collections

问题 Please describe the reason if you know. I Googled it, but didn't find well explained answers. Is it for making index of bucket positive when your hashCode is negative? 回答1: For HashMap , the index in the array that stores the entries of the Map is calculated this way (where h is calculated from the hashCode of the key): static int indexFor(int h, int length) { return h & (length-1); } Where length is the length of the array. This only works when length is a power of 2. If length wasn't power

问题 I am sick of doing blocks of code like this for various bits of code I have: if (dict.ContainsKey[key]) { dict[key] = value; } else { dict.Add(key,value); } and for lookups (i.e. key -> list of value) if (lookup.ContainsKey[key]) { lookup[key].Add(value); } else { lookup.Add(new List<valuetype>); lookup[key].Add(value); } Is there another collections lib or extension method I should use to do this in one line of code no matter what the key and value types are? e.g. dict.AddOrUpdate(key,value)

问题 If I have a collection of an object in Kotlin, is there a quick way to get a collection of a certain property of those objects? I looked at a list of collection operations for Kotlin, but nothing stood out for me (but I may have overlooked something) In python it would be akin to: [person.name for person in persons] And I'd prefer to use a collections function instead of doing: var nameMap = mutableListOf<String>() persons.forEach{person -> nameMap.add(person.name)} I'm pretty lacking in

问题 If I have a collection of an object in Kotlin, is there a quick way to get a collection of a certain property of those objects? I looked at a list of collection operations for Kotlin, but nothing stood out for me (but I may have overlooked something) In python it would be akin to: [person.name for person in persons] And I'd prefer to use a collections function instead of doing: var nameMap = mutableListOf<String>() persons.forEach{person -> nameMap.add(person.name)} I'm pretty lacking in

问题 I want to make a data class which can accept both list and mutable-list and if the list is instance of MutableList then directly make it a property else if it is a List then convert it into a MutableList and then store it. data class SidebarCategory(val title: String, val groups: MutableList<SidebarGroup>) { constructor(title: String, groups: List<SidebarGroup>) : this(title, if (groups is MutableList<SidebarGroup>) groups else groups.toMutableList()) } In the above code Platform declaration

问题 I want to make a data class which can accept both list and mutable-list and if the list is instance of MutableList then directly make it a property else if it is a List then convert it into a MutableList and then store it. data class SidebarCategory(val title: String, val groups: MutableList<SidebarGroup>) { constructor(title: String, groups: List<SidebarGroup>) : this(title, if (groups is MutableList<SidebarGroup>) groups else groups.toMutableList()) } In the above code Platform declaration

问题 I recently found out that you can initialize Lists in Java by calling the Collections.nCopies() method rather than using a for-loop. But that got me wondering, is there a performance advantage/disadvantage in using this method over a for-loop or is it just a simpler way of doing the same thing? 回答1: Since the collection returned by nCopies is immutable, the entries in this collection need not be "materialized". In other words, all that is needed is a space for a single object of type T ;

问题 I have just started implementing data structures in Java and was wondering can we have a situation like this. Map<HashMap<String,String>,String> map = new HashMap<HashMap<String,String>,String>(); And if yes ,, please give a small example. If you don't found question relevant ,, please mention in comments, 回答1: You can do this, but you should not do so in most cases. The key to a map needs to be constant and its equals and hashcode need to be set up to give the correct behavior. If you modify

问题 Is foreach by-definition guaranteed to iterate the subject collection (if it defines order) sequentially from the very first to the very last (unless accidentally interrupted) element? Aren't there any compiler optimization switches which can brake it (shuffle the sequence) or plans to make the ordinary foreach parallel in future versions? 回答1: Foreach is guaranteed to be sequential for sequential collections (that is, the normal hierarchy, or for anything transformed by .seq ). The parallel

问题 The bounty expires in 2 days . Answers to this question are eligible for a +50 reputation bounty. Concurrent Bhai is looking for a more detailed answer to this question. I had a interview today and the person taking my interview puzzled me with his statement asking if it possible that TreeSet equals HashSet but not HashSet equals TreeSet . I said "no" but according to him the answer is "yes". How is it even possible? 回答1: Your interviewer is right, they do not hold equivalence relation for