hashmap

3分钟搞掂Set集合

由 醉酒当歌 提交于 2019-12-29 20:32:01
前言 声明，本文用的是jdk1.8 前面章节回顾： Collection总览 List集合就这么简单【源码剖析】 Map集合、散列表、红黑树介绍 HashMap就是这么简单【源码剖析】 LinkedHashMap就这么简单【源码剖析】 TreeMap就这么简单【源码剖析】 ConcurrentHashMap基于JDK1.8源码剖析 现在这篇主要讲Set集合的三个子类： HashSet集合 A:底层数据结构是哈希表(是一个元素为链表的数组) + 红黑树 TreeSet集合 A:底层数据结构是红黑树(是一个自平衡的二叉树) B:保证元素的排序方式 LinkedHashSet集合 A:：底层数据结构由哈希表(是一个元素为链表的数组)和双向链表组成。 这篇主要来看看它们比较重要的方法是如何实现的，需要注意些什么，最后比较一下哪个时候用哪个～ 强调： 在学习本文之前，最好是看过Map系列的文章 看这篇文章之前最好是有点数据结构的基础： Java实现单向链表 栈和队列就是这么简单 二叉树就这么简单 当然了，如果讲得有错的地方还请大家多多包涵并不吝在评论去指正～ 一、HashSet剖析 首先，我们来看一下HashSet的继承结构图： 按照惯例，我们来看看HashSet顶部注释： 从顶部注释来看，我们就可以归纳HashSet的要点了： 实现Set接口 不保证迭代顺序 允许元素为null

Java HashMap key value storage and retrieval

由 走远了吗. 提交于 2019-12-29 14:19:13
问题 I want to store values and retrieve them from a Java HashMap. This is what I have so far: public void processHashMap() { HashMap hm = new HashMap(); hm.put(1,"godric gryfindor"); hm.put(2,"helga hufflepuff"); hm.put(3,"rowena ravenclaw"); hm.put(4,"salazaar slytherin"); } I want to retrieve all Keys and Values from the HashMap as a Java Collection or utility set (for example LinkedList ). I know I can get the value if I know the key, like this: hm.get(1); Is there a way to retrieve key values

How to save my Arraylist into SQLite database?

由 岁酱吖の 提交于 2019-12-29 09:50:33
问题 I want to save my ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>>() in SQLite database and later retrieve it to display the items stored in it. How can I achieve this? Please provide examples. Thanks in Advance :) 回答1: Tutorial Link Please refer to the link above. I has a working example of using HashMap for SqLite operations using SQliteOpenHelper. It has one insertion at a time, but you might wanna create a loop, if you wish to insert them all in one

Get first element from HashMap

由 我与影子孤独终老i 提交于 2019-12-29 08:55:16
问题 I have a HashMap and need to get the first element: type VarIdx = std::collections::HashMap<u16, u8>; fn get_first_elem(idx: VarIdx) -> u16 { let it = idx.iter(); let ret = match it.next() { Some(x) => x, None => -1, }; ret } fn main() {} but the code doesn't compile: error[E0308]: match arms have incompatible types --> src/main.rs:5:15 | 5 | let ret = match it.next() { | _______________^ 6 | | Some(x) => x, 7 | | None => -1, 8 | | }; | |_____^ expected tuple, found integral variable | = note

Hashmap and how this works behind the scene [duplicate]

由 混江龙づ霸主 提交于 2019-12-29 08:32:25
问题 This question already has answers here : How does Java HashMap store entries internally (3 answers) Closed 6 years ago . quick question to be sure I understood well how HashMap in java works. Here is an example of code: //String key = new String("key"); //String val = new String("value"); String key = "key"; String val = "value"; HashMap map = new HashMap(); map.put(key, val); System.out.println("hashmap object created. Its key hashcode = "+key.hashCode()); // the hashcode is 106079 System

What are the differences between Hashmap vs Hashtable in theory?

由 ぐ巨炮叔叔 提交于 2019-12-29 08:11:54
问题 Are there are differences between hashmap and hashtable in theory? I don't mean in the concrete definitions given in Java (or the implementation), but in theory. Isn't a hashtable a map that uses hashing ... hence a hashmap? 回答1: According to Wikipedia, they are the same: In computing, a hash table (hash map) is a data structure used to implement an associative array (...) According to Wikibooks, it's the same: A hash table, or a hash map, is a data structure that associates keys with values.

Nested Maps or combined keys in java

由 蹲街弑〆低调 提交于 2019-12-29 06:48:10
问题 I need a Map to make a cache in Java for same values that I have two String keys. My question it's better to make nested Maps (one for each key) or make some type of custom key maked with the two Strings? Access to data on cache will all time accessed with the two keys and I don't need group it by any of that two keys. Then if is better combine string key in only one what it's better? Custom class with custom getHash method. But then the problem is what hash function implement? Simply

How to get a HashMap value with three values

由 走远了吗. 提交于 2019-12-29 04:30:35
问题 If I have a HashMap with a such key: [pubDate, title, link] and such value(example): [Thu, 03 Jan 2013 21:50:02 +0100, Transferts - YBX : ''Je change de dimension'', http://www.link.fr/info/link_informations.html] Can I retrieve the link http://www.link.fr/info/link_informations.html ? Code: for (int i = 0; i < nl.getLength(); i++) { // creating new HashMap map = new HashMap<String, String>(); Element e = (Element) nl.item(i); // adding each child node to HashMap key => value map.put(KEY

How to iterate Arraylist<HashMap<String,String>>?

由 霸气de小男生 提交于 2019-12-29 04:24:27
问题 I have an ArrayList object like this: ArrayList<HashMap<String, String>> data = new ArrayList<HashMap<String, String>>(); How to iterate through the list? I want to display the value in a TextView which comes from the data of ArrayList object. 回答1: Simplest is to iterate over all the HashMap s in the ArrayList and then iterate over all the keys in the Map : TextView view = (TextView) view.findViewById(R.id.view); for (HashMap<String, String> map : data) for (Entry<String, String> entry : map

Java中HashMap底层实现原理(JDK1.8)源码分析

由 ＊爱你＆永不变心＊ 提交于 2019-12-28 17:46:16
Java中HashMap底层实现原理(JDK1.8)源码分析 ​ 在JDK1.6，JDK1.7中，HashMap采用位桶+链表实现，即使用链表处理冲突，同一hash值的链表都存储在一个链表里。但是当位于一个桶中的元素较多，即hash值相等的元素较多时，通过key值依次查找的效率较低。而JDK1.8中，HashMap采用位桶+链表+红黑树实现，当链表长度超过阈值（8）时，将链表转换为红黑树**，这样大大减少了查找时间。 简单说下HashMap的实现原理： 首先有一个每个元素都是链表(可能表述不准确)的数组，当添加一个元素(key-value)时，就首先计算元素key的hash值，以此确定插入数组中的位置，但是可能存在同一个hash值的元素已经被放在数组同一位置了，这时就添加到同一hash值的元素的后面，它们在数组的同一位置，但是形成了链表，同一个链表上的Hash值是相同的，所以说数组存放的是链表。而当链表长度太长时，链表就转换为红黑树，这样大大提高了查找的效率。 当链表数组的容量超过初始容量的0.75时，再散列表将链表数组扩大2倍，把原链表数组搬移到新的数组中，即HashMap的原理图是: 一.JDK1.8中涉及到的数据结构 1.位桶数组 transient Node < k , v > [ ] table ; //存储（位桶）的数组</k,v> 2，数组元素Node<K,V
订阅 hashmap