java-stream

问题 Assume we have a list as follows. CoreResult has a field of type List<Double> . final List<CoreResult> list = new LinkedList<>(SOME_DATA); The objective is to flatten the list after extracting that specific field from each CoreResult object. Here are 3 possible options. Is any one of them preferable to the others? Option 1 : extract a field via map() and flatten inside collector final List<Double> A = list.stream().map(CoreResult::getField) .collect(ArrayList::new, ArrayList::addAll,

问题 In a unit test, I want to verify that two lists contain the same elements. The list to test is build of a list of Person objects, where one field of type String is extracted. The other list contains String literals. One often finds the following code snippet to accomplish this task (see this answer): List<Person> people = getPeopleFromDatabasePseudoMethod(); List<String> expectedValues = Arrays.asList("john", "joe", "bill"); assertTrue(people.stream().map(person -> person.getName()).collect

问题 In a unit test, I want to verify that two lists contain the same elements. The list to test is build of a list of Person objects, where one field of type String is extracted. The other list contains String literals. One often finds the following code snippet to accomplish this task (see this answer): List<Person> people = getPeopleFromDatabasePseudoMethod(); List<String> expectedValues = Arrays.asList("john", "joe", "bill"); assertTrue(people.stream().map(person -> person.getName()).collect

问题 To better understand the new stream API I'm trying to convert some old code, but I'm stuck on this one. public Collection<? extends File> asDestSet() { HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>(); //... Set<File> result = new HashSet<File>(); for (Set<File> v : map.values()) { result.addAll(v); } return result; } I can't seem to create a valid Collector for it: public Collection<? extends File> asDestSet() { HashMap<IFileSourceInfo, Set<File>> map = new

问题 To better understand the new stream API I'm trying to convert some old code, but I'm stuck on this one. public Collection<? extends File> asDestSet() { HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>(); //... Set<File> result = new HashSet<File>(); for (Set<File> v : map.values()) { result.addAll(v); } return result; } I can't seem to create a valid Collector for it: public Collection<? extends File> asDestSet() { HashMap<IFileSourceInfo, Set<File>> map = new

问题 Having the next original map: G1=[7,8,45,6,9] G2=[3,9,34,2,1,65] G3=[6,5,9,1,67,5] Where G1, G2 and G3 are groups of people's ages, How can I create a new map like this: 45=[7,8,45,6,9] 65=[3,9,34,2,1,65] 67=[6,5,9,1,67,5] Where the new keys are the max people's age in each group. I have tried this: Map<Integer, List<Integer>> newMap = originalMap.entrySet().stream() .collect(Collectors.toMap(Collections.max(x -> x.getValue()), x -> x.getValue())); But the compiler say me: "The target type of

问题 EDITED on September, 2015 When I initially asked this question on February, 2015, the behaviour reported in the linked question was counter-intuitive, though kind of allowed by the specification (despite some little inconsistencies in the docs). However, Tagir Valeev asked a new question on June, 2015, where I think he clearly demonstrated that the behaviour reported in this question was actually a bug . Brain Goetz answered his question, and admitted that it was a bug to not stop the back

问题 With stream API I could easily check whether all elements satify a given condition, using allMatch(e -> predicate(e)) method. I could also check if any of multiple conditions is satified allMatch(e -> predicateA(e) || predicateB(e) || predicateC(e)) . But is it possible to check if all elements satisfy one of those predicates (either one)? In the previous case it is possible that some elements satisfy A and some of them not, but they satisfy B or C (and vice versa). I could perform allMatch

问题 I have two arrays of objects. I want to update one array with updated objects from the second array if the object matches a certain criteria. For example, I have this: public class Foobar { private String name; // Other methods here... public String getName() { return this.name; } } Foobar [] original = new Foobar[8]; // Instantiate them here and set their field values Foobar [] updated = new Foobar[8]; // Instantiate them here and set their field values /* Use Java8 stream operation here * -

问题 I have two arrays of objects. I want to update one array with updated objects from the second array if the object matches a certain criteria. For example, I have this: public class Foobar { private String name; // Other methods here... public String getName() { return this.name; } } Foobar [] original = new Foobar[8]; // Instantiate them here and set their field values Foobar [] updated = new Foobar[8]; // Instantiate them here and set their field values /* Use Java8 stream operation here * -