hashcode

HashMap with Null Key and Null Value

由 有些话、适合烂在心里 提交于 2019-12-18 03:08:29
问题 Consider the following Code : import java.util.*; class Employee { String name; public Employee(String nm) { this.name=nm; } } public class HashMapKeyNullValue { Employee e1; public void display(){ Employee e2=null; Map map=new HashMap(); map.put(e2, "25"); System.out.println("Getting the Value When e2 is set as KEY"); System.out.println("e2 : "+map.get(e2)); System.out.println("e1 : "+map.get(e1)); System.out.println("null : "+map.get(null)); map.put(e1, ""); System.out.println("Getting the

How does Object.GetHashCode work when the GC moves an object?

由 笑着哭i 提交于 2019-12-18 03:03:48
问题 If I understand correctly, in .NET the default implementation of Object.GetHashCode() returns a value based on an object's memory address (at least for reference-types). However, the garbage collector is free to move objects around in memory. Presumably the hash code doesn't change just because the GC moves an object, so is there special handling for this interaction, or are my assumptions wrong? 回答1: It doesn't return a value based on the address. It returns a value based on the sync block

Technique to automatically check consistency of equals, hashCode, and compareTo?

由 不羁的心 提交于 2019-12-18 02:09:08
问题 I'm well aware of the contractual needs to make sure that hashCode is consistent with equals and that equals is consistent with compareTo . However, this is often violated in practice. Are there any tools, techniques, or libraries that can test for this consistency automatically? I suspect unfortunately that the answer is "no," but it would be useful to be able to have a unit test for this sort of thing that could make use of a library call or framework rather than needing to write a custom

What is the standard idiom for implementing equals and hashCode in Scala?

由 霸气de小男生 提交于 2019-12-17 22:58:23
问题 What is the standard idiom for implementing the equals and hashCode methods in Scala? I know the preferred approach is discussed in Programming in Scala , but I don't currently have access to the book. 回答1: There's a free 1st edition of PinS that discuss this subject just as well. However, I think the best source is this article by Odersky discussing equality in Java. The discussion in PinS is, iirc, an abbreviated version of this article. 回答2: After doing quite a bit of research, I couldn't

hashmap实现原理浅析

由 谁说我不能喝 提交于 2019-12-17 22:55:42
看了下JAVA里面有HashMap、Hashtable、HashSet三种hash集合的实现源码，这里总结下，理解错误的地方还望指正 HashMap和Hashtable的区别 HashSet和HashMap、Hashtable的区别 HashMap和Hashtable的实现原理 HashMap的简化实现MyHashMap HashMap和Hashtable的区别 两者最主要的区别在于Hashtable是线程安全，而HashMap则非线程安全 Hashtable的实现方法里面都添加了synchronized关键字来确保线程同步，因此相对而言HashMap性能会高一些，我们平时使用时若无特殊需求建议使用HashMap，在多线程环境下若使用HashMap需要使用Collections.synchronizedMap()方法来获取一个线程安全的集合（ Collections.synchronizedMap()实现原理是Collections定义了一个SynchronizedMap的内部类，这个类实现了Map接口，在调用方法时使用synchronized来保证线程同步,当然了实际上操作的还是我们传入的HashMap实例，简单的说就是Collections.synchronizedMap()方法帮我们在操作HashMap时自动添加了synchronized来实现线程同步

Why setting HashTable's length to a Prime Number is a good practice?

由 試著忘記壹切 提交于 2019-12-17 22:24:17
问题 I was going through Eric Lippert's latest Blog post for Guidelines and rules for GetHashCode when i hit this para: We could be even more clever here; just as a List resizes itself when it gets full, the bucket set could resize itself as well, to ensure that the average bucket length stays low. Also, for technical reasons it is often a good idea to make the bucket set length a prime number, rather than 100. There are plenty of improvements we could make to this hash table. But this quick

Default hashCode() implementation for Java Objects

由 ℡╲_俬逩灬. 提交于 2019-12-17 18:31:18
问题 I was trying to understand the hashCode() for Java's Object and saw the following code for Java Object's hashCode() method: package java.lang; public class Object { // Some more code public native int hashCode(); // Some other code } Now, we know that if we create a class it implicitly extends the Object class, and to do this I wrote a sample example: package com.example.entity; public class FirstClass { private int id; private String name; // getters and setters } So, this class viz:

What is the default implementation of `hashCode`? [duplicate]

由 六月ゝ 毕业季﹏ 提交于 2019-12-17 17:53:21
问题 This question already has answers here : What is an object's hash code if hashCode() is not overridden? (11 answers) Closed 6 years ago . If one does not override the hashCode method, what is the default implementation of hashCode ? 回答1: Then this class inherits hashCode from one of its ancestors. If non of them overrides it, then Object.hashCode is used. From the docs: As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct

Does Object.toString or Object.hashCode ever give the memory address of the object

由 风格不统一 提交于 2019-12-17 16:52:34
问题 It is often claimed that the implementation of Object.hashCode() (the default implementation for all objects) gives the memory address of the object. That claim is often attached to an explanation of the peculiar output produced by Object.to String(). See here for an example. This is certainly not the case for any JVMs/JREs I am aware of. Not least because addresses are usually 64 bits long now. But also, garbage collectors relocate objects, so the address changes. I've seen claims it can be

Does hashcode return the memory address? [duplicate]

由 霸气de小男生 提交于 2019-12-17 16:52:09
问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: what is an objects hashcode How do hashCode() and identityHashCode() work at the back end? I'm not talking about String class or any other class where the hashcode is overridden. Say if I just create a new object of the Object class, then will the hashcode() or to be true in any case, identityHashCode(Object x) return the memory address of that object? 回答1: Not necessarily. From the documentation (emphasis mine)
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