java-stream

问题 I'm new to Java8 and I can't use streams to map one array into another 2 dimensional array. I have one 2-dimensional array which is a pattern: boolean[][] pattern = { {true, true, false}, {true, false, true}, {false, true, true} }; And second array which contains keys. 0 means: take 0-element from pattern 1 means: take 1-element from pattern and so on int[] keys = {2, 1, 0}; From these 2 arrays I'd like to produce another 2-dimensional array. In this case the result will look like this:

问题 I have a class like this ObjectA id type where id is unique but the type can be duplicate so if I have a list of ObjectA like this (id, type) = (1, "A") , (2, "B"), (3, "C"), (4, "A") then I want Map<type, List<id>> where map is < "A",[1,4], "B",[2], "C",[3] > It is working with Java 7 but can't find a way to do it in Java 8. In Java 8 I can create key, value pair but not able to create a list if a type is same. 回答1: yourTypes.stream() .collect(Collectors.groupingBy( ObjectA::getType,

问题 I have a class like this ObjectA id type where id is unique but the type can be duplicate so if I have a list of ObjectA like this (id, type) = (1, "A") , (2, "B"), (3, "C"), (4, "A") then I want Map<type, List<id>> where map is < "A",[1,4], "B",[2], "C",[3] > It is working with Java 7 but can't find a way to do it in Java 8. In Java 8 I can create key, value pair but not able to create a list if a type is same. 回答1: yourTypes.stream() .collect(Collectors.groupingBy( ObjectA::getType,

问题 I have collection List<Foo> of Foo elements: class Foo { private TypeEnum type; private int amount; //getters, setters ... } Foo type can be TypeEnum.A and TypeEnum.B . I would like to get only those Foo elements from list which if the element have type == TypeEnum.B then amount is greater than zero ( amount > 0 ). How can I do it by Java 8 Streams filter() method? If I use: List<Foo> l = list.stream() .filter(i -> i.getType().equals(TypeEnum.B) && i.getAmount() > 0) .collect(Collectors.<Foo

问题 I am trying to calculate the sum of squares of values in the list. Below are three variations which all calculates the required value. I want to know which one is the most efficient. I am expecting the third one to be more efficient as auto boxing is only done once. // sum of squares int sum = list.stream().map(x -> x * x).reduce((x, y) -> x + y).get(); System.out.println("sum of squares: " + sum); sum = list.stream().mapToInt(x -> x * x).sum(); System.out.println("sum of squares: " + sum);

问题 I am trying to calculate the sum of squares of values in the list. Below are three variations which all calculates the required value. I want to know which one is the most efficient. I am expecting the third one to be more efficient as auto boxing is only done once. // sum of squares int sum = list.stream().map(x -> x * x).reduce((x, y) -> x + y).get(); System.out.println("sum of squares: " + sum); sum = list.stream().mapToInt(x -> x * x).sum(); System.out.println("sum of squares: " + sum);

问题 This question already has answers here : Variable used in lambda expression should be final or effectively final (6 answers) Lambdas: local variables need final, instance variables don't (10 answers) Closed 7 months ago . I'm studying java 8 streams and the only problem I have is understanding lambdas is why the effectively final warning in lambdas is ignored for instance(and static) variables. I can't seem to find any reference to it online, as most pages will just talk about the definition

问题 I'm new to Java 8, not sure how to use streams and it's methods to sort. If I have map as below, how to sort this map by value to take only top 10 entries using Java 8. HashMap<String, Integer> map = new HashMap<String, Integer>(); map.put("a", 10); map.put("b", 30); map.put("c", 50); map.put("d", 40); map.put("e", 100); map.put("f", 60); map.put("g", 110); map.put("h", 50); map.put("i", 90); map.put("k", 70); map.put("L", 80); I know before Java 8, we can sort as this link: https:/

问题 I'm new to Java 8, not sure how to use streams and it's methods to sort. If I have map as below, how to sort this map by value to take only top 10 entries using Java 8. HashMap<String, Integer> map = new HashMap<String, Integer>(); map.put("a", 10); map.put("b", 30); map.put("c", 50); map.put("d", 40); map.put("e", 100); map.put("f", 60); map.put("g", 110); map.put("h", 50); map.put("i", 90); map.put("k", 70); map.put("L", 80); I know before Java 8, we can sort as this link: https:/

问题 I'm new to Java 8, not sure how to use streams and it's methods to sort. If I have map as below, how to sort this map by value to take only top 10 entries using Java 8. HashMap<String, Integer> map = new HashMap<String, Integer>(); map.put("a", 10); map.put("b", 30); map.put("c", 50); map.put("d", 40); map.put("e", 100); map.put("f", 60); map.put("g", 110); map.put("h", 50); map.put("i", 90); map.put("k", 70); map.put("L", 80); I know before Java 8, we can sort as this link: https:/