hashcode

I agree with the statement from this post What issues should be considered when overriding equals and hashCode in Java? Use the same set of fields that you use to compute equals() to compute hashCode(). But i've some doubts : Is this absolutely necessary to have same fields ? If yes, what if I don't use same field ? Will it affect HashMap performance or HashMap Accuracy ? The fields don't have to be the same. The requirement is for two objects that are equal, they must have the same hash code. If they have the same hash code, they don't have to be equal. From the javadocs: Whenever it is

1.& 和 && 的区别？ & ：逻辑与（ and ） , 运算符两边的表达式均为 true 时，整个结果才为 true 。 && ：短路与，如果第一个表达式为 false 时，第二个表达式就不会计算了。 2. ” == ”和 equals 方法究竟有什么区别？ == ：表示两个变量的值是否相等，用于比较两个基本数据类型的数据或者引用变量。 equals: 用于比较两个独立对象的内容是否相同。字符串的比较也用 equals 。 == 对于基本类型来说是值比较，对于引用类型来说是比较的是引用；而 equals 默认情况下是引用比较，只是很多类重新了 equals 方法，比如 String 、 Integer 等把它变成了值比较，所以一般情况下 equals 比较的是值是否相等。 3. 比较字符串相等时用 equals 和 == 的区别 String 是对象，如果用 == 号就是比较两个 String 对象内存地址是否一样， equals() 比较的是 String 内容时候一样，不过不同的编程语言两者不太一样 4.Int 和 integer 的区别？ Int 是 Java 的 8 中基本数据类型之一， integer 是 int 的封装类。 Int 类型的默认值为 0 ， integer 默认值为 null ，所以区别在于， integer 能区分出 null 值和 0 的区别。 5

One of the advantages of String immutability is hashcode caching for faster access. In this case how cache is handled for string which has same hashcode? Does it really improve the performance in this case? In this case how cache is handled for string which has same hashcode? I don't understand the first part of your question. Cache is handled for all Strings the same, whether hashcodes are the same or not (since two different Strings can theoretically have the same hashCode, and so if hashCodes are equal, it doesn't mean that the Strings are equal). But if the same String object is used, the

How likely is it to get a HashCode collision with the function below in following scenarios. With random int values for key[0],key[1], key[2], key[3] With random key values with the following constraints key[0] <1,000,000 key[1] <10,000 key[2] <1,000 key[3] <1,000 Assume we have 10 Million objects. int[] key=new int[4]; public override int GetHashCode() { // Use large prime multiples to create a unique hash key // Create the hash offsets using a "even powers of 2 minus 1" method, which gives // primes most of the time. int hashKey = 0; hashKey += 2047 * key[0]; hashKey += 8191 * key[1];

This question already has an answer here: How does native implementation of ValueType.GetHashCode work? 3 answers I saw this code recently in a struct and I was wondering what base.GetHashCode actually does. public override int GetHashCode() { var hashCode = -592410294; hashCode = hashCode * -1521134295 + base.GetHashCode(); hashCode = hashCode * -1521134295 + m_Value.GetHashCode(); return hashCode; } The coreclr repo has this comment : Action: Our algorithm for returning the hashcode is a little bit complex. We look for the first non-static field and get it's hashcode. If the type has no non

hashing的概念 HashMap中解决碰撞的方法 equals()和hashCode()的应用，以及它们在HashMap中的重要性 不可变对象的好处 HashMap多线程的条件竞争 重新调整HashMap的大小 “什么是HashMap？你为什么用到它？” 数组和链表组合成的链表散列结构，通过hash算法，尽量将数组中的数据分布均匀，如果hashcode相同再比较equals方法，如果equals方法返回false，那么就将数据以链表的形式存储在数组的对应位置，并将之前在该位置的数据往链表的后面移动，并记录一个next属性，来指示后移的那个数据。注意数组中保存的是entry,其中保存的是键值. HashMap可以接受null键值和值，而HashTable则不能；HashMap是非synchronized;HashMap很快；以及HashMap储存的是键值对等等 “为什么String, Interger这样的wrapper类适合作为键？” String, Interger这样的wrapper类作为HashMap的键是再适合不过了，而且String最为常用。因为String是不可变的，也是final的，而且已经重写了equals()和hashCode()方法了。其他的wrapper类也有这个特点。不可变性是必要的，因为为了要计算hashCode()，就要防止键值改变