hashmap

Dozer: Hibernate PersistentMap is not mapped to java.util.HashMap

由 早过忘川 提交于 2020-01-05 00:55:27
问题 I have a Hibernate annotated entity with a field: @OneToMany(mappedBy="templateInstance", fetch = FetchType.EAGER) @MapKey(name = "attributeName") private Map<String, Component> components; Hibernate makes a PersistentMap out of this. Then I want Dozer to map this to an object with such a field: private Map<String, ComponentDto> components; After having LazyInitializationExceptions and some debugging, I found out, that it´s not a problem of a closed Hibernate session, but that Dozer tries not

Dozer: Hibernate PersistentMap is not mapped to java.util.HashMap

由 泄露秘密 提交于 2020-01-05 00:55:11
问题 I have a Hibernate annotated entity with a field: @OneToMany(mappedBy="templateInstance", fetch = FetchType.EAGER) @MapKey(name = "attributeName") private Map<String, Component> components; Hibernate makes a PersistentMap out of this. Then I want Dozer to map this to an object with such a field: private Map<String, ComponentDto> components; After having LazyInitializationExceptions and some debugging, I found out, that it´s not a problem of a closed Hibernate session, but that Dozer tries not

Dozer: Hibernate PersistentMap is not mapped to java.util.HashMap

由 拈花ヽ惹草 提交于 2020-01-05 00:55:09
问题 I have a Hibernate annotated entity with a field: @OneToMany(mappedBy="templateInstance", fetch = FetchType.EAGER) @MapKey(name = "attributeName") private Map<String, Component> components; Hibernate makes a PersistentMap out of this. Then I want Dozer to map this to an object with such a field: private Map<String, ComponentDto> components; After having LazyInitializationExceptions and some debugging, I found out, that it´s not a problem of a closed Hibernate session, but that Dozer tries not

Java系列之HashMap篇

由 廉价感情. 提交于 2020-01-04 09:45:24
1 概述 HashMap是基于哈希表实现的,每一个元素是一个key-value对,其内部通过单链表解决冲突问题,容量不足(超过了阀值)时,同样会自动增长. HashMap是非线程安全的,只适用于单线程环境,多线程环境可以采用并发包下的concurrentHashMap HashMap 实现了Serializable接口，因此它支持序列化，实现了Cloneable接口，能被克隆 HashMap是基于哈希表的Map接口的非同步实现.此实现提供所有可选的映射操作,并允许使用null值和null键.此类不保证映射的顺序,特别是它不保证该顺序恒久不变. Java8中又对此类底层实现进行了优化，比如引入了红黑树的结构以解决哈希碰撞 2 HashMap的数据结构 在Java中,最基本的结构就是两种,一个是数组,另外一个是模拟指针(引用),所有的数据结构都可以用这两个基本结构来构造,HashMap也不例外. HashMap实际上是一个"链表散列"的数据结构,即数组和链表的结合体. HashMap的主结构类似于一个数组,添加值时通过key确定储存位置. 每个位置是一个Entry的数据结构,该结构可组成链表. 当发生冲突时,相同hash值的键值对会组成链表. 这种数组+链表的组合形式大部分情况下都能有不错的性能效果,Java6、7就是这样设计的. 然而,在极端情况下,一组（比如经过精心设计的

HashMap vs. ArrayList insertion performance confusion

由 只谈情不闲聊 提交于 2020-01-04 09:14:11
问题 From my understanding a hashmap insertion is O(1) and for an arraylist the insertion is O(n) since for the hashmap the hashfunction computes the hashcode and index and inserts the entry and an array list does a comparison every time it enters a new element. 回答1: Firstly, an operation of complexity O(1) does not always take lesser time than an operation of complexity O(n). O(1) only means that the operation takes a constant time (which could be any value), regardless of the size of the input.

Altering hashCode of object inside of HashSet / HashMap

由 半腔热情 提交于 2020-01-04 06:24:20
问题 I am relatively new to Java and am puzzled about the following thing: I usually add objects to an ArrayList before setting its content. I.e., List<Bla> list = new ArrayList<>(); Bla bla = new Bla(); list.add(bla); bla.setContent(); // content influences hashCode This approach works great. I am concerned whether this approach will give me trouble when used with HashSet s or HashMap s. The internal hash table get set at the time the object is added. What will happen if setContent() gets called

Usage of HashMap in a multi-threaded environment for regular update

由 倾然丶 夕夏残阳落幕 提交于 2020-01-04 06:14:15
问题 I have a Java application in which I maintain set of IPs of my other servers in a hash map in memory. The hash map contains mapping between servers instance ids to servers ip address. I also maintain these servers information in a database for persistence. I am trying to solve a simple problem where I just need to cache the servers information in memory for faster access. So I have used hashmap for that. And I need to make sure that the server information in memory are NOT stale and all the

Convert map key from collection to double - java

由 不羁的心 提交于 2020-01-04 05:36:17
问题 I have a Map<String, List<Double>> and from this I want to get a SORTED Map<String, Double> where the keys in both maps are the same but the values in the second map are the averages of the values from the first map and then sorted. I’ve tried: public class ListComparator implements Comparator<List<Double>> { public int compare(List<Double> lhs, List<Double>rhs){ Double lhs_sum = lhs.stream().mapToDouble(a -> a).sum()/lhs.size(); Double rhs_sum = rhs.stream().mapToDouble(a -> a).sum()/rhs

Best way to synchronize infrequently updated hashmap

由 旧城冷巷雨未停 提交于 2020-01-04 03:11:46
问题 I have a HashMap that we use in our application. The data is populated from database during application initial load and then it's always just read and never updated. There will be multiple threads constantly reading the data. Since the data is never updated, we currently don't use any synchronization and is using just the HashMap. The way we define this now is: private volatile Map<Integer, MyData> myMap = new HashMap<>(); Now we want to update this data in map once a day by re-populating

Java generics Pair<String, String> stored in HashMap not retrieving key->value properly

由 扶醉桌前 提交于 2020-01-04 02:54:08
问题 Here's Pair.java import java.lang.*; import java.util.*; public class Pair<TYPEA, TYPEB> implements Comparable< Pair<TYPEA, TYPEB> > { protected final TYPEA Key_; protected final TYPEB Value_; public Pair(TYPEA key, TYPEB value) { Key_ = key; Value_ = value; } public TYPEA getKey() { return Key_; } public TYPEB getValue() { return Value_; } public String toString() { System.out.println("in toString()"); StringBuffer buff = new StringBuffer(); buff.append("Key: "); buff.append(Key_); buff
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