hashmap | 易学教程

How to create a hashmap for a specific object

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问题 EDIT: @Oscar Lopez I have added the code you specified, and now have the following: import java.util.HashMap; import java.util.Map; public class Character{ public String name; private HashMap<String, String> stats; public Character(String charName){ name = charName; stats.put("Strength", "5"); stats.put("Dexterity", "5"); stats.put("Constitution", "5"); stats.put("Intelligence", "5"); stats.put("Strength", "5"); stats.put("Wisdom", "5"); } public String getStat(String name) { return stats.get

Spark dataframe to nested map

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问题 How can I convert a rather small data frame in spark (max 300 MB) to a nested map in order to improve spark's DAG. I believe this operation will be quicker than a join later on (Spark dynamic DAG is a lot slower and different from hard coded DAG) as the transformed values were created during the train step of a custom estimator. Now I just want to apply them really quick during predict step of the pipeline. val inputSmall = Seq( ("A", 0.3, "B", 0.25), ("A", 0.3, "g", 0.4), ("d", 0.0, "f", 0.1

How can I use a custom type for keys in a boost::unordered_map?

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问题 I'm using Boost's implementation of a hash map in a project right now, and I'm trying to implement a custom type for keys. I have four unsigned integers which I'd like to combine into a single 128-bit datatype to use as a key. I've created a struct with a 32-bit integer array of four elements, which serves as my storage. To be honest, I'm not sure how Boost's hash map works, so I'm not sure what I'm doing here, but I followed the Boost documentation (http://www.boost.org/doc/libs/1_37_0/doc

hashmap keyset automatically sorted

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问题 HashMap<String, String[]> content = new HashMap<String, String[]>(); content.put("Help", new String[]{"About"}); content.put("View", new String[]{"Grid"}); content.put("File", new String[]{"Import", "Export"}); for(String key : content.keySet()){ System.out.println(key); } The above code logs: View Help File But why is this sorted? 回答1: HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added. LinkedHashMap will

《吊打面试官》系列-HashMap

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你知道的越多，你不知道的越多点赞再看，养成习惯本文 GitHub https://github.com/JavaFamily 上已经收录，有一线大厂面试点思维导图，也整理了很多我的文档，欢迎Star和完善，大家面试可以参照考点复习，希望我们一起有点东西。前言作为一个在互联网公司面一次拿一次Offer的面霸，打败了无数竞争对手，每次都只能看到无数落寞的身影失望的离开，略感愧疚（请允许我使用一下夸张的修辞手法）。于是在一个寂寞难耐的夜晚，我痛定思痛，决定开始写互联网技术栈面试相关的文章，希望能帮助各位读者以后面试势如破竹，对面试官进行360°的反击，吊打问你的面试官，让一同面试的同僚瞠目结舌，疯狂收割大厂Offer！所有文章的名字只是我的噱头，我们应该有一颗谦逊的心，所以希望大家怀着空杯心态好好学，一起进步。正文一个婀娜多姿，穿着衬衣的小姐姐，拿着一个精致的小笔记本，径直走过来坐在我的面前。看着眼前这个美丽的女人，心想这不会就是Java基础系列的面试官吧，真香。不过看样子这么年轻应该问不出什么深度的吧，嘻嘻。（哦？是么😏）小伙子，听前面的面试官说了，你Redis和消息队列都回答得不错，看来还是有点东西。美丽迷人的面试官您好，您见笑了，全靠看了敖丙的《吊打面试官》系列，不然我还真的回答不上很多原本的知识盲区，他真的有点东西。面试官心想：哦，吊打面试官是么

Java thread-safe write-only hashmap

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问题 In my Java class I include a Hashmap variable (class property) and run some Threads which write-only into that HashMap using put() : each time the write happens it stores a unique key (which is done by design). Is the synchronized keyword on a class method write-only sufficient for thead-safe conditions? My HashMap is simple and not a ConcurrentHashMap ? 回答1: No, it is not sufficient to only synchronize the writes. Synchronization must be applied to both reads and writes to memory. Some other

《吊打面试官》系列-HashMap

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Strange Java HashMap behavior - can't find matching object

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问题 I've been encountering some strange behavior when trying to find a key inside a java.util.HashMap , and I guess I'm missing something. The code segment is basically: HashMap<Key, Value> data = ... Key k1 = ... Value v = data.get(k1); boolean bool1 = data.containsKey(k1); for (Key k2 : data.keySet()) { boolean bool2 = k1.equals(k2); boolean bool3 = k2.equals(k1); boolean bool4 = k1.hashCode() == k2.hashCode(); break; } That strange for loop is there because for a specific execution I happen to

ModCount in map and list

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问题 While Debugging the collections in eclispse I just Inspect that there is thing called modCount for example if we debug list we will see while inspecting in debugging what this modCount represents..!!please advise 回答1: See the javadoc The number of times this list has been structurally modified . Structural modifications are those that change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results. This field is used by the

hashcode的理解

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============================================================ 如何理解hashCode的作用: ============================================================ 以java.lang.Object来理解,JVM每new一个Object,它都会将这个Object丢到一个Hash哈希表中去,这样的话,下次做Object的比较或者取这个对象的时候,它会根据对象的hashcode再从Hash表中取这个对象。这样做的目的是提高取对象的效率。具体过程是这样: 1.new Object(),JVM根据这个对象的Hashcode值,放入到对应的Hash表对应的Key上,如果不同的对象确产生了相同的hash值,也就是发生了Hash key相同导致冲突的情况,那么就在这个Hash key的地方产生一个链表,将所有产生相同hashcode的对象放到这个单链表上去,串在一起。 2.比较两个对象的时候,首先根据他们的hashcode去hash表中找他的对象,当两个对象的hashcode相同,那么就是说他们这两个对象放在Hash表中的同一个key上,那么他们一定在这个key上的链表上。那么此时就只能根据Object的equal方法来比较这个对象是否equal。当两个对象的hashcode不同的话

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