hashtable

Hello I am using a dictionary in Python storing some cities and their population like that: population = { 'Shanghai' : 17.8, 'Istanbul' : 13.3, 'Karachi' : 13.0, 'mumbai' : 12.5 } Now if I use the command print population , I get the result: {'Karachi': 13.0, 'Shanghai': 17.800000000000001, 'Istanbul': 13.300000000000001, 'mumbai': 12.5} whereas if I use the command print population['Shanghai'] I get the initial input of 17.8 . My question to you is how does the 17.8 and the 13.3 turned into 17.800000000000001 and 13.300000000000001 respectively? How was all that information produced? And why

I am really confused on how these 2 collections behave in multithreaded environment. Hash table is synchronized that means no 2 threads will be updating its value simultaneously right? Look at ConcurrentHashMap s for Thread safe Maps. They offer all the features of HashTable with a performance very close to a HashMap. Performance is gained by instead of using a map wide lock, the collection maintains a list of 16 locks by default, each of which is used to lock a single bucket of the map. You can even configure the number of buckets :) Tweaking this can help performance depending on your data.

Are there are differences between hashmap and hashtable in theory? I don't mean in the concrete definitions given in Java (or the implementation), but in theory. Isn't a hashtable a map that uses hashing ... hence a hashmap? woliveirajr According to Wikipedia , they are the same: In computing, a hash table (hash map) is a data structure used to implement an associative array (...) According to Wikibooks , it's the same: A hash table, or a hash map, is a data structure that associates keys with values. Some answer on StackOverflow also states: Hashtable is often useful (they are also called

When we put a class Object (that has say three data members) in a hashtable, how can I prevent putting another entry into the hash table whose key has the same three data members ? Cos I am guessing this will be a new object. So hashtable.containsKey() will return false even when there is a key (this class object) that has the very same data members as the one that is waiting to be inserted. More clearly: I have a class like class Triplet { private Curr curr; private Prev prev; private Next next; } I have a hashtable structure like: Hashtable<Triplet, Integer> table = new Hashtable<Triplet,

By trie map I mean an associative array, where the payloads are stored in a trie instead of a hash table. When I'm using a hash map/table, the keys I use are typically strings. What are the advantages of a hash map over some trie based map? I have read that a hash map is faster - but it seems to me that a consistent hash functions would have to check each element of the (char) array for the final hash - iterating over the array once. In a trie you would similarly have to iterate over the array just once. It does seem to me that this would use a lot more memory when encoding small objects (even

前几天被一家公司电面的时候被问到HashMap和HashTable的区别，当时就懵逼了，hashTable是个啥？从来没用过啊，于是电面完之后马上google了一把，这回涨姿势了； HashMap和HashTable同属于Java.util包下的集合类，Hashtable是个过时的集合类，存在于Java API中很久了。在Java 4中被重写了，实现了Map接口，所以自此以后也成了Java集合框架中的一部分。 HashMap和Hashtable都实现了Map接口，但决定用哪一个之前先要弄清楚它们之间的分别。主要的区别有：线程安全性，同步(synchronization)，以及速度。 对比项 HashMap HashTable 是否线程安全 否 是 是否synchronized 否 是 是否支持多线程共享 在有正确的同步机制的前提下可以支持多线程共享 支持 单线程效率 高 底 只随时间推移元素次序是否不变 不一定 是 是否支持null的key和value 支持 不支持 另一个区别是HashMap的迭代器(Iterator)是fail-fast迭代器，而Hashtable的enumerator迭代器不是fail-fast的。所以当有其它线程改变了HashMap的结构（增加或者移除元素），将会抛出ConcurrentModificationException，但迭代器本身的remove(