hashmap | 易学教程

String Vs Stringbuffer as HashMap key

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问题 I am trying to understand why String and Stringbuilder/StringBuffer are treated differently when used as Hashmap keys. Let me make my confusion clearer with the following illustrations: Example #1, using String: String s1 = new String("abc"); String s2 = new String("abc"); HashMap hm = new HashMap(); hm.put(s1, 1); hm.put(s2, 2); System.out.println(hm.size()); Above code snippet prints '1'. Example #2, using StringBuilder(or StringBuffer): StringBuilder sb1 = new StringBuilder("abc");

Hashmap single key holding a class. count the key and retrieve counter

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问题 I am working on a database self project. I have an input file got from: http://ir.dcs.gla.ac.uk/resources/test_collections/cran/ After processing into 1400 separate file, each named 00001.txt ,... 01400.txt ...) and after applying Stemming on them, I will store them separately in a specific folder lets call it StemmedFolder with the following format: in StemmedFolder: 00001.txt includes: investig aerodynam wing slipstream brenckman experiment investig aerodynam wing in StemmedFolder: 00756

Remove a key in hashmap when the value's hashset is Empty

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问题 I have a hashmap that maps strings keys to hashsets values, and I want to remove a key from the hashmap when the hashmaps's hashset value is empty. I'm having trouble approaching this. Here's what I've tried but I'm very stuck: for(Map.Entry<String, HashSet<Integer>> entr : stringIDMap.entrySet()) { String key = entr.getKey(); if (stringIDMap.get(key).isEmpty()) { stringIDMap.remove(key); continue; } //few print statements... } 回答1: In order to avoid ConcurrentModificationException, you need

In Java, sort hash map by its key.length()

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问题 i have a hashmap like this: HashMap<String,Integer> map = new HashMap<String,Integer>(); map.put("java",4); map.put("go",2); map.put("objective-c",11); map.put("c#",2); now i want to sort this map by its key length, if two keys length are equal (e.g go and c# both length 2), then sorted by alphba order. so the outcome i expect to get is something like: printed result: objective-c, 11 java, 4 c#, 2 go, 2 here is my own attamp, but it doesnt work at all... HashMap<String,Integer> map = new

Order of items in a HashMap differ when the same program is run in JVM5 vs JVM6

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问题 I have an application which displays a collection of objects in rows, one object = one row. The objects are stored in a HashMap. The order of the rows does not affect the functionality of the application (that is why a HashMap was used instead of a sortable collection). However I have noticed that the same application runs differently when run using two different versions of the Java Virtual Machine. The application is compiled using JDK 5, and can be run using either Java 5 or Java 6

Are size(), put(), remove(), get() atomic in Java synchronized HashMap?

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问题 I am declaring a Java Map as Map<String, String> map = Collections.synchronizedMap(new HashMap<String, String>()); to deal with the concurrency issues, and synchronizing on the map for all the operations on it. However, I read that synchronization isn't necessary on a synchronizedMap when the operations are atomic. I checked the Java API and the documentation of HashMap doesn't seem to mention which are atomic, so I'm not sure which are. I'm synchronizing on the following calls to the map:

generic map value

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问题 I have run into this problem a few times when wanting to use keys of maps in a similar way but the values in the maps are different. I thought I could write a function that takes the key type I want with interface{} as the value type but it doesn't work. func main() { mapOne := map[string]int mapTwo := map[string]double mapThree := map[string]SomeStruct useKeys(mapOne) } func useKeys(m map[string]interface{}) { //something with keys here } Not sure if there is an elegant way to do this I just

Java Hash Multi Map (key with multiple values) Implementation

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问题 From here, I found that Colt's OpenIntIntHashMap and Trove's TIntIntHashMap give better performance and memory uses than Java's built in HashMap or Guava's HashMultimap . Do Colt's OpenIntIntHashMap or Trove's TIntIntHashMap allow keys with multiple values, as with HashMultimap ? If not what is a nice way to implement a HashMultimap that can achieve Colt's or Trove's performance and memory efficiency? Note: I have tested Guava's HashMultimap , but its performance and memory efficiency seems

Best way to initialize a HashMap

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问题 I usually do e.g. HashMap<String,String> dictionary = new HashMap<String,String>(); I started to think about it, and as far as I know a HashMap is implemented under the hood via a hash table. The objects are stored in the table using a hash to find where they should be stored in the table. Does the fact that I do not set a size on the construction of the dictionary makes the performace decrease? I.e. what would be the size of the hash table during construction? Would it need to allocate new

java的集合框架最全详解

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java的集合框架最全详解（图）前言：数据结构对程序设计有着深远的影响，在面向过程的C语言中，数据库结构用struct来描述，而在面向对象的编程中，数据结构是用类来描述的，并且包含有对该数据结构操作的方法。在Java语言中，Java语言的设计者对常用的数据结构和算法做了一些规范（接口）和实现（具体实现接口的类）。所有抽象出来的数据结构和操作（算法）统称为Java集合框架（JavaCollectionFramework）。 Java程序员在具体应用时，不必考虑数据结构和算法实现细节，只需要用这些类创建出来一些对象，然后直接应用就可以了，这样就大大提高了编程效率。 1. 先说Set和List: 1.1. Set子接口:无序，不允许重复。List子接口:有序，可以有重复元素。具体区别是 Set：检索元素效率低下，删除和插入效率高，插入和删除不会引起元素位置改变。<对应类有 HashSet,TreeSet> List：和数组类似，List可以动态增长，查找元素效率高，插入删除元素效率低，因为会引起其他元素位置改变。<相应类有 ArrayList,LinkedList，Vector> Set和List具体子类： 2.2. < 实例比较> HashSet：以哈希表的形式存放元素，插入删除速度很快。 ArrayList：动态数组，LinkedList：链表、队列、堆栈。