hashmap | 易学教程

JAVA中重写equals()方法为什么要重写hashcode()方法说明

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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 重写hashCode（）时最重要的原因就是：无论何时，对同一个对象调用hashCode()都应该生成同样的值。如果在将一个对象用put（）方法添加进HashMap时产生一个hashCode()值，而用get()取出时却产生了另外一个 hashCode()值，那么就无法重新取得该对象了。所以，如果你的hashCode()方法依赖于对象中易变的数据，那用户就要小心了，因为此数据发生变化时，hashCode()就会产生一个不同的hash码，相当于产生了一个不同的“键”。 Object的hashCode（）方法，返回的是当前对象的内存地址。下次如果我们需要取一个一样的“键”对应的键值对的时候，我们就无法得到一样的 hashCode值了。因为我们后来创建的“键”对象已经不是存入HashMap中的那个内存地址的对象了。我们看一个简单的例子，就能更加清楚的理解上面的意思。假定我们写了一个类：Person （人），我们判断一个对象“人”是否指向同一个人，只要知道这个人的身份证号一直就可以了。先来个没有重写Code类的hashcode()的例子吧，看看是什么效果： Java代码 package com.fit; import java.util.HashMap; /** * 身份证类 * * @author ZYD *

adding data to hashmap from database

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问题 Hello I am creating an application where I can store data in a Hashmap and List for a shopping cart module.But I want to fetch this data from my MS-Access database.I tried the following code but its not compiling.Please give me guidance. Code: import java.sql.*; import java.util.ArrayList; import java.util.HashMap; import java.util.List; public class Program { public static void main(String [] args){} public static HashMap getProductsAsMap() { try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver

map 对象转 list对象

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List list = new ArrayList<>(); HashMap map = new HashMap<String,Object>(); map.put("name", "张三"); map.put("age", 20); list.add(map); JSONArray result = JSONArray.fromObject(list); List<User> jsonDtosList = (List<User>) JSONArray.toCollection(result, User.class); 导包 import net.sf.json.JSONArray 来源： CSDN 作者：那一刻哎哎链接： https://blog.csdn.net/qq_36213455/article/details/103489632

Malformed binary serialization of HashMap<String,Double>

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问题 I wrote some code to serialize a HashMap<String,Double> by iterating entries and serializing each of them instead of using ObjectOutputStream.readObject() . The reason is just efficiency: the resulting file is much smaller and it is much faster to write and read (eg. 23 MB in 0.6 seconds vs. 29 MB in 9.9 seconds). This is what I did to serialize: ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("test.bin")); oos.writeInt(map.size()); // write size of the map for (Map.Entry

how to get hashmap values depends on key

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问题 i have this hashmap with string and arraylis: Map<String, ArrayList<String>> devNameType = new HashMap<String, ArrayList<String>>(); public void kimenetPV(String name){ //létrehozom a nevet String devName = name; //létrehozom a kimeneteket tartalmazó arraylistet ArrayList<String> outputs = new ArrayList<String>(); // 2. hozzáadaom az elemeket outputs.add("EM A"); outputs.add("EM B"); outputs.add("AOUT1"); outputs.add("AOUT2"); devNameType.put(devName, outputs); Iterator iter = devNameType

Consistency of contains method on a HashSet and HashMap

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问题 Why does containsAll method on a HashSet does not remain consistent if remove is called on the Set whereas a containsValue method on a HashMap remains consistent after a value is removed After a value is removed from a HashSet containsAll returns false even if all values were present where as in case of HashMap the containsValue method returns correct value public static void main(String[] args) { HashSet<String> lookup=new HashSet<String>(); HashMap<Integer,String> findup=new HashMap<Integer

issue with hashmap getting concurrent modification exception

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问题 I am getting the below error while using map and performing some remove.How to avoid this ? Caused by: java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793) at java.util.HashMap$EntryIterator.next(HashMap.java:834) at java.util.HashMap$EntryIterator.next(HashMap.java:832) Map<FormField, Object> ItemMap = domainItem.getValues(); for (Map.Entry<FormField, Object> ValMap : ItemMap.entrySet()) { List<Field> groupIdList = Mapper.getGroupId

Iterate through a hashmap with an arraylist?

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问题 So I have a basic hashmap with an arraylist: Map<Text, ArrayList<Text>> map = new HashMap<Text, ArrayList<Text>>(); Say I have a key value pair: Key: Apple, Value: orange, red, blue I already understand how to iterate through to print the key and it’s values like so: Apple, orange, red, blue but is there a way to break up the values/iterate through the inner ArrayList and print the key/value pair three separate times/print the key with each value separately like: Apple orange Apple red Apple

Java usage of HashMap or Map with multidimensional arrays

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问题 I would like to use HashMaps<String, V> where the value type V should be a two dimensional int array ( int[][] ). Map<String,int[][]> map = new HashMap<>(); int[][] a = new int[][]{ {1, 2} }; map.put("test", a); System.out.println("Test:" + map.containsKey("test")); // Test: true System.out.println("key ==== " + map.get("test")); // Key === [[I@19a8942 I am trying to have a key that access a unique value i.e. a 2-dimensional array, the size of the array is fixed array[1][3] , this is constant

What is a fast alternative to HashMap for mapping to primitive types?

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问题 First of all let me tell you that i have read the following questions that has been asked before Java HashMap performance optimization / alternative and i have a similar question. What i want to do is take a LOT of dependencies from New york times text that will be processed by stanford parser to give dependencies and store the dependencies in a hashmap along with their scores, i.e. if i see a dependency twice i will increment the score from the hashmap by 1. The task starts off really