java-8

问题 Can we somehow split stream into substreams with no more than N elements in Java? For example Stream<Integer> s = Stream.of(1,2,3,4,5); Stream<Stream<Integer>> separated = split(s, 2); // after that separated should contain stream(1,2), stream(3,4), stream(5) splitting by two streams solution is correct only for 2 streams, the same for N streams will be very ugly and write-only. 回答1: You can't split a Stream into 2 or more Streas s easily and directly. The only way the procedural one

问题 I have a simple snippet as below. I referred this List<Document> list = new LinkedList<Document>(); FindIterable<Document> itr = collection.find(findQuery) .forEach((Document doc) -> list.add(doc)); return list; It compiles without any issues. I guess that we are telling compiler that doc is of type Document . Why is it needed? But If I do the below, it throws ambiguous error. I referred this But couldn't relate and understand exactly. collection.find(findQuery).forEach(list::add); Could

问题 The Java 8 coding style preferred by my colleagues is chaining asynchronous calls all the way through, e.g. CompletionStage<E> someMethod() { return doSomething().thenCompose(a -> { // ... return b; }).thenCompose(b -> { // ... return c; }).thenCompose(c -> { // ... return d; }).thenApply(d -> { // ... return e; }); } I have something like the above, but with an added challenge: I need to recall values retrieved within some of the lambdas, in later lambdas. For example, CompletionStage<E>

问题 Collection class comes with a new method " parallelStream " in Java SDK 8. It is obvious that this new method provides a mechanism to consume collections in parallel. But, I wonder about how Java achieve this parallelism. What is the underlying mechanism? Is it simply a multithreaded execution? Or does fork/join framework (coming with Java SDK 7) step in? If the answer is neither then, how does it work and what are the advantages of it over the other two mechanisms? 回答1: Looking at the stream

问题 Collection class comes with a new method " parallelStream " in Java SDK 8. It is obvious that this new method provides a mechanism to consume collections in parallel. But, I wonder about how Java achieve this parallelism. What is the underlying mechanism? Is it simply a multithreaded execution? Or does fork/join framework (coming with Java SDK 7) step in? If the answer is neither then, how does it work and what are the advantages of it over the other two mechanisms? 回答1: Looking at the stream

问题 This question already has answers here : When I need to use Optional.orElseGet() over Optional.orElse() (3 answers) Closed 2 years ago . The below code prints: should Not have called hello world Why is orElse getting executed even when we have an Optional that contains a value? public static void main(String[] args){ String o = Optional.ofNullable("world").map(str -> "hello" + str).orElse(dontCallMe()); System.out.println(o); } private static String dontCallMe() { System.out.println("should

问题 Why does the below code return Predicate<String> and not boolean ? My understanding is that the !s.isEmpty() check here is going against the Predicate boolean test(T t); The return type here is boolean . So in my lambda should my nonEmptyStringPredicate not be of type boolean ? Obviously, it's not, I'm just trying to understand why it's not. Predicate<String> nonEmptyStringPredicate = (String s) -> !s.isEmpty(); 回答1: A Predicate gets in this case a String as parameter and returns a boolean .

问题 Why does the below code return Predicate<String> and not boolean ? My understanding is that the !s.isEmpty() check here is going against the Predicate boolean test(T t); The return type here is boolean . So in my lambda should my nonEmptyStringPredicate not be of type boolean ? Obviously, it's not, I'm just trying to understand why it's not. Predicate<String> nonEmptyStringPredicate = (String s) -> !s.isEmpty(); 回答1: A Predicate gets in this case a String as parameter and returns a boolean .

问题 I have a Report {String name, Date date, int score } class. I want to be able to sort a list of reports for any member variable using the new java 8 syntax So java 8 provides this new list.sort(Comparator.comparing(report -> report.name)) to sort the list on name. Lets say instead of name I want to provide a variable field name to this method eg. something like list.sort(Comparator.comparing(report -> report.anyField)) where anyField can be name or date or score. How do I achieve this

问题 I have a Report {String name, Date date, int score } class. I want to be able to sort a list of reports for any member variable using the new java 8 syntax So java 8 provides this new list.sort(Comparator.comparing(report -> report.name)) to sort the list on name. Lets say instead of name I want to provide a variable field name to this method eg. something like list.sort(Comparator.comparing(report -> report.anyField)) where anyField can be name or date or score. How do I achieve this