java-8

问题 The application is basically a Servlet on top of Apaches Tomcat 8 and crashes every 3 to n days. The most interesting part is, that the last 3 crashes happened at 1.30 PM + 3 to 4 seconds. Every other crash happens at 15, 30, 45 or the full hour. The same application worked for months on Windows 7 with Java 7 something. I found this report so I checked the available space at /tmp and I have plenty of space here. However, the next time I ran the VM with: -XX:-UsePerfData -XX:

问题 I want to change all items in list . What is the correct way to do it with java8 ? public class TestIt { public static void main(String[] args) { ArrayList<String> l = new ArrayList<>(); l.add("AB"); l.add("A"); l.add("AA"); l.forEach(x -> x = "b" + x); System.out.println(l); } } 回答1: You can use replaceAll. Replaces each element of this list with the result of applying the operator to that element. ArrayList<String> l = new ArrayList<>(Arrays.asList("AB","A","AA")); l.replaceAll(x -> "b" + x

问题 I want to change all items in list . What is the correct way to do it with java8 ? public class TestIt { public static void main(String[] args) { ArrayList<String> l = new ArrayList<>(); l.add("AB"); l.add("A"); l.add("AA"); l.forEach(x -> x = "b" + x); System.out.println(l); } } 回答1: You can use replaceAll. Replaces each element of this list with the result of applying the operator to that element. ArrayList<String> l = new ArrayList<>(Arrays.asList("AB","A","AA")); l.replaceAll(x -> "b" + x

问题 This question already has answers here : In Java8 functional style, how can i map the values to already existing key value pair (4 answers) Closed 4 years ago . I want to use Map<String, List<String>> to record something, e.g. each city have how many users of some kind . Now my code is Map<String, List<String>> map = new HashMap<>(); if(map.get("city_1")==null){ map.put("city_1", new ArrayList<>()); } map.get("city_1").add("aaa"); but I feel it's a little cumbersome, I want this effect Map

问题 If I have a hierarchical structure that is still sorted, but flattened - how can I create a parent/child structure with Java Streams API? An example: How do I go from -,A,Foo A,A1,Alpha1 A,A2,Alpha2 -,B,Bar B,B1,Bravo1 B,B2,Bravo2 to - A A1,Alpha1 A2,Alpha2 B B1,Bravo1 B2,Bravo2 A simple non-stream way would be to keep track of the parent column and see if it has changed. I have tried various ways with Collectors and groupingBy, but have not yet found how to do. List<Row> list = new ArrayList

问题 I was going through a tutorial of Optional class here - https://www.geeksforgeeks.org/java-8-optional-class/ which has the following String[] words = new String[10]; Optional<String> checkNull = Optional.ofNullable(words[5]); if (checkNull.isPresent()) { String word = words[5].toLowerCase(); System.out.print(word); } else{ System.out.println("word is null"); } I am trying to make it of less lines using ifPresent check of Optional as Optional.ofNullable(words[5]).ifPresent(a -> System.out

问题 The question is about java.util.stream.Stream.reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner) method. From the API docs: accumulator – an associative , non-interfering, stateless function for incorporating an additional element into a result Now, the definition of associative is: (a op b) op c = a op (b op c) for all a, b, c As we can see, the above definition requires a BinaryOperator, not only a BiFunction, because a op b requires b of type T while b

问题 I have three class i.e. Engine , Wheel and AutoMobile . The contents of these classes are as follows:- class Engine { String modelId; } class Wheel { int numSpokes; } class AutoMobile { String make; String id; } I have a List<Engine> , a List<Wheel> and a List<Automobile> which I have to iterate through and check for a particular condition. If there is one entity that satisfies this condition, I have to return true; otherwise the function returns false. The function is as follows: Boolean

问题 The api for Stream.max requires an argument of type Comparator<? super T> , and for Comparator , the only abstract method is int compare(T o1, T o2) but Double::compareTo , the compareTo api is public int compareTo(Double anotherDouble) why just provide one argument, so why can Double::compareTo use as argument of Stream Optional<T> max(Comparator<? super T> comparator) 回答1: The below MyComparator implement a Comparator . It takes two arguments. It's the same as the lambda expression (d1,d2)

问题 I recently started experimenting with both Spark and Java. I initially went through the famous WordCount example using RDD and everything went as expected. Now I am trying to implement my own example but using DataFrames and not RDDs. So I am reading a dataset from a file with DataFrame df = sqlContext.read() .format("com.databricks.spark.csv") .option("inferSchema", "true") .option("delimiter", ";") .option("header", "true") .load(inputFilePath); and then I try to select a specific column