collections

问题 I have the following array and I would like to be able to organize it through collection $collection = Collection($total); $filter = $collection->groupBy('date_general')->toArray(); [2017-11-14] => Array ( [0] => Array ( [status] => pending [date_general] => 2017-11-14 [user_id] => 164 ) [1] => Array ( [status] => pending [date_general] => 2017-11-14 [user_id] => 112 ) ) Up to this point I already have the array organized by dates. Now within each date I need to organize it by user, that is

问题 I like to implement a collection (something like List<T> ) which would hold all my objects that I have created in the entire life span of my application as if its an array of pointers in C++. The idea is that when my process starts I can use a central factory to create all objects and then periodically validate/invalidate their state. Basically I want to make sure that my process only deals with valid instances and I don't re-fetch information I already fetched from the database. So all my

问题 I am well versed with pros and cons of ArrayList and LinkedList. ArrayList is preferred for random access, when additions and removals are less, and vice versa. What if I need a data structure where I need to do both random access, and need to add & remove items from the list often? Which one to choose? 回答1: These data structures are API-compatible, just benchmark/profile your code with both. Another hint: with ArrayList assume you perform N lookups and N mutations. This totals to O(N) + O(N

问题 I would like to split a sequence of size > 2 into alternating sequences like this: def splitAlt(s: Seq[Char]): (Seq[Char], Seq[Char]) = ??? splitAlt(Nil) // raise an exception splitAlt("a") // raise an exception splitAlt("ab") // (Seq('a'), Seq('b')) splitAlt("abc") // (Seq('a', 'c'), Seq('b')) I found an elegant solution with grouped and transpose I'd like to use. Unfortunately it works only i fthe input sequence has even size. How would you modify that solution to work for input of any size

问题 why does Guava doesn't have the following factory call to create a MultiMap from a normal Map? public static <K,V> MultiMap<K,V> invertMap(Map<V,K> map); I have program-names mapped to an integer of how often they were called. I'd like to invert this, so that i can ultimately construct a TreeMap, sorted by call-count, which then are the keys leading to one or multiple program-names. 回答1: How about: public static <K,V> Multimap<K,V> invertMap(Map<V,K> map) { return Multimaps.invertFrom

问题 why does Guava doesn't have the following factory call to create a MultiMap from a normal Map? public static <K,V> MultiMap<K,V> invertMap(Map<V,K> map); I have program-names mapped to an integer of how often they were called. I'd like to invert this, so that i can ultimately construct a TreeMap, sorted by call-count, which then are the keys leading to one or multiple program-names. 回答1: How about: public static <K,V> Multimap<K,V> invertMap(Map<V,K> map) { return Multimaps.invertFrom

问题 what is the most elegant and simple algorithm to map a sequential collection such that contiguous elements that satisfy some predicate are collapsed into another element, and those that do not satisfy the predicate are mapped 1:1 into another element? here is an example: sealed trait A // say the input elements are of this type sealed trait B // say the output elements are of this type case class C(i: Int) extends A // these are the input elements satisfying the predicate case class D(s: C*)

问题 I have a collection of objects and I know that I can sort by NAME (string type) by saying collEquipment.Sort((x, y) => string.Compare(x.ItemName, y.ItemName)); that WORKS. But I want to sort by a ID (integer type) and there is no such thing as Int32.Compare So how do I do this? This doesn't work collEquipment.Sort((x, y) => (x.ID < y.ID)); //error I know the answer is going to be really simple. Lambda expressions confuse me. 回答1: try this collEquipment.Sort((x, y) => y.ID - x.ID); 回答2:

问题 java.util.concurrent provides many thread safe collections like ConcurrentHashMap , ConcurrentSkipListMap , ConcurrentSkipListSet and ConcurrentLinkedQueue . These collections are supposed to minimize contention by allowing concurrent access to different parts of the data structure. Java also has synchronized wrappers to allow concurrent access to non thread safe collections like HashMap and Arraylist . Map<KeyType, ValType> m = Collections.synchronizedMap(new HashMap<KeyType, ValType>()); Do

问题 I'm trying an example in kotlin, like: fun test(){ val harfler = listOf("a","b",'c','d') println(harfler.all { it.javaClass == String::class.java || it.javaClass == Char::class.java }) } List contains Char or String but all function in this expression returns false ,Why return false ? Can anybody explain it? Edit for @JBNizet 回答1: As @JB Nizet has already told you how to analyze the problem. According to the Mapped Types, The Kotlin Char will be mapped to Java Type decide on its declaration.