hashcode

This question already has answers here : Closed 6 years ago . Possible Duplicate: What’s the implementation of hashCode in java Object? While I was browsing through the Object class, I found that there is only a declaration of the hashCode() method. Where is the implementation part? If there is no implementation how does the hashCode() method return me a result? It's implemented in the native code. As for implementation, it's a bit more tricky - you can alter default implementation. If you look at the "Open JDK" sources you will see the following options: -XX:hashCode=n (from 0 to 5). 0 – Park

difference between hash code and reference or address of an object? In JavaME, these three things are unrelated. An object's hashCode is a semi-unique identifier for it. A reference to an object is a scoped handle on that object. An object's address is (probably) unobtainable, and certainly useless. The address of/reference to an object points to the location in memory that an Object lives. A Hash Code is generated via a hashing algorithm and is used to identify objects in a hash collection. Different algorithms will produce different codes (poor algorithms will result in poor hash codes and

In my program, I handle a lot of lists of size n that all are permutations of [1,..., n ]. My problem is that I put these permutations in HashMap s and HashSet s, and I need a good hashCode() that avoids too many collisions. All the solutions I have thought of lead to either a lot of collisions or an overflow. How can I write a good hashCode for permutations ? Have you tried a ' rotating hash '? You could adjust the barrel rotate amount to see if it makes much difference to the hash distribution. Please see link in karmakaze's answer. It shows a rotating shift in much neater code, and

I have a scenario where I am synchronizing data between multiple VERY dissimilar systems. (The data itself is similar but the tables on the different systems have quite different formats.) To assist with this synchronization, I have a database table which stores object hashes from each of the systems along with item keys and other relevant information. When the hash of an object from either system changes, I update the other. My database table looks something like this. CREATE TABLE [dbo].[SyncHashes]( [SyncHashId] [int] IDENTITY(1,1) NOT NULL, [ObjectName] [nvarchar](50) NULL,

Had a discussion with an interviewer regarding internal implementation of Java Hashmaps and how it would behave if we overrode equals() but not the HashCode() method for an Employee object. He told me that hashCode for two different objects would never be the same for the default object.hashCode() implementation, unless we overrode the hashCode() ourselves. Also, I was told that one bucket can only have unique Hashcode in it, and objects with same hashcodes went in one bucket. Which I know contradicts point one. Duh! From what I remembered, i told him that Java Hashcode contracts says that two

I have an application which displays a collection of objects in rows, one object = one row. The objects are stored in a HashMap. The order of the rows does not affect the functionality of the application (that is why a HashMap was used instead of a sortable collection). However I have noticed that the same application runs differently when run using two different versions of the Java Virtual Machine. The application is compiled using JDK 5, and can be run using either Java 5 or Java 6 runtimes, without any functional difference. The object in question overrides java.lang.Object#hashCode() and

*面向对象语言对事物的体现都是以对象的形式，所以为了方便对多个对象的操作，就对对象进行存储，集合就是存储对象最常用的一种方式。 *数组虽然也可以存储对象，但长度是固定的；集合长度是可变的，数组中可以存储基本数据类型，集合只能存储对象。 *集合类的特点：集合只用于存储对象，集合长度是可变的，集合可以存储不同类型的对象。 集合框架图 1、Collection （集合的最大接口）继承关系 　　——List　可以存放重复的内容 　　——Set　　不能存放重复的内容，所以的重复内容靠hashCode()和equals()两个方法区分 　　——Queue　　队列接口 　　——SortedSet　　可以对集合中的数据进行排序 Collection定义了集合框架的共性功能。 1，添加 add(e); addAll(collection); 2，删除 remove(e); removeAll(collection); clear(); 3，判断。 contains(e); isEmpty(); 4，获取 iterator(); size(); 5，获取交集。 retainAll(); 6，集合变数组。 toArray(); *add方法的参数类型是Object。以便于接收任意类型对象。 *集合中存储的都是对象的引用(地址) 2、List的常用子类 List： 特有方法

1、为什么用HashMap？ 1.1、HashMap是一个散列桶（数组和链表），它存储的内容是键值对(key-value)映射 1.2、HashMap采用了数组和链表的数据结构，能在查询和修改方便继承了数组的线性查找和链表的寻址修改 1.3、HashMap是非synchronized，所以HashMap很快 1.4、HashMap可以接受null键和值，而Hashtable则不能（原因就是equlas()方法需要对象，因为HashMap是后出的API经过处理才可以） 2、HashMap的工作原理是什么？ HashMap是基于hashing的原理，我们使用put(key, value)存储对象到HashMap中，使用get(key)从HashMap中获取对象。 当我们给 put()方法传递键和值时，我们先对键调用hashCode()方法，计算并返回的hashCode是用于找到Map数组的bucket位置来储存Node 对象。 这里关键点在于指出， HashMap是在bucket中储存键对象和值对象，作为Map.Node 。 以下是 HashMap初始化 ，简单模拟数据结构 以下是具体的 put过程（JDK1.8版） 1、对Key求Hash值，然后再计算下标 2、如果没有碰撞，直接放入桶中（碰撞的意思是计算得到的Hash值相同，需要放到同一个bucket中） 3、如果碰撞了

转自：https://www.toutiao.com/i6544826418210013700/ 了解hashmap底层原理，理解为什么hashmap是线程不安全的。 老师：JDK中我们最常用的一个数据类是HashMap。那么，谁可以回答一下HashMap的底层数据结构原理是什么呢？ 小明：老师，我知道。众所周知，HashMap是一个用于存储Key-Value键值对的集合，每一个键值对也叫做Entry。这些个键值对（Entry）分散存储在一个数组当中，这个数组就是HashMap的主干。 HashMap数组每一个元素的初始值都是Null。 对于HashMap，我们最常使用的是两个方法：Get 和 Put。 1.Put方法的原理 调用Put方法的时候发生了什么呢？ 比如调用 hashMap.put("apple", 0) ，插入一个Key为“apple"的元素。这时候我们需要利用一个哈希函数来确定Entry的插入位置（index）： index = Hash（“apple”） 假定最后计算出的index是2，那么结果如下： 但是，因为HashMap的长度是有限的，当插入的Entry越来越多时，再完美的Hash函数也难免会出现index冲突的情况。比如下面这样： 这时候该怎么办呢？我们可以利用链表来解决。 HashMap数组的每一个元素不止是一个Entry对象，也是一个链表的头节点

Ive got one question. What happens when I try to add the "same" object twice to an ArrayList. With "the same" I mean an object of an individual class, which is identified as the same with equals() and hashCode(). It has different values for most of the member variables and was created from maybe different threads, but for equals() and hashCode() its the "same". Does the second object then replace the first object? Also, what happens if two threads try to add those objects exactly at the same time to the ArrayList? Is this even possible? If yes, what happens? Thank you! :-) [EDIT] Thanks for