https://www.javacodegeeks.com/2014/03/how-hashmap-works-in-java.html
Most common interview questions are “How HashMap works in java”, “How get and put method of HashMap work internally”. Here I am trying to explain internal functionality with an easy example. Rather than going through theory, we will start with example first, so that you will get better understanding and then we will see how get and put function work in java.
Lets take a very simple example. I have a Country class, we are going to use Country class object as key and its capital name(string) as value. Below example will help you to understand, how these key value pair will be stored in hashmap.
1. Country.java
01 |
package org.arpit.javapostsforlearning; |
02 |
public class Country { |
07 |
public Country(String name, long population) { |
10 |
this .population = population; |
12 |
public String getName() { |
15 |
public void setName(String name) { |
18 |
public long getPopulation() { |
21 |
public void setPopulation( long population) { |
22 |
this .population = population; |
25 |
// If length of name in country object is even then return 31(any random number) and if odd then return 95(any random number). |
26 |
// This is not a good practice to generate hashcode as below method but I am doing so to give better and easy understanding of hashmap. |
28 |
public int hashCode() { |
29 |
if ( this .name.length()% 2 == 0 ) |
35 |
public boolean equals(Object obj) { |
37 |
Country other = (Country) obj; |
38 |
if (name.equalsIgnoreCase((other.name))) |
If you want to understand more about hashcode and equals method of object, you may refer hashcode() and equals() method in java
2. HashMapStructure.java(main class)
01 |
import java.util.HashMap; |
02 |
import java.util.Iterator; |
04 |
public class HashMapStructure { |
07 |
* @author Arpit Mandliya |
09 |
public static void main(String[] args) { |
11 |
Country india= new Country( "India" , 1000 ); |
12 |
Country japan= new Country( "Japan" , 10000 ); |
14 |
Country france= new Country( "France" , 2000 ); |
15 |
Country russia= new Country( "Russia" , 20000 ); |
17 |
HashMap<country,string> countryCapitalMap= new HashMap<country,string>(); |
18 |
countryCapitalMap.put(india, "Delhi" ); |
19 |
countryCapitalMap.put(japan, "Tokyo" ); |
20 |
countryCapitalMap.put(france, "Paris" ); |
21 |
countryCapitalMap.put(russia, "Moscow" ); |
23 |
Iterator<country> countryCapitalIter=countryCapitalMap.keySet().iterator(); //put debug point at this line |
24 |
while (countryCapitalIter.hasNext()) |
26 |
Country countryObj=countryCapitalIter.next(); |
27 |
String capital=countryCapitalMap.get(countryObj); |
28 |
System.out.println(countryObj.getName()+ "----" +capital); |
34 |
</country></country,string></country,string> |
Now put debug point at line 23 and right click on project->debug as-> java application. Program will stop execution at line 23 then right click on countryCapitalMap then select watch.You will be able to see structure as below.
Now From above diagram, you can observe following points
- There is an Entry[] array called table which has size 16.
- This table stores Entry class’s object. HashMap class has a inner class called Entry.This Entry have key value as instance variable. Lets see structure of entry class Entry Structure.
1 |
static class Entry implements Map.Entry |
7 |
... //More code goes here |
- Whenever we try to put any key value pair in hashmap, Entry class object is instantiated for key value and that object will be stored in above mentioned Entry[](table). Now you must be wondering, where will above created Enrty object get stored(exact position in table). The answer is, hash code is calculated for a key by calling Hascode() method. This hashcode is used to calculate index for above Entry[] table.
- Now, If you see at array index 10 in above diagram, It has an Entry object named HashMap$Entry.
- We have put 4 key-values in hashmap but it seems to have only 2!!!!This is because if two objects have same hashcode, they will be stored at same index. Now question arises how? It stores objects in a form of LinkedList(logically).
So how hashcode of above country key-value pairs are calculated.
1 |
Hashcode for Japan = 95 as its length is odd. |
2 |
Hashcode for India = 95 as its length is odd |
3 |
HashCode for Russia= 31 as its length is even. |
4 |
HashCode for France= 31 as its length is even. |
Below diagram will explain LinkedList concept clearly.
So now if you have good understanding of hashmap structure,Lets go through put and get method.
Put :
Lets see implementation of put method:
02 |
* Associates the specified value with the specified key in this map. If the |
03 |
* map previously contained a mapping for the key, the old value is |
07 |
* key with which the specified value is to be associated |
09 |
* value to be associated with the specified key |
10 |
* @return the previous value associated with <tt>key</tt>, or <tt>null</tt> |
11 |
* if there was no mapping for <tt>key</tt>. (A <tt>null</tt> return |
12 |
* can also indicate that the map previously associated |
13 |
* <tt>null</tt> with <tt>key</tt>.) |
15 |
public V put(K key, V value) { |
17 |
return putForNullKey(value); |
18 |
int hash = hash(key.hashCode()); |
19 |
int i = indexFor(hash, table.length); |
20 |
for (Entry<k , V> e = table[i]; e != null ; e = e.next) { |
22 |
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { |
31 |
addEntry(hash, key, value, i); |
now lets understand above code step by step
- Key object is checked for null. If key is null then it will be stored at table[0] because hashcode for null is always 0.
- Key object’s hashcode() method is called and hash code is calculated. This hashcode is used to find index of array for storing Entry object. It may happen sometimes that, this hashcode function is poorly written so JDK designer has put another function called hash() which takes above calculated hash value as argument.If you want to learn more about hash() function, you can refer hash and indexFor method in hashmap.
- indexFor(hash,table.length) is used to calculate exact index in table array for storing the Entry object.
- As we have seen in our example, if two key objects have same hashcode(which is known as collision) then it will be stored in form of linkedlist.So here, we will iterate through our linkedlist.
- If there is no element present at that index which we have just calculated then it will directly put our Entry object at that index.
- If There is element present at that index then it will iterate until it gets Entry->next as null.Then current Entry object become next node in that linkedlist
- What if we are putting same key again, logically it should replace old value. Yes,it will do that.While iterating it will check key equality by calling equals() method(key.equals(k)), if this method returns true then it replaces value object with current Entry’s value object.
Get:
Lets see implementation of get now:
02 |
* Returns the value to which the specified key is mapped, or {@code null} |
03 |
* if this map contains no mapping for the key. |
06 |
* More formally, if this map contains a mapping from a key {@code k} to a |
07 |
* value {@code v} such that {@code (key==null ? k==null : |
08 |
* key.equals(k))}, then this method returns {@code v}; otherwise it returns |
09 |
* {@code null}. (There can be at most one such mapping.) |
12 |
* A return value of {@code null} does not <i>necessarily</i> indicate that |
13 |
* the map contains no mapping for the key; it's also possible that the map |
14 |
* explicitly maps the key to {@code null}. The {@link #containsKey |
15 |
* containsKey} operation may be used to distinguish these two cases. |
17 |
* @see #put(Object, Object) |
19 |
public V get(Object key) { |
21 |
return getForNullKey(); |
22 |
int hash = hash(key.hashCode()); |
23 |
for (Entry<k , V> e = table[indexFor(hash, table.length)]; e != null ; e = e.next) { |
25 |
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) |
As you got the understanding on put functionality of hashmap. So to understand get functionality is quite simple. If you pass any key to get value object from hashmap.
- Key object is checked for null. If key is null then value of Object resides at table[0] will be returned.
- Key object’s hashcode() method is called and hash code is calculated.
- indexFor(hash,table.length) is used to calculate exact index in table array using generated hashcode for getting the Entry object.
- After getting index in table array, it will iterate through linkedlist and check for key equality by calling equals() method and if it returns true then it returns the value of Entry object else returns null.
Key points to Remeber:
- HashMap has a inner class called Entry which stores key-value pairs.
- Above Entry object is stored in Entry[ ](Array) called table
- An index of table is logically known as bucket and it stores first element of linkedlist
- Key object’s hashcode() is used to find bucket of that Entry object.
- If two key object ‘s have same hashcode , they will go in same bucket of table array.
- Key object ‘s equals() method is used to ensure uniqueness of key object.
- Value object ‘s equals() and hashcode() method is not used at all
- How HashMap works in Java
https://www.javainterviewpoint.com/hashmap-works-internally-java/ How a HashMap Works internally has ...
- HashMap如何工作 - Java
大多数人应该会同意HashMap是现在面试最喜欢问的主题之一.我和同事常常进行讨论,并很有帮助.现在,我继续和大家讨论. 我假设你对HashMap的内部工作原理感兴趣,并且你已经知道了基本的HashM ...
- SpringMvc中Hashmap操作遇到 java.util.ConcurrentModificationException: null
代码按照网上修改为类似,还不能解决问题 for (Iterator<String> it = target.keySet().iterator(); it.hasNext(); ) { i ...
- HashTable HashMap HashSet区别(java)
Hashtable: 1. key和value都不许有null值 2. 使用enumeration遍历 3. 同步的,每次只有一个线程能够访问 4. 在java中Hashtable是H大写,t小写,而 ...
- HashMap如何在Java中工作?
通过优锐课学习笔记分享,我们可以看到HashMap问题在工作面试中很常见. 这也是HashMaps在Java内部如何工作的一些深入说明,分享给大家参考学习. HashMap在内部如何工作已成为几乎所有 ...
- JSONObject JSONArray json字符串 HashMap ArryList 在java开发中用到的数据结构
1.JSONObject 长成这样的: { "key1":value1, "key2":value2, "key3":value3} ...
- Understanding How Graal Works - a Java JIT Compiler Written in Java
https://chrisseaton.com/truffleruby/jokerconf17/ https://chrisseaton.com/truffleruby/tenthings/ http ...
- LRU hashMap(拉链) + 双向链表 java实现
//基于 hash (拉链法) + 双向链表,LRUcache //若改为开放寻址,线性探测法能更好使用cpuCache public class LRU { private class Node { ...
- Java集合框架(Collection Framework)学习之 HashMap
从API文档可以得到HashMap的以下几个特点: 基于哈希表(hash table)实现,并且是链式哈希表 允许空值和空键(null=null 键值对) HashMap与Hashtable基本相同, ...
随机推荐
- VMware12 克隆虚拟机并且重新分配ip、mac
记录如何快速拷贝一台虚拟机,并且重新分配IP.mac等方便自己做实验: 环境:VMWare12 .Redhat6.8 Step1.克隆虚拟机 虚拟机处于未打开状态,右击管理—>克隆 弹出克隆界 ...
- zoj 2818 Root of the Problem(数学思维题)
题目链接: http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemCode=2818 题目描述: Given positive integer ...
- 【转载】Java8 HashMap之tableSizeFor
Java8对许多内置的容器进行了优化与拓展,其中对HashMap的改变尤其大.之后将进行总结. 最近在看HashMap的源码时,发现了里面好多很不错的算法,相比Java7从性能上提高了许多.其中tab ...
- 并发编程——ConcurrentHashMap#transfer() 扩容逐行分析
前言 ConcurrentHashMap 是并发中的重中之重,也是最常用的数据结果,之前的文章中,我们介绍了 putVal 方法.并发编程之 ConcurrentHashMap(JDK 1.8) pu ...
- Nodejs微信公众号开发
概览 key value 项目名称 node微信公众号开发 项目描述 使用node编写接口,前后端分离获取签名数据 开发者 leinov 发布日期 2018-11-07 仓库 github地址 安装& ...
- 下拉加载更多DEMO(js实现)
项目的一个前端页面展示已购买商品时,要求能下拉加载更多.花了点时间研究这个功能,以前没做过. 首先需要给div加scroll事件,监听滚动条滚动动作.那何时触发加载动作呢?当滚动条滚到底的时候.如何判 ...
- python学习之内存机制
不可变对象(字符串.元组) 1. a = 1 首先在内存中创建对象1,并记录对象的引用计数为1次. id(a) 查看变量a引用的对象的内存地址 2. b = 1 内存中已存在对象1,变量b引用对象1, ...
- MVC应用程序实现会员登录功能
实现之前,我们已经把验证成功的信息存在cookie里<MVC登录前准备写好cookie>http://www.cnblogs.com/insus/p/3464105.html.现在就可以实 ...
- Spring基础(9) : 自动扫描
一 配置xml方式:扫描com包下的bean <?xml version="1.0" encoding="UTF-8" ?> <beans ...
- JS生成指定范围内的随机数(支持随机小数)
直接需要函数的话,直接到文章的最后面找. ============================================================= 转载:https://www.cn ...