java-stream

问题 Consider this snippet - String a = "hello" , b = null, c = "guru"; boolean value = Stream .of(a, b, b.substring(2),c) .allMatch(x -> x != null); System.out.println(value); This results in NPE. It seems to be doing b.substring(2) and since b is null , NPE is thrown. Why is this condition evaluated? The second expression b is null and hence evaluates to false . So, allMatch will be false regardless of the truth values of the subsequent operations. In that case, why is it trying to evaluate b

问题 Consider this snippet - String a = "hello" , b = null, c = "guru"; boolean value = Stream .of(a, b, b.substring(2),c) .allMatch(x -> x != null); System.out.println(value); This results in NPE. It seems to be doing b.substring(2) and since b is null , NPE is thrown. Why is this condition evaluated? The second expression b is null and hence evaluates to false . So, allMatch will be false regardless of the truth values of the subsequent operations. In that case, why is it trying to evaluate b

问题 This question already has answers here : How to force max to return ALL maximum values in a Java Stream? (5 answers) Closed 7 months ago . I'm trying to retrieve a list of those with lowest salary from a list of employees. So far I've managed to find the employee with lowest salary, but I want to retrieve multiple if several employees have the same salary. I think the solution is supposed to be in one line. So I can't create a variable with the lowest salary and just check each one for

问题 I wrote some simple code like below. This class works fine without any errors. public class Test { public static void main(String[] args) { List<Integer> intList = IntStream.of(1,2,3,4,5,6,7,8,9,10).boxed().collect(Collectors.toList()); int value = intList.stream().max(Integer::compareTo).get(); //int value = intList.stream().max(<Comparator<? super T> comparator type should pass here>).get(); System.out.println("value :"+value); } } As the code comment shows the max() method should pass an

问题 I love Java 8 streams. They are intuitive, powerful and elegant. But they do have one major drawback IMO: they make debugging much harder (unless you can solve your problem by just debugging lambda expressions, which is answered here). Consider the following two equivalent fragments: int smallElementBitCount = intList.stream() .filter(n -> n < 50) .mapToInt(Integer::bitCount) .sum(); and int smallElementBitCount = 0; for (int n: intList) { if (n < 50) { smallElementBitCount += Integer

问题 So I have a list from which I obtain a parallel stream to fill out a map, as follows: Map<Integer, TreeNode> map = new HashMap<>(); List<NodeData> list = some_filled_list; //Putting data from the list into the map list.parallelStream().forEach(d -> { TreeNode node = new TreeNode(d); map.put(node.getId(), node); }); //print out map map.entrySet().stream().forEach(entry -> { System.out.println("Processing node with ID = " + entry.getValue().getId()); }); The problem with this code is that the

问题 So I have a list from which I obtain a parallel stream to fill out a map, as follows: Map<Integer, TreeNode> map = new HashMap<>(); List<NodeData> list = some_filled_list; //Putting data from the list into the map list.parallelStream().forEach(d -> { TreeNode node = new TreeNode(d); map.put(node.getId(), node); }); //print out map map.entrySet().stream().forEach(entry -> { System.out.println("Processing node with ID = " + entry.getValue().getId()); }); The problem with this code is that the

问题 The default "paralellStream()" in Java 8 uses the common ForkJoinPool which may be a latency problem if the common Pool threads are exhausted when a task is submitted. However in many cases enough CPU power is available and the tasks are short enough so that this is not a problem. If we do have some long running tasks this will of course need some careful consideration, but for this question let's assume that this is not the problem. However filling the ForkJoinPool with I/O tasks that don't

问题 I am new to Java 8. I have a list of custom objects of type A, where A is like below: class A { int id; String name; } I would like to determine if all the objects in that list have same name. I can do it by iterating over the list and capturing previous and current value of names. In that context, I found How to count number of custom objects in list which have same value for one of its attribute. But is there any better way to do the same in java 8 using stream? 回答1: One way is to get the

问题 I have a stream of strings: Stream<String> stream = ...; I want to create a string using stream.collect(Collectors.joining(',', '[', ']')) only I want to return "No Strings" if the stream does not contain any elements. I notice that String java.util.stream.Stream.collect(Collector<? super String, ?, String> collector) method takes an argument of type java.util.stream.Collector For my project I need to this functionality in many places so I would need a class the implements the Collector