hashmap

collect values of a hashmap into a vector

由 和自甴很熟 提交于 2020-01-04 01:56:07
问题 I am new to Rust and can not find the way of collecting the values of a hashmap into a vector in the documentation. Let's say I have a hashmap: score_table: HashMap<Id, Score> and I want to get all the Score into a Vec all_scores: Vec<Score> I was tempted to use the values but it does not work since values is not a vec: all_scores = score_table.values() I know that Values implement the ExactSizeIterator trait, but I do not know how to collect all values of an iterator into a vector without

Data Structure Behind Amazon S3s Keys (Filtering Data Structure)

由 此生再无相见时 提交于 2020-01-04 01:44:49
问题 I'd like to implement a data structure similar to the lookup functionality of Amazon S3. For context, Amazon S3 stores all files in a flat namespace, but allows you to look up groups of files by common prefixes in their names, therefore replicating the power of a directory tree without the complexity of it. The catch is, both lookup and filter operations are O(1) (or close enough that even on very large buckets - S3's disk equivalents - both operations might as well be O(1))). So in short, I

LinkedHashMap ordering

由 …衆ロ難τιáo~ 提交于 2020-01-04 01:37:09
问题 As specified in the javadoc for LinkedHashMap, insertion order is not affected if a key is re-inserted into the map but While running the below program,I notice that inserting the same key again in changing the access order. Map<Integer, String> map = new LinkedHashMap<Integer,String>(16, .75f, true); map.put(new Integer(1), "Ajay"); map.put(new Integer(2), "Vijay"); map.put(new Integer(3), "Kiran"); map.put(new Integer(4), "Faiz"); for(String value:map.values()){ System.out.println(value); }

ConcurrentHashMap的原理详解

由 假装没事ソ 提交于 2020-01-04 00:30:54
版本JDK 1.7 ConcurrentHashMap的数据结构视图 数据结构图解析 上面就是ConcurrentHashMap的数据结构图，对上面的数据结构进行解析，包含的主要元素： Segment table HashEntry Segment static final class Segment < K , V > extends ReentrantLock implements Serializable { private static final long serialVersionUID = 2249069246763182397 L ; static final int MAX_SCAN_RETRIES = Runtime . getRuntime ( ) . availableProcessors ( ) > 1 ? 64 : 1 ; transient volatile HashEntry < K , V > [ ] table ; transient int count ; transient int modCount ; transient int threshold ; final float loadFactor ; Segment ( float lf , int threshold , HashEntry < K , V > [ ] tab ) {

Java, HashMaps and using Strings as the keys - does the string value get stored twice?

由 眉间皱痕 提交于 2020-01-03 17:22:07
问题 If I have a HashMap that looks like this: HashMap<String, MyObject> where the String key is a field in MyObject , does this string value get stored twice? So when I add entries: _myMap.put(myObj.getName(), myObj); Am I using double the String size in terms of memory? Or does Java do something clever behind the scenes? Thanks 回答1: Java uses the reference, so it is just a pointer to the string that it stores twice. So you don't have to worry if your string is huge, it will still be the same

Go's maps under the hood

由 巧了我就是萌 提交于 2020-01-03 16:59:12
问题 After reading Dave Cheney's blogpost about Go's maps there is still few things unclear to me. TLDR: Why are they unordered? Where actual values are stored in memory? After digging in runtime package I found out that underlying map structure is following: // A header for a Go map. type hmap struct { // Note: the format of the hmap is also encoded in cmd/compile/internal/gc/reflect.go. // Make sure this stays in sync with the compiler's definition. count int // # live cells == size of map. Must

能快速理解Java_集合类_的文章

由 落花浮王杯 提交于 2020-01-03 15:14:05
这篇文章是我学习完Java集合类做的笔记和总结，也是用来记录自己从大一开始的IT生涯，如果你想认真细读这篇文章，请做好受虐的准备(建议电脑看)，因为这篇文章有点长，ヽ(ー_ー)ノ。 如果在看我这篇文章过程中，发现了错误，望指点。 一、什么是集合？ 举个例子：当你有很多书时，你会考虑买一个书柜，将你的书分门别类摆放进入。使用了书柜不仅仅使房间变得整洁，也便于以后使用书时方便查找。在计算机中管理对象亦是如此，当获得多个对象后，也需要一个容器将它们管理起来，这个容器就是集合。 集合本质是基于某种数据结构数据容器。常见的数据结构：数组（Array）、集（Set）、队列（Queue）、链表（Linkedlist）、树（Tree）、堆（Heap）、栈（Stack）和映射（Map）等结构。 下面便 一 . 一 介绍： 其中在两大接口中会有框架图，以方便大家学前、学后在大脑里可以形成一个思维导图，也方便大家检查自己对各各知识点的熟悉程度。 注意： 由于在集合中是有一个参数化类型的，所以在下面的代码里我会 指定成Object 。为什么我要指定为Object呢？因为Java中的Object类是所有类的超类。 先涉及一下泛型的定义：集合类 对象 = new 集合类 (); 也可以先前往 四、泛型，了解U•ェ•*U 二、Collection接口 1、集合类中Collection接口的介绍

How to implement n:m relation in Java?

由 怎甘沉沦 提交于 2020-01-03 10:56:08
问题 I need to implement an n:m relation in Java. The use case is a catalog. a product can be in multiple categories a category can hold multiple products My current solution is to have a mapping class that has two hashmaps. The key of the first hashmap is the product id and the value is a list of category ids The key to the second hashmap is the category id and the value is a list of product ids This is totally redundant an I need a setting class that always takes care that the data is stored

How to implement n:m relation in Java?

由 ╄→尐↘猪︶ㄣ 提交于 2020-01-03 10:56:05
问题 I need to implement an n:m relation in Java. The use case is a catalog. a product can be in multiple categories a category can hold multiple products My current solution is to have a mapping class that has two hashmaps. The key of the first hashmap is the product id and the value is a list of category ids The key to the second hashmap is the category id and the value is a list of product ids This is totally redundant an I need a setting class that always takes care that the data is stored

How HashMap works internally

由 家住魔仙堡 提交于 2020-01-03 06:42:08
问题 HashMap objHashMap = new HashMap(); objHashMap.put("key1", "Value1"); objHashMap.put("key1", "Value2"); System.out.println(objHashMap.get("key1")); Above code displaying "Value2" how and why 回答1: check this /** * Associates the specified value with the specified key in this map. If the * map previously contained a mapping for the key, the old value is * replaced. * * @param key * key with which the specified value is to be associated * @param value * value to be associated with the specified
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