hashmap

hashCode, implementation, and relation to HashMap

由 白昼怎懂夜的黑 提交于 2019-12-19 06:18:12
问题 So I asked another related question here: java string hash function with avalanche effect, but I have a different, related question now. What I established in that question was that the hashCode() function for String does not have an avalanche effect. This means, for example, that if I have strings "k1", "k2", "k3", and I call hashCode() on each, the values returned will be contiguous. Now, based on my recollection of data structures 101, I was under the impression that this is a bad thing.

Hashcode for NULL key in HashMap

由 隐身守侯 提交于 2019-12-19 05:13:17
问题 I was just reading about the difference between HashMap and HashTable class in java. There I found a difference that former allow null key and later doesn't privileges for the same. As far as the working of HashMap is concern I know that, it calls hashcode method on key for finding the bucket in which that key value pair is to be placed. Here comes my question: How hashcode for a null value is computed or Is there any default value for hashcode of null key (if so please specify the value)?

How to sum values from Java Hashmap [duplicate]

由 蹲街弑〆低调 提交于 2019-12-19 05:04:07
问题 This question already has answers here : How do I efficiently iterate over each entry in a Java Map? (41 answers) Closed 5 years ago . I need some help, I'm learning by myself how to deal with maps in Java ando today i was trying to get the sum of the values from a Hashmap but now Im stuck. This are the map values that I want to sum. HashMap<String, Float> map = new HashMap<String, Float>(); map.put("First Val", (float) 33.0); map.put("Second Val", (float) 24.0); Ass an additional question,

How to sum values from Java Hashmap [duplicate]

由 不羁的心 提交于 2019-12-19 05:03:59
问题 This question already has answers here : How do I efficiently iterate over each entry in a Java Map? (41 answers) Closed 5 years ago . I need some help, I'm learning by myself how to deal with maps in Java ando today i was trying to get the sum of the values from a Hashmap but now Im stuck. This are the map values that I want to sum. HashMap<String, Float> map = new HashMap<String, Float>(); map.put("First Val", (float) 33.0); map.put("Second Val", (float) 24.0); Ass an additional question,

Recursive iteration of a Map Java

由 久未见 提交于 2019-12-19 05:00:49
问题 I am writing a recursive function whose purpose is to iterate over the pList File. My code is public static void HashMapper(Map lhm1) throws ParseException { //Set<Object> set = jsonObject.keySet(); for (Object entry : lhm1.entrySet()) { if(entry instanceof String) { System.out.println(entry.toString()); } else { HashMapper((Map) ((Map) entry).keySet()); //getting Exception java.util.HashMap$HashMap Entry cannot be cast to java.util.Map } } } But when i am calling my function "HashMapper((Map

Segmentation in ConcurrentHashMap

由 ￡可爱￡侵袭症＋ 提交于 2019-12-19 04:07:42
问题 I am a newbie to the world of Java and I was exploring the ConcurrentHashMap API in which I discovered this: static final int DEFAULT_INITIAL_CAPACITY = 16; static final float DEFAULT_LOAD_FACTOR = 0.75F; static final int DEFAULT_CONCURRENCY_LEVEL = 16; static final int MAXIMUM_CAPACITY = 1073741824; static final int MAX_SEGMENTS = 65536; static final int RETRIES_BEFORE_LOCK = 2; final Segment<K, V>[] segments; final Segment<K, V> segmentFor(int paramInt) { return this.segments[(paramInt >>>

Scalable way to access every element of ConcurrentHashMap<Element, Boolean> exactly once

由 こ雲淡風輕ζ 提交于 2019-12-19 04:04:52
问题 I have 32 machine threads and one ConcurrentHashMap<Key,Value> map , which contains a lot of keys. Key has defined a public method visit() . I want to visit() every element of map exactly once using the processing power I have available and possibly some sort of thread pooling. Things I could try: I could use the method map.keys() . The resulting Enumeration<Key> could be iterated over using nextElement() , but since a call to key.visit() is very brief I won't manage to keep threads busy. The

2020年Java面试100题

由 别来无恙 提交于 2019-12-19 01:54:10
一、Java 基础 1. JDK 和 JRE 有什么区别？ JDK：Java Development Kit 的简称，java 开发工具包，提供了 java 的开发环境和运行环境。 JRE：Java Runtime Environment 的简称，java 运行环境，为 java 的运行提供了所需环境。 具体来说 JDK 其实包含了 JRE，同时还包含了编译 java 源码的编译器 javac，还包含了很多 java 程序调试和分析的工具。简单来说：如果你需要运行 java 程序，只需安装 JRE 就可以了，如果你需要编写 java 程序，需要安装 JDK。 2. == 和 equals 的区别是什么？ == 解读 对于基本类型和引用类型 == 的作用效果是不同的，如下所示： 基本类型：比较的是值是否相同； 引用类型：比较的是引用是否相同； 代码示例： String x = "string"; String y = "string"; String z = new String("string"); System.out.println(x==y); // true System.out.println(x==z); // false System.out.println(x.equals(y)); // true System.out.println(x.equals(z));

java集合常见问题

由 别等时光非礼了梦想. 提交于 2019-12-19 00:09:29
Java集合 常见的集合问题 HashMap与HashTable的区别 HashMap的put方法的具体流程？ HashMap 解决哈希冲突 什么是哈希？ 什么是哈希冲突？ HashMap为什么不直接使用hashCode()处理后的哈希值直接作为table的下标？ HashMap在JDK1.7和JDK1.8中有哪些不同？ 为什么HashMap中String、Integer这样的包装类适合作为K？ ConcurrentHashMap和Hashtable的区别？ Java集合的快速失败机制 “fail-fast” ArrayList 和 Vector 的区别？ ArrayList和LinkedList的区别？ Array 和 ArrayList 有什么区别？什么时候该应 Array 而不是 ArrayList 呢？ HashSet是如何保证数据不可重复的？ BlockingQueue是什么？ 转自 https://zhuanlan.zhihu.com/p/82714518 常见的集合问题 Map 接口和 Collection 接口是所有集合框架的父接口： Collection 接口的子接口包括： Set 接口和 List 接口 Map 接口的实现类主要有： HashMap 、 TreeMap 、 Hashtable 、 ConcurrentHashMap 以及 Properties 等

HashMap.containsValue - What's the point?

由 |▌冷眼眸甩不掉的悲伤 提交于 2019-12-18 22:36:42
问题 I've got a HashMap and I need to fetch an item by its integer value. I notice there's a containsValue() function, but it would appear I still have to iterate through the map to find the correct index anyway. My question is; why use containsValue() if I'm required to traverse it afterwards? Also, am I missing the point completely? ;-) 回答1: A map maps a key to a value. If you have a value and you know the map contains this value, why do you need the key anymore? On the other hand, if you really
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