hashcode

Java - Is the Point class's hashCode() method any good, or should I override it and write my own?

由 微笑、不失礼 提交于 2019-12-10 15:03:39
问题 Is there any way to actually see the source code of standard java classes by the way? I'm making a hash table of points ( HashSet<Point> ) and I want to make sure that it will hash well, but I can't see what Point's hashCode() method actually looks like, so I don't know how good it really is. Can anyone help me? Should I override it? And if so, is there an easy way to do this without creating a whole new java file/class? 回答1: If you're searching for the hashCode() of java.awt.Point , it is

Why it is necessary to override hashcode and equals method of key for Hashmap? [duplicate]

由 我只是一个虾纸丫 提交于 2019-12-10 14:35:14
问题 This question already has answers here : What issues should be considered when overriding equals and hashCode in Java? (11 answers) Closed 6 years ago . I created a HashMap having Student as key and String as value. Now everywhere i have read It is necessary to override equals and hashcode method if using as a key for hashmap. But I did not override it. And insert multiple key value pairs in hashmap. I am also able to fetch it back. So why it is necessary? 回答1: They are required when you want

About Object.hashcode() and collisions

由 牧云@^-^@ 提交于 2019-12-10 14:29:37
问题 I was reading the JavaDoc for Object.hashCode method, it says that As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer [...]) But whatever its implementation is, hashCode method always returns a (let's assume positive) integer, so given Integer.MAX+1 different objects, two of them are going to have the same hashcode.

Any way to see HashCode of primitive types?

由 别等时光非礼了梦想. 提交于 2019-12-10 14:26:40
问题 Mugging up basic D.S....I read that, no hashCode() method for primitive type int is available and if called on int , it will throw error error: int cannot be dereferenced Does this means that if int n = 10 then its HashCode will also be 10?? If i still need to see hascode for int in below program, is there a method to see it, like Integer Wrapper ??? public static void main(String []args){ String me = "hello"; int n = 10; int h1 = me.hashCode(); int h2 = n.hashCode(); System.out.println(h1);

How to implement equals() and hashcode() methods in BaseEntity of JPA?

由 橙三吉。 提交于 2019-12-10 14:16:41
问题 I have a BaseEntity class which is a superclass of all JPA entities in my application. @MappedSuperclass public abstract class BaseEntity implements Serializable { private static final long serialVersionUID = -3307436748176180347L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Column(name = "ID", nullable=false, updatable=false) protected long id; @Version @Column(name="VERSION", nullable=false, updatable=false, unique=false) protected long version; } Every JPA entity extends from

hashcode()方法

由 妖精的绣舞 提交于 2019-12-10 08:08:38
　浅谈Java中的hashcode方法 　　哈希表这个数据结构想必大多数人都不陌生，而且在很多地方都会利用到hash表来提高查找效率。在Java的Object类中有一个方法: public native int hashCode(); 　　为何Object类需要这样一个方法？它有什么作用呢？今天我们就来具体探讨一下hashCode方法。　　根据这个方法的声明可知，该方法返回一个int类型的数值，并且是本地方法，因此在Object类中并没有给出具体的实现。 一.hashCode方法的作用 　　对于包含容器类型的程序设计语言来说，基本上都会涉及到hashCode。在Java中也一样，hashCode方法的主要作用是为了配合基于散列的集合一起正常运行，这样的散列集合包括HashSet、HashMap以及HashTable。 　　为什么这么说呢？考虑一种情况，当向集合中插入对象时，如何判别在集合中是否已经存在该对象了？（注意：集合中不允许重复的元素存在） 　　也许大多数人都会想到调用equals方法来逐个进行比较，这个方法确实可行。但是如果集合中已经存在一万条数据或者更多的数据，如果采用equals方法去逐一比较，效率必然是一个问题。此时hashCode方法的作用就体现出来了，当集合要添加新的对象时，先调用这个对象的hashCode方法，得到对应的hashcode值

浅谈equals和hashcode

由 自古美人都是妖i 提交于 2019-12-10 07:50:25
转自：http://www.sunxin.org/forum/thread/19720.html 先谈equals。 equals是Object类提供的方法之一，众所周知，每一个java类都继承自Object类， 所以说每一个对象都有equals这个方法。而我们在用这个方法时却一般都重写这个方法，why? Ok，先看一个Object类中equals()方法的源代码： public boolean equals(Object obj) { return (this == obj); } 从这个方法中可以看出，只有当一个实例等于它本身的时候，equals()才会返回true值。通俗地说，此时比较的是两个引用是否指向内存中的同一个对象，也可以称做是否实例相等。而我们在使用equals()来比较两个指向值对象的引用的时候，往往希望知道它们逻辑上是否相等，而不是它们是否指向同一个对象——这就是我们通常重写这个方法的原因。 在程序员之家论坛中有这样一篇文章《全面理解Java中的String数据类型》(链接http://www.phome.asia/forum/thread/19667.html) ，在这篇文章中,String s1 = new String(“kvill”),String s2 = new String(“kvill”); s1.equals(s2)为ture

【转】hashcode()与equals()

由 依然范特西╮ 提交于 2019-12-10 07:50:03
源地址： http://lxj8495138.iteye.com/blog/286024 java.lnag.Object中对hashCode的约定： 在一个应用程序执行期间，如果一个对象的equals方法做比较所用到的信息没有被修改的话，则对该对象调用hashCode方法多次，它必须始终如一地返回同一个整数。 如果两个对象根据equals(Object o)方法是相等的，则调用这两个对象中任一对象的hashCode方法必须产生相同的整数结果。 如果两个对象根据equals(Object o)方法是不相等的，则调用这两个对象中任一个对象的hashCode方法，不要求产生不同的整数结果。但如果能不同，则可能提高散列表的性能。 来源： oschina 链接： https://my.oschina.net/u/192322/blog/53244

转的~== 和 Equals

由 萝らか妹 提交于 2019-12-10 07:32:51
值类型是存储在内存中的堆栈（以后简称栈），而引用类型的变量在栈中仅仅是存储引用类型变量的地址，而其本身则存储在堆中。 ==操作比较的是两个变量的值是否相等，对于引用型变量表示的是两个变量在堆中存储的地址是否相同，即栈中的内容是否相同。 equals操作表示的两个变量是否是对同一个对象的引用，即堆中的内容是否相同。 ==比较的是2个对象的地址，而equals比较的是2个对象的内容。 显然，当equals为true时，==不一定为true； 一、String中的equals和== 1、 public class TestString { public static void main(String[] args) { String s1 = "Monday"; String s2 = "Monday"; } } 上面这段程序中，到底有几个对象呢？ 来检测一下吧，稍微改动一下程序 public class TestString { public static void main(String[] args) { String s1 = "Monday"; String s2 = "Monday"; if (s1 == s2) System.out.println("s1 == s2"); else System.out.println("s1 != s2"); } } 编译并运行程序

ArrayList - add “same” objects (same => equals, hashCode), Threads

由 不打扰是莪最后的温柔 提交于 2019-12-10 03:06:14
问题 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
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