collections

问题 Is there any difference between passing int and lambda: 0 as arguments? Or between list and lambda: [] ? It looks like they do the same thing: from collections import defaultdict dint1 = defaultdict(lambda: 0) dint2 = defaultdict(int) dlist1 = defaultdict(lambda: []) dlist2 = defaultdict(list) for ch in 'abracadabra': dint1[ch] += 1 dint2[ch] += 1 dlist1[ch].append(1) dlist2[ch].append(1) print dint1.items() print dint2.items() print dlist1.items() print dlist2.items() ## -- Output: -- [('a',

问题 What's the standard way to get a typed, readonly empty list in C#, or is there one? ETA: For those asking "why?": I have a virtual method that returns an IList (or rather, post-answers, an IEnumerable ), and the default implementation is empty. Whatever the list returns should be readonly because writing to it would be a bug, and if somebody tries to, I want to halt and catch fire immediately, rather than wait for the bug to show up in some subtle way later. 回答1: Personally, I think this is

问题 I already know how to do it the hard way and got it working - iterating over entries and swapping "manually". But i wonder if, like so many tasks, this one can be solved in a more elegant way. I have read this post, unfortunately it does not feature elegant solutions. I also have no possibility to use any fancy Guava BiMaps or anything outside the jdk (project stack is already defined). I can assume that my map is bijective, btw :) 回答1: The standard API / Java runtime doesn't offer a bi

问题 I have two containers in a view. The top one has a collection view. I want to update my collection view from a button when a button is hit from the below container. My button is also changing the value of an array, which my collection view uses. I thought didSet would do the job but unfortunately did not work. Top: class TopViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate { @IBOutlet weak var favoritesCV: UICollectionView! var myFavorites = [] { didSet {

问题 I would like to store a group of objects in a hashmap , where the key shall be a composite of two string values. is there a way to achieve this? i can simply concatenate the two strings , but im sure there is a better way to do this. 回答1: You could have a custom object containing the two strings: class StringKey { private String str1; private String str2; } Problem is, you need to determine the equality test and the hash code for two such objects. Equality could be the match on both strings

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 5 years ago . Did they add a runtime List<> and/or Map<> type class to typepad 1.0? And if not, is there a solid library out there someone wrote that provides this functionality? And in the case of List<>, is there a linked list where the elements in the list have the next/prev property? We need a list where from an element

问题 Is there any practical difference between a Set and Collection in Java, besides the fact that a Collection can include the same element twice? They have the same methods. (For example, does Set give me more options to use libraries which accept Set s but not Collection s?) edit: I can think of at least 5 different situations to judge this question. Can anyone else come up with more? I want to make sure I understand the subtleties here. designing a method which accepts an argument of Set or

问题 Consider a class User public class User{ int userId; String name; Date date; } Now I have a List<User> of size 20, how can I find the max date in the list without using manual iterator? 回答1: Since you are asking for lambdas, you can use the following syntax with Java 8: Date maxDate = list.stream().map(u -> u.date).max(Date::compareTo).get(); or, if you have a getter for the date: Date maxDate = list.stream().map(User::getDate).max(Date::compareTo).get(); 回答2: Comparator<User> cmp = new

问题 I'm putting values into the hashmap which is of the form, Map<Long, Double> highLowValueMap=new HashMap<Long, Double>(); highLowValueMap.put(1l, 10.0); highLowValueMap.put(2l, 20.0); I want to create a list by using values() method of map. List<Double> valuesToMatch=new ArrayList<>(); valuesToMatch=(List<Double>) highLowValueMap.values(); or List<Double> valuesToMatch=(List<Double>) highLowValueMap.values(); However, it throws an exception: Exception in thread "main" java.lang

问题 Like just about everyone, I'm still learning the intricacies (and loving them) of the new Java 8 Streams API. I have a question concerning usage of streams. I'll provide a simplified example. Java Streams allows us to take a Collection , and use the stream() method on it to receive a stream of all of its elements. Within it, there are a number of useful methods, such as filter() , map() , and forEach() , which allow us to use lambda operations on the contents. I have code that looks something