hashmap | 易学教程

ANSI C hash table implementation with data in one memory block

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问题 I am looking for an open source C implementation of a hash table that keeps all the data in one memory block, so it can be easily send over a network let say. I can only find ones that allocate small pieces of memory for every key-value pair added to it. Thank you very much in advance for all the inputs. EDIT: It doesn't necessarily need to be a hash table, whatever key-value pair table would probably do. 回答1: On a unix system I'd probably utilise a shared memory buffer (see shm_open()), or

Why is the maximum capacity of a Java HashMap 1<<30 and not 1<<31?

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问题 Why is the maximum capacity of a Java HashMap 1<<30 and not 1<<31, even though the max value of an int is 2 31 -1? The maximum capacity is initialized as static final int MAXIMUM_CAPACITY = 1 << 30; 回答1: Java uses signed integers which means the first bit is used to store the sign of the number (positive/negative). A four byte integer has 32 bits in which the numerical portion may only span 31 bits due to the signing bit. This limits the range of the number to 2^31 - 1 (due to inclusion of 0)

HashSet和TreeSet源码分析

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目录 HashSet底层 HashSet构造函数 HaseSet add分析 TreeSet底层 TreeSet构造函数 TreeSet add分析 HashSet底层是一个HashMap,只不过HashMap里面每个key对应的value值都是一样的PRESENT private static final Object PRESENT = new Object(); HashSet构造函数 public HashSet() { map = new HashMap<>();//new HashSet实际上是new HashMap(); } HaseSet add分析 // Dummy value to associate with an Object in the backing Map private static final Object PRESENT = new Object(); public boolean add(E e) { return map.put(e, PRESENT)==null;//add其实是在HashMap里put一个key,对一个的value是固定的PRESENT } TreeSet底层是一个TreeMap,只不过HashMap里面每个key对应的value值都是一样的PRESENT private static final Object

cannot borrow variable as mutable because it is also borrowed as immutable while building a self-referential HashMap

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问题 I'm trying to build a self-referential HashMap : use std::collections::HashMap; struct Node<'a> { byte: u8, map: HashMap<i32, &'a Node<'a>>, } fn main() { let mut network = HashMap::<u32, Node>::new(); network.insert(0, Node { byte: 0, map: HashMap::<i32, &Node>::new() }); network.insert(1, Node { byte: 1, map: HashMap::<i32, &Node>::new() }); let zeroeth_node = network.get(&0).unwrap(); let mut first_node = network.get_mut(&1).unwrap(); first_node.map.insert(-1, zeroeth_node); } I'm running

Saving records into the database from the hashmap in android

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问题 I just wanted to ask help for this one. I have a hashmap populate in the listview. the code below is my code for my hashmap: mylist = new ArrayList<HashMap<String, String>>(); for (int i = 0; i < listSelectedFileNames.size(); i++) { HashMap<String, String> map = new HashMap<String, String>(); map.put(FILE_NAME, selectedFileNames[i]); map.put(DESC, ""); map.put(UPLOADED_BY, "User"); map.put(DATE_UPLOADED, myDate); map.put(ACTION, "Delete"); map.put(ID, String.valueOf(i)); map.put(FILE_URI,

Saving records into the database from the hashmap in android

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LinkedHashMap是什么？

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大家都知道， HashMap 是一个无序的集合，所有存入HashMap的元素都会根据它的 key 值对应的 hashCode 来映射到一个 Entity 数组上，而不是根据存入的顺序进行排列的。对于一些需要使用排序的情况， HashMap 是无能为力的，为了可以应对这种需要 HashMap 并且需要排序的情况，JDK 便推出了 LinkedHashMap 。 LinkedHashMap 继承了 HashMap ，通过使用一个双向链表来实现。由于是继承了 HashMap ，因此对于 HashMap 存在的问题， LinkedHashMap 也自然会存在，比如不支持并发。既然 LinkedHashMap 是可以排序的，那么它到底是怎么排序的呢？其实，它有两种排序的方式：根据写入的顺序排序根据读取的顺序排序根据写入顺序排序，这个很好理解，几乎其他所有的有序集合都是按照这种方式来做的。对于第二种情况，是在每次使用 get 方法来访问 LinkedHashMap 中的元素时，将该元素放到集合的最后，这样多次操作之后，就会得到一个根据读取的顺序排列的集合了。来源： CSDN 作者：昨日的桥链接： https://blog.csdn.net/weixin_42829639/article/details/103866750

how to instantiate Unit in Scala?

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问题 All I desire is to use some concurrent Set (that appears not to exist at all). Java uses java.util.concurrent.ConcurrentHashMap<K, Void> to achieve that behavior. I'd like to do sth similar in Scala so I created instance of Scala HashMap (or Java ConcurrentHashMap) and tried to add some tuples: val myMap = new HashMap[String, Unit]() myMap + (("myStringKey", Unit)) This of course crashed the process of compilation as Unit is abstract and final. How to make this work? Should I use Any / AnyRef

Java中的HashMap详解

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文章目录一、什么是HashMap 二、HashMap数据结构解析三、HashMap中hashCode的作用四、HashMap的数组长度为什么是2的n次幂五、HashMap何时扩容以及它的扩容机制？六、HashMap的key一般用字符串,能用其他对象吗？七、HashMap的key和value都能为null么?如果key能为null,那么它是怎么样查找值的？八、HashMap是线程安全的吗？如何实现线程安全？九、HashMap的实现原理及它与HashTable、 ConcurrentHashMap的区别一、什么是HashMap HashMap继承了AbstractMap，实现了Map接口，存储的是一个键值对对象。二、HashMap数据结构解析 1、继承关系：HashMap继承了AbstractMap，实现了Map接口。 public abstract class AbstractMap < K , V > implements Map < K , V > { 2、常量及构造方法 //这两个是限定值当节点数大于8时会转为红黑树存储 static final int TREEIFY_THRESHOLD = 8 ; //当节点数小于6时会转为单向链表存储 static final int UNTREEIFY_THRESHOLD = 6 ; //红黑树最小长度为 64

Is it possible to create a HashMap that is Parcelable on Android?

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问题 I am trying to extend HashMap as a Parcelable and I got the syntax to compile, however, at runtime it throws an exception and returns a null pointer trying to un-marshal the data. The sender has to cast to (Parcelable) to resolve ambiguity, however, the receiver complains that is expected Parcelable but found HashMap. Has anyone been successful with this? Is my syntax wrong? Is there a better solution? Following is the code: HomeActivity.java - the sender ContentViewActivity.java - the