hashmap

How do HashMap.values() and HashMap.keySet() return values and keys?

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问题 The source code of HashMap.values() is shown as follows public Collection<V> values() { Collection<V> vs = values; return (vs != null ? vs : (values = new Values())); } As you can see, when the values() method first called, it just returns a Values object. The Values object is a subclass of AbstractCollection with no constructor, and of course contains no element. But when I called the method, it returned a collection rapidly Collection<String> values = map.values(); System.out.println(values

HashMap with weak values

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问题 I'm implementing a cache for Objects stored persistently. The idea is: Method getObjectFromPersistence(long id); ///Takes about 3 seconds Method getObjectFromCache(long id) //Instantly And have a method: getObject(long id) with the following pseudocode: synchronized(this){ CustomObject result= getObjectFromCache(id) if (result==null){ result=getObjectFromPersistence(id); addToCache(result); } return result; } But I need to allow the CustomObject to be collected by the garbage collector. Until

c++ pthread - How to make map access threadsafe?

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问题 I have a map as member variable and multiple threads that access the map (read and write access). Now I have to ensure that only ONE thread have access to the map. But how do I dot that? What is the best solution for that? 回答1: Actually, the premise that only a single thread should access to the map at a given time is slightly off. Concurrent reads are okay, what you want to avoid is having a thread modifying the map while others are reading it. Depending on the level of granularity you need,

How to iterate Hashmap with containing arraylist [duplicate]

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问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: How do I iterate over each Entry in a Map? i have map like HashMap<Integer, ArrayList<String>> map = = new HashMap<Integer, ArrayList<String>>(); i want iterate this type of map please give me example how to iterate this map 回答1: for (Map.Entry<String, ArrayList<String>> entry : map.entrySet()) { String key = entry.getKey(); ArrayList<String> value = entry.getValue(); for(String aString : value){ System.out

Fast C++ container like the C# HashSet<T> and Dictionary<K,V>?

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问题 I've used HashSet and Dictionary a lot in C#, and found them very fast... I've tried using std::map and std::hash_map and am finding them very slow in comparision. Does this sound like expected behaviour? Is there something I might be doing wrong in my use of std::hash_map? Or, is there a better C++ Hash container out there? I'm hashing int32s, usually around 100,000 of them. Update: I created a repro in C# and C++. It runs two trials, they take 19ms and 13ms in C#, and about 11,000ms in C++.

Clojure: How do I apply a function to a subset of the entries in a hash-map?

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问题 I am not to Clojure and attempting to figure out how to do this. I want to create a new hash-map that for a subset of the keys in the hash-map applies a function to the elements. What is the best way to do this? (let [my-map {:hello "World" :try "This" :foo "bar"}] (println (doToMap my-map [:hello :foo] (fn [k] (.toUpperCase k))) This should then result a map with something like {:hello "WORLD" :try "This" :foo "BAR"} 回答1: (defn do-to-map [amap keyseq f] (reduce #(assoc %1 %2 (f (%1 %2)))

阅读更多关于 HashMap

HashMap也是我们使用非常多的Collection，它是基于哈希表的 Map 接口的实现，以key-value的形式存在。在HashMap中，key-value总是会当做一个整体来处理，系统会根据hash算法来来计算key-value的存储位置，我们总是可以通过key快速地存、取value。下面就来分析HashMap的存取。一、定义 HashMap实现了Map接口，继承AbstractMap。其中Map接口定义了键映射到值的规则，而AbstractMap类提供 Map 接口的骨干实现，以最大限度地减少实现此接口所需的工作，其实AbstractMap类已经实现了Map，这里标注Map LZ觉得应该是更加清晰吧！ public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable 二、构造函数 HashMap提供了三个构造函数： HashMap()：构造一个具有默认初始容量 (16) 和默认加载因子 (0.75) 的空 HashMap。 HashMap(int initialCapacity)：构造一个带指定初始容量和默认加载因子 (0.75) 的空 HashMap。 HashMap(int initialCapacity, float loadFactor

Json String to map convertor,

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问题 I am trying to write a generic code for nested JsonObject to map conversion. I have a sample JSONObject as { "glossary": { "title": "example glossary", "GlossDiv": { "title": "S", "GlossList": { "GlossEntry": { "ID": "SGML", "SortAs": "SGML", "GlossTerm": "Standard Generalized \n Markup Language", "GlossDef": { "para": "A DocBook.", "GlossSeeAlso": [ "GML", "XML" ] }, "GlossSee": "markup" } } } } } I want to convert it into map having key value as glossary.title = "example glossary", glossary

not sure if I should search or sort my hashmap

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问题 Hi I have a list of people with their ages, I need to find those who are more than 30 years old, is there any possibility to search in a hashmap ? (please note that I may need to look for those in other age ranges as well so I prefer not to use two different lists for the sake of simplicity of code) In short: My goal is to find a way to search for elements with specific values in HashMap Sample list is element1 40 element2 4 element3 66 element4 5 I want to find those with values more than 40

Creating a CSV File in java from a HashMap

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问题 I have a hashMap in java in terms of some keys which each key is indicating a flow. Then each value showing statics about each packet belongs to that flow. What I need to do is, to draw graphs for each flow based on those values. for example: Flow1: {[length, time],[],[],...} Flow2: {[length, time],[length, time],[],...} i need to create a CSV file that then can be read from MS excel. Can anyone has the idea to give me some clues please? Edited: here is my hashMap: Iterator<Flows> iterator =