java-stream

问题 import java.util.ArrayList; import java.util.List; public class IterationBenchmark { public static void main(String args[]){ List<String> persons = new ArrayList<String>(); persons.add("AAA"); persons.add("BBB"); persons.add("CCC"); persons.add("DDD"); long timeMillis = System.currentTimeMillis(); for(String person : persons) System.out.println(person); System.out.println("Time taken for legacy for loop : "+ (System.currentTimeMillis() - timeMillis)); timeMillis = System.currentTimeMillis();

问题 I have a Stream< Map< K, V > > and I'm trying to merge those maps together, but preserve duplicate values in a list, so the final type would be Map< K, List<V> > . Is there a way to do this? I know the toMap collector has a binary function to basically choose which value is returned, but can it keep track of the converted list? i.e. if a is a Stream< Map< String, Int > > a.flatMap(map -> map.entrySet().stream()).collect( Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (val1, val2) ->

问题 Let's say that I want to remove all the non-letters from my String . String s = "abc-de3-2fg"; I can use an IntStream in order to do that: s.stream().filter(ch -> Character.isLetter(ch)). // But then what? What can I do in order to convert this stream back to a String instance? On a different note, why can't I treat a String as a stream of objects of type Character ? String s = "abc-de3-2fg"; // Yields a Stream of char[], therefore doesn't compile Stream<Character> stream = Stream.of(s

问题 I wanted to filter list of Person class and finally map to some anonymous class in Java using Streams. I am able to do the same thing very easily in C#. Person class class Person { public int Id { get; set; } public string Name { get; set; } public string Address { get; set; } } Code to map the result in desire format. List<Person> lst = new List<Person>(); lst.Add(new Person() { Name = "Pava", Address = "India", Id = 1 }); lst.Add(new Person() { Name = "tiwari", Address = "USA", Id = 2 });

问题 Basically I look for a way to avoid working with entry -> entry.getValue and entry -> entry.getKey similar to what Map.forEach() does. If only I could get a way to work as map.stream().filter((k,v) -> ) ... and so on It seems the interface is called BiConsumer. Perhaps with a converter to BiConsumer or a Stream.generate() someway 回答1: Since this is a repeating question, I will throw a full solution into the ring. It’s a PairStream type that is by default a simple wrapper around an ordinary

问题 For the Collectors.groupingBy() that returns Map<K,List<T>> is it implied that the List<T> is in order that the stream is evaluated? I see no explicit description of the ordering of the list, whereas the concurrent version explicitly states no ordering. If it weren't ordered somehow, I'd expect it to be a Collection though, and I don't see what other ordering it could possibly be, other than order received. I'm hoping it's guaranteed that the last value in each list is the last value received

问题 I have a List of Integer say list1, and I want to get another list list2 which will contain the cumulative sum up until the current index from start. How can I do this using Stream API java 8 ? List<Integer> list1 = new ArrayList<>(); list1.addAll(Arrays.asList(1, 2, 3, 4)); List<Integer> list2 = new ArrayList<>(); // initialization list2.add(list1.get(0)); for(int i=1;i<list1.size();i++) { // increment step list2.add(list2.get(i-1) + list1.get(i)); } How can I change above imperative style

问题 I would like to merge two Map with JAVA 8 Stream: Map<String, List<String>> mapGlobal = new HashMap<String, List<String>>(); Map<String, List<String>> mapAdded = new HashMap<String, List<String>>(); I try to use this implementation: mapGlobal = Stream.of(mapGlobal, mapAdded) .flatMap(m -> m.entrySet().stream()) .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList()) )); However, this implementation only create a result like: Map<String,

问题 This question already has answers here : Limit a stream by a predicate (19 answers) Closed 5 years ago . I am writing a code to calculate Fibonacci numbers. With this code I can generate first n numbers of the Fibonacci sequence. Stream.generate(new Supplier<Long>() { private long n1 = 1; private long n2 = 2; @Override public Long get() { long fibonacci = n1; long n3 = n2 + n1; n1 = n2; n2 = n3; return fibonacci; } }).limit(50).forEach(System.out::println); The method limit returns the Stream

问题 The following fails to compile: @NotNull String defaultFormatter(@Nullable Object value) { if (value instanceof Collection) { return ((Collection) value).stream() .map(MyClass::defaultFormatter) .collect(Collectors.joining(eol)); } return String.valueOf(value); } In particular, when compiled with javac, the error would be: Error:(809, 94) java: incompatible types: java.lang.Object cannot be converted to @org.jetbrains.annotations.NotNull java.lang.String But the following compiles just fine: