that stream consists of, what's the generic way to reverse any stream? Specific question: IntStream provides range method to generate Integers in specific range IntStream.range(-range, 0) , now that I want to reverse it switching range from 0 to negative won't work, also I can't use Integer::compare List<Integer> list = Arrays.asList(1,2,3,4); list.stream().sorted(Integer::compare)