java-stream

问题 I have JPA entity with list like this: @OneToMany(mappedBy = "scadaElement", orphanRemoval = true) private List<ElementParameter> elementParameters; and map form ElementParameter @ManyToOne @JoinColumn(name = "SCADAELEMENT_ID") ScadaElement scadaElement; when i get entity with elementParameters list and do stream on it stream do nothing, even when I trigger list with .size() but when I do the same with a for loop it work. System.out.println("elements size: " + s.getElementParameters().size())

问题 Does the non-interference requirement for using streams of non-concurrent data structure sources mean that we can't change the state of an element of the data structure during the execution of a stream pipeline (in addition to that we can't change the source data structure itself)? (Question 1) In the section about non-interference, in the stream package description, its said: "For most data sources, preventing interference means ensuring that the data source is not modified at all during the

问题 Stream doesn't have a last() method: Stream<T> stream; T last = stream.last(); // No such method What's the most elegant and/or efficient way to get the last element (or null for an empty Stream)? 回答1: Do a reduction that simply returns the current value: Stream<T> stream; T last = stream.reduce((a, b) -> b).orElse(null); 回答2: This heavily depends on the nature of the Stream . Keep in mind that “simple” doesn’t necessarily mean “efficient”. If you suspect the stream to be very large, carrying

问题 In order to try to deeply understand java streams and spliterators, I have some subtle questions about spliterator characteristics : Q1: Stream.empty() vs Stream.of() (Stream.of() without args) Stream.empty() : SUBSIZED, SIZED Stream.of() : SUBSIZED, IMMUTABLE , SIZED, ORDERED Why Stream.empty() doesn't have the same characteristics of Stream.of() ? Note that it has impacts when using in conjunction with Stream.concat() (specially not having ORDERED ). I would say that Stream.empty() should

问题 In order to try to deeply understand java streams and spliterators, I have some subtle questions about spliterator characteristics : Q1: Stream.empty() vs Stream.of() (Stream.of() without args) Stream.empty() : SUBSIZED, SIZED Stream.of() : SUBSIZED, IMMUTABLE , SIZED, ORDERED Why Stream.empty() doesn't have the same characteristics of Stream.of() ? Note that it has impacts when using in conjunction with Stream.concat() (specially not having ORDERED ). I would say that Stream.empty() should

问题 I'd like to use java.util.Optional.orElseThrow() with an Exception type that asks for a constructor parameter. Something like this: orElseThrow(MyException::new(someArgument)) // obviously NOT working Is there a way to create a Supplier that passes my argument value in? 回答1: Sure. orElseThrow(() -> new MyException(someArgument)) . 回答2: It appears that you can throw only RuntimeException from the method orElseThrow . Otherwise you will get an error message like MyException cannot be converted

问题 I'm trying to generate Order instances using the Stream API. I have a factory function that creates the order, and a DoubleStream is used to initialize the amount of the order. private DoubleStream doubleStream = new Random().doubles(50.0, 200.0); private Order createOrder() { return new Order(doubleStream.findFirst().getAsDouble()); } @Test public void test() { Stream<Order> orderStream = Stream.generate(() -> { return createOrder(); }); orderStream.limit(10).forEach(System.out::println); If

问题 I'm trying to generate Order instances using the Stream API. I have a factory function that creates the order, and a DoubleStream is used to initialize the amount of the order. private DoubleStream doubleStream = new Random().doubles(50.0, 200.0); private Order createOrder() { return new Order(doubleStream.findFirst().getAsDouble()); } @Test public void test() { Stream<Order> orderStream = Stream.generate(() -> { return createOrder(); }); orderStream.limit(10).forEach(System.out::println); If

问题 I would like to create a method which creates a stream of elements which are cartesian products of multiple given streams (aggregated to the same type at the end by a binary operator). Please note that both arguments and results are streams, not collections. For example, for two streams of {A, B} and {X, Y} I would like it produce stream of values {AX, AY, BX, BY} (simple concatenation is used for aggregating the strings). So far, I have came up with this code: private static <T> Stream<T>

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