hashmap | 易学教程

Java Map sort by value

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问题 I was looking for ways of sorting Map<String, Integer> by values. I found this post, which solved my sorting problem, but not exactly. According to the post, I wrote the following code: import java.util.*; public class Sort { static class ValueComparator implements Comparator<String> { Map<String, Integer> base; ValueComparator(Map<String, Integer> base) { this.base = base; } @Override public int compare(String a, String b) { if (base.get(a) >= base.get(b)) { return 1; } else { return -1; } }

std::map difference between index and insert calls

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问题 What is the difference between the index overloaded operator and the insert method call for std::map? ie: some_map["x"] = 500; vs. some_map.insert(pair<std::string, int>("x", 500)); 回答1: I believe insert() will not overwrite an existing value, and the result of the operation can be checked by testing the bool value in the iterator/pair value returned The assignment to the subscript operator [] just overwrites whatever's there (inserting an entry if there isn't one there already) Either of the

std::map difference between index and insert calls

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Java putting Hashmap into Treemap

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问题 I am currently reading 2 million lines from a textfile as asked in the previous question Java Fastest way to read through text file with 2 million lines Now I store these information into HashMap and I want to sort it via TreeMap because I want to use ceilingkey. Is the following method correct? private HashMap<Integer, String> hMap = new HashMap(); private TreeMap<Integer, String> tMap = new TreeMap<Integer, String>(hMap); 回答1: HashMap<Integer, String> hashMap = new HashMap<Integer, String>(

Why not allow an external interface to provide hashCode/equals for a HashMap?

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问题 With a TreeMap it's trivial to provide a custom Comparator , thus overriding the semantics provided by Comparable objects added to the map. HashMap s however cannot be controlled in this manner; the functions providing hash values and equality checks cannot be 'side-loaded'. I suspect it would be both easy and useful to design an interface and to retrofit this into HashMap (or a new class)? Something like this, except with better names: interface Hasharator<T> { int alternativeHashCode(T t);

How to iterate over a nested map in <c:forEach>

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问题 I've a Map in a bean as follows: public class TaskListData { private Map<String, String[]> srcMasks = new HashMap<String, String[]>(); private Map<Integer, Map<String, String[]>> ftqSet = new HashMap<Integer, Map<String, String[]>>(); public void setFTQSet(Integer ftqid, String[] src, String[] masks) { srcMasks.put("srcDir", src); srcMasks.put("masks", masks); ftqSet.put(ftqid, srcMasks); } This ftqSet fits in below datastructure: feedId = "5", feedName = "myFeedName", ftqSet => { 1 => {

JAXB HashMap unmappable

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问题 I want to convert a HashMap in a POJO class to XML. I tried using the XmlAdapter but it results in only the key and value pairs of the HashMap being the attributes of the XML Elements. I need the Key to be the Element itself and the value of the HashMap to be the value of the element. For instance, I need the following XML: <?xml version="1.0" encoding="UTF-8" standalone="yes"?> <cart> <supervisor_id>555</supervisor_id> <payments> <payment sequence="1"> <amount>123.45</amount> <billing_method

delphi hashmap?

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问题 i have java-code filling a hashmap from a textfile. HashMap<String, String[]> data = new HashMap<String, String[]>(); i use this to make key-value-pairs. the values are an array of string. i have to iterate over every possible combo of the key-value-pairs (so also have to iterate over the String[]-array). This works with java but now i have to port this to delphi. is it possible to do so? and how? thanks! 回答1: In Delphi 2009 and higher, you can use TDictionary<string, TStringlist> using

Remove duplicate values from HashMap in Java

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问题 I have a map with duplicate values: ("A", "1"); ("B", "2"); ("C", "2"); ("D", "3"); ("E", "3"); I would like to the map to have ("A", "1"); ("B", "2"); ("D", "3"); Do you know how to get rid of the duplicate values? At present, I get 'java.util.ConcurrentModificationException' error. Thank you. public static void main(String[] args) { HashMap<String, String> map = new HashMap<String, String>(); map.put("A", "1"); map.put("B", "2"); map.put("C", "2"); map.put("D", "3"); map.put("E", "3"); Set

GC HASHMAP JVM

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JVM、GC与HashMap 阿里巴巴突然来了个面试邀请电话，问了些java底层的东西，不知所措，所以专门花了些时间做了下学习，顺便记录下，好记性不如烂笔头。一、对JAVA的垃圾回收机制（GC）的理解不同于C/C++需要手工释放对象所占的内存，JAVA全部委托给了GC进行处理，能更有效的防止内存泄漏的情况。一个程序对应着一个JVM，每个JVM会单独有一个堆，java中创建的对象与数组是存放在堆中的，堆中的内存由GC进行管理（栈中存储引用变量、局部变量一级基本数据类型，超出作用域就会立即释放内存）。当一个对象没有再被引用时就会被GC标记为可回收状态，然后在一个不确定的时间对其进行回收（一般是在应用程序空闲或者java堆内存不足是被调用），当然在对一个对象进行回收时，首先会执行其finalize()方法，在finalize()方法中你可再次将该对象变为活跃状态，阻止其回收。一个对象还有没有再被引用一般有两种判断方式： 1、（JDK1.2之前）每个对象都有一个引用计数器，每多一个引用，计数器+1，少一个则-1，当计数器为0时，则表示该对象没有再被引用了。 2、根搜索算法。这里要盗一张图了：从GC ROOT开始，寻找对应的引用节点，找到这个节点以后，继续寻找这个节点的引用节点，当所有的引用节点寻找完毕之后，剩余的节点则被认为是没有被引用到的节点，即无用的节点。