hashmap

Best way to create a hashmap of arraylist

由 谁说我不能喝 提交于 2019-12-27 17:38:26
问题 I have one million rows of data in .txt format. the format is very simple. For each row: user1,value1 user2,value2 user3,value3 user1,value4 ... You know what I mean. For each user, it could appear many times, or appear only once (you never know). I need to find out all the values for each user. Because user may appear randomly, I used Hashmap to do it. That is: HashMap(key: String, value: ArrayList). But to add data to the arrayList, I have to constantly use HashMap get(key) to get the

Best way to create a hashmap of arraylist

由 天涯浪子 提交于 2019-12-27 17:38:07
问题 I have one million rows of data in .txt format. the format is very simple. For each row: user1,value1 user2,value2 user3,value3 user1,value4 ... You know what I mean. For each user, it could appear many times, or appear only once (you never know). I need to find out all the values for each user. Because user may appear randomly, I used Hashmap to do it. That is: HashMap(key: String, value: ArrayList). But to add data to the arrayList, I have to constantly use HashMap get(key) to get the

How does Java order items in a HashMap or a HashTable?

由 帅比萌擦擦* 提交于 2019-12-27 13:43:07
问题 I was wondering how Java orders items in the Map ( HashMap or Hashtable ) when they are added. Are the keys ordered by the hashcode, memory reference or by allocation precedence...? It's because I've noticed same pairs in the Map are not always in the same order 回答1: java.util.HashMap is unordered; you can't and shouldn't assume anything beyond that. This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

Reverse HashMap keys and values in Java

由 懵懂的女人 提交于 2019-12-27 12:01:28
问题 It's a simple question, I have a simple HashMap of which i want to reverse the keys and values. HashMap<Character, String> myHashMap = new HashMap<Character, String>(); myHashMap.put('a', "test one"); myHashMap.put('b', "test two"); and I want to create a new HashMap in which i put the opposites. HashMap<String, Character> reversedHashMap = new HashMap<String, Character>(); e.g. Keys "test one" & "test two" and values 'a' & 'b'. 回答1: They all are unique, yes If you're sure that your values

How do I copy a hash in Ruby?

由 佐手、 提交于 2019-12-27 11:11:54
问题 I'll admit that I'm a bit of a ruby newbie (writing rake scripts, now). In most languages, copy constructors are easy to find. Half an hour of searching didn't find it in ruby. I want to create a copy of the hash so that I can modify it without affecting the original instance. Some expected methods that don't work as intended: h0 = { "John"=>"Adams","Thomas"=>"Jefferson","Johny"=>"Appleseed"} h1=Hash.new(h0) h2=h1.to_hash In the meantime, I've resorted to this inelegant workaround def

java核心数据结构总结

由 こ雲淡風輕ζ 提交于 2019-12-27 11:02:31
　　JDK提供了一组主要的数据结构的实现，如List、Set、Map等常用结构，这些结构都继承自java.util.collection接口。 List接口 　　List有三种不同的实现，ArrayList和Vector使用数组实现，其封装了对内部数组的操作。LinkedList使用了循环双向链表的数据结构，LinkedList链表是由一系列的链表项连接而成，一个链表项包括三部分：链表内容、前驱表项和后驱表项。 　　LinkedList的表项结构如图： 　　LinkedList表项间的连接关系如图： 　　 　　可以看出，无论LinkedList是否为空，链表都有一个header表项，它即表示链表的开头也表示链表的结尾。表项header的后驱表项便是链表的第一个元素，其前驱表项就是链表的最后一个元素。 　　对基于链表和基于数组的两种List的不同实现做一些比较： 　　1、增加元素到列表的末尾： 　　在ArrayList中源代码如下： 1 public boolean add(E e) { 2 ensureCapacityInternal(size + 1); // Increments modCount!! 3 elementData[size++] = e; 4 return true; 5 } 　　add()方法性能的好坏取决于grow()方法的性能： 1 private

What is the difference between the HashMap and Map objects in Java?

由 一世执手 提交于 2019-12-27 09:01:19
问题 What is the difference between the following maps I create (in another question, people answered using them seemingly interchangeably and I'm wondering if/how they are different): HashMap<String, Object> map = new HashMap<String, Object>(); Map<String, Object> map = new HashMap<String, Object>(); 回答1: There is no difference between the objects; you have a HashMap<String, Object> in both cases. There is a difference in the interface you have to the object. In the first case, the interface is

Integer cannot be cast to java.util.HashMap

由 浪子不回头ぞ 提交于 2019-12-27 06:38:46
问题 Im using listview on android, below code is giving error after clicking on a list item for another new activity @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { // getting list view size int size = parent.getCount(); // initializing array according to list view size String[] all_pid = new String[size]; String[] all_name = new String[size]; String[] all_address = new String[size]; String[] all_latitude = new String[size]; String[] all_longitude = new

Thinking in Java：容器深入研究

由 て烟熏妆下的殇ゞ 提交于 2019-12-27 04:56:07
1.虚线框表示Abstract类，图中大量的类的名字都是以Abstract开头的，它们仅仅是部分实现了特定接口的工具，因此创建时能够选择从Abstract继承。 Collections中的实用方法：挑几个经常使用的： 1. reverse(List)：逆转次序 2. rotate(List,int distance)全部元素向后移动distance个位置，将末尾元素循环到前面来(用了三次reverse) 3.sort(List,Comparator) 排序，可依照自己的Comparator 4.copy(List dest,List src) 复制元素（引用） 5.fill(List,T x)同Arrays一样。复制的是同一引用 6.disjoint(Collection。Collection) 两个集合没有不论什么元素时返回ture 7.frequency(Collection, Object x)返回Collection中等于x的元素个数 8.binarySearch()折半查找(要求有序) Collection的功能方法： boolean add(T) 可选方法 。若没将參数加入进容器，则false boolean addAll(Collection ) 可选方法 ，仅仅要加入了随意元素就true void clear() 可选方法 boolean contains(T)

Can I get all the values of HashMap elements, having the same keys? (Java)

由 為{幸葍}努か 提交于 2019-12-27 04:48:20
问题 I have a task given me on the Java programming course. According to this task I have to create a method that returns a number of the HashMap elements with identical keys. But the problem is that the iterator goes through elements with different keys only, so anyway, the method returns 1. What`s the way out? package com.javarush.test.level08.lesson08.task03; import java.util.*; /* Одинаковые имя и фамилия Создать словарь (Map<String, String>) занести в него десять записей по принципу «Фамилия»
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