java-stream

问题 int [] numbers = {1,2,3,4,5,6,7,8}; int [] doubleNumbers = new int[numbers.length]; int [] tripleNumbers = new int[numbers.length]; for(int index = 0; index < numbers.length; index++) { doubleNumbers[index] = numbers[index] * 2; tripleNumbers[index] = numbers[index] * 3; } System.out.println("Double Numbers"); Arrays.stream(doubleNumbers).forEach(System.out::println); System.out.println("Triple Numbers"); Arrays.stream(tripleNumbers).forEach(System.out::println); I have above code where I

问题 There is a list of Person objects. List<Person> persons = generatePersons(); An unmodifiableList is created with it. List<Person> unmodifiableList = Collections.unmodifiableList(persons); I understand that unmodifiableList doesn't support add/remove/set operations. At the same time it is not immutable since it has a reference to an existing modifiable list persons and whenever changes are made to the persons list, the changes are reflected in unmodifiableList too. An immutable list is created

问题 I have a List<SingleDay> where SingleDay is: class SingleDay{ private Date date; private String County; // otherstuff } I would like to convert this into a Map<LocalDate, Map<String,SingleDay>> by the following transformation: Key: 2020-05-27, Value: {'County1'=SingleDay [date=2020-05-27, County='County1'], 'County2'=SingleDay [date=2020-05-27, County='County2'], 'County3'=SingleDay [date=2020-05-27, County='County3'] } Key: 2020-05-28, Value: {'County4'=SingleDay [date=2020-05-28, County=

问题 I am learning many new features of lambda and wondering how can I group by my custom object list based on certain properties as key? For example, I have list of object like this in json. [{ "account" : "checking", "source" : "BOA" }, { "account" : "checking", "source" : "TD" }, { "account" : "saving", "source" : "WS" } ] I am looking for way to group using java 8 feature to get output like this (grouping source as comma separated for same account. [{ "account" : "checking", "source" : "BOA,

问题 This question already has answers here : Concatenating two int[] (3 answers) Combine two integer arrays [duplicate] (9 answers) Closed last month . This page shows how to combine two arrays of Integer objects into an array of Object objects. Integer[] firstArray = new Integer[] { 10 , 20 , 30 , 40 }; Integer[] secondArray = new Integer[] { 50 , 60 , 70 , 80 }; Object[] merged = Stream .of( firstArray , secondArray ) .flatMap( Stream :: of ) .toArray() ; Arrays.toString( merged ): [10, 20, 30,

问题 This question already has answers here : Concatenating two int[] (3 answers) Combine two integer arrays [duplicate] (9 answers) Closed last month . This page shows how to combine two arrays of Integer objects into an array of Object objects. Integer[] firstArray = new Integer[] { 10 , 20 , 30 , 40 }; Integer[] secondArray = new Integer[] { 50 , 60 , 70 , 80 }; Object[] merged = Stream .of( firstArray , secondArray ) .flatMap( Stream :: of ) .toArray() ; Arrays.toString( merged ): [10, 20, 30,

问题 I have an assignment where we're reading textfiles and counting the occurrences of each word (ignoring punctuation). We don't have to use streams but I want to practice using them. So far I am able to read a text file and put each line in a string, and all the strings in a list using this: try (Stream<String> p = Files.lines(FOLDER_OF_TEXT_FILES)) { list = p.map(line -> line.replaceAll("[^A-Za-z0-9 ]", "")) .collect(Collectors.toList()); } However, so far, it simply makes all the lines a

问题 This question already has answers here : Java 8 Streams : get non repeated counts (5 answers) Closed 2 months ago . I need to get only unique values from the collection( values which do not have any dublicates in collection). For example, this code: ArrayList<Integer> g = new ArrayList<>(Arrays.asList(1,1,2,2,3,4,5,5,5,6,6)); System.out.println(Arrays.toString(g.stream().mapToInt(Integer::intValue).distinct().toArray())); gives me this result: [1, 2, 3, 4, 5, 6] However I want the result: [3,

问题 This question already has answers here : Java 8 Streams : get non repeated counts (5 answers) Closed 2 months ago . I need to get only unique values from the collection( values which do not have any dublicates in collection). For example, this code: ArrayList<Integer> g = new ArrayList<>(Arrays.asList(1,1,2,2,3,4,5,5,5,6,6)); System.out.println(Arrays.toString(g.stream().mapToInt(Integer::intValue).distinct().toArray())); gives me this result: [1, 2, 3, 4, 5, 6] However I want the result: [3,

问题 Consider Below 2 examples. 1 With Streams myList.stream().map(this::getInt).max(Integer::compareTo); 2 Old way int max = Integer.MIN_VALUE; for (MyItem item : myList) { max = Math.max(max, getInt(item)); } Above getInt method accepts a MyItem argument and returns an int result. Here, #2 gives me a much lower latency compared to #1. Does anyone have an idea why or anything going wrong for me? 回答1: myList.stream().mapToInt(this::getInt).max() Try mapping to an IntStream. An IntStream works with