hashcode | 易学教程

Influence of HashMap optimization that caches the hash code associated with each entry to its get method

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问题 from p.46 "Effective Java" Joshua Bloch. Item 9: ALways override hashCode when you override equals Some class PhoneNumber overrides equals() and doesn't override hashCode() "two instances are involved: one is used for insertion into the HashMap, and a second, equal, instance is used for (attempted) retrieval." ... "... Even if the two instances happen to hash to the same bucket, the get method will almost certainly return null , as HashMap has an optimization that caches the hash code

deepHashCode with byte array

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问题 For some reason, Arrays.deepHashCode() cannot work with byte[] . Is there any other equivalent? 回答1: First off, no need for "Deep". It's a primitive. You don't need Deep. Just use Arrays.hashCode(byte[] yourArray) Edit: To clarify, Deep implies delving into the Objects contained within the array. Given that you are dealing with a primitive, you just need to use the primitive value itself in the calculation. That's why none of the Deep methods revolve around primitives. 回答2: The accepted

Why chose 31 to do the multiplication in the hashcode() implementation ? [duplicate]

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问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Why does Java's hashCode() in String use 31 as a multiplier? Why use a prime number in hashCode? From Effective Java Item 9: Always override hashCode when you override equals consider the following relevant code snippet that overrides the hashcode() defined in Object class. public final class PhoneNumber { private final short areaCode; private final short prefix; private final short lineNumber; ......//Rest

how does hashing in java works?

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问题 I am trying to figure something out about hashing in java. If i want to store some data in a hashmap for example, will it have some kind of underlying hashtable with the hashvalues? Or if someone could give a good and simple explanation of how hashing work, I would really appreciate it. 回答1: HashMap is basically implemented internally as an array of Entry[] . If you understand what is linkedList, this Entry type is nothing but a linkedlist implementation. This type actually stores both key

difference between hash code and reference or address of an object?

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问题 difference between hash code and reference or address of an object? 回答1: 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. 回答2: 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

Can we define hashcode method within a class in C++

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问题 I am trying to implement a class in C++, and I want each class to have its own implementation of hashcode(basically to use it as a key in unordered_map & unordered_set ) for eg: class CustomClass{ int a; vector<int> b; string c; bool operator ==(const CustomClass& o) const{ return ((a == o.a) && (b == o.b) && (c == o.c)); } /* Is it possible to define the hashcode function here instead of defining it outside the class. size_t operator()() const { // Some custom logic for calculating hash of

Converting Non-Serializable Classes to a Byte Array

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问题 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].

hashcode and equals contract vice versa

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I know the contract says "if two objects are equal then they should return the same hash code". That is so that those objects can be place in the same hash bucket and to improve hash code related collection functions know. Then again why it says "if two objects have same hash code those should not always be equals". I mean if it is true in the contract we should say "if two objects are equals they may return same hash code but which is not mandatory" I mean if it is true in the contract we should say "if two objects are equals they may return same hash code but which is not mandatory" No, it

How do I prove that Object.hashCode() can produce similar hash code for two different objects in Java?

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问题 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

Java 基础复习 -- Enum 类

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一、枚举类基本语法在 Java SE5 中添加了一个看似很小的特性，即 enum 关键字，它使得我们在需要群组并使用枚举类型集时，可以很方便的处理。创建一个简单的ColorEnum public enum ColorEnum { RED,YELLOW,BULE,GREED,BLACK; } 调用的收直接在方法中使用 ColorEnum.RED 即可 ColorEnum red = ColorEnum.RED; 而且 Enum 有一个特别实用的特性，它可以在 switch 语句内使用： ColorEnum red = ColorEnum.RED; switch (red) { case RED: System.out.println("this's red color"); break; case BULE: System.out.println("this's bule color"); break; default: System.out.println("this's other color"); } 由于 switch 是要在有限的可能值集合中进行选择，因此它和 enum 是绝佳的组合。自定义构造方法枚举类如何定义？ public enum RedisEnum { ADD(1, "add"), DEL(2, "delete"), query(3, "query");