hashmap

问题 I want to be able to use a variety of different types as keys in a HashMap , all of which would implement Hash . This seems like it should be possible: from reading the docs it seems that every Hasher will produce a u64 result, so they eventually get reduced down to a common type. Effectively I want to do: use std::{collections::HashMap, hash::Hash}; fn x(_: HashMap<Box<dyn Hash>, ()>) {} which I'm not allowed to do: error[E0038]: the trait `std::hash::Hash` cannot be made into an object -->

问题 I need a HashMap that is accessible from multiple threads. There are two simple options, using a normal HashMap and synchronizing on it or using a ConcurrentHashMap. Since ConcurrentHashMap does not block on read operations it seems much better suited for my needs (almost exclusively reads, almost never updates). On the other hand, I expect very low concurrency anyway, so there should be no blocking (just the cost of managing the lock). The Map will also be very small (under ten entries), if

问题 I am learning about HashMap class and wrote this simple program. this code works good for adding elements to the hashmap and while removing elements from the hashmap , I am encountering java.util.ConcurrentModificationException for example here is a copy of my terminal, [ravi@doom test]$ java TestHashMap .....MENU..... 1. Add 2. remove key 3. remove value 4. display 7. Exit Your choice :1 Key : A Value : 1 Key/Value : (A,1) added to storage. .....MENU..... 1. Add 2. remove key 3. remove value

问题 I am in big trouble, created a hashMap and inserted two values with same key using StringBuilder as a key of map. Now, while trying to retrieve the data using StringBuilder object is working fine, but in other case its fails to return any value. I have listed three cases in the below given code, ` class MainClass { public static void main(String[] args) { MainClass m = new MainClass(); StringBuilder sb = new StringBuilder("sb"); StringBuilder sb1 = new StringBuilder("sb"); Map<StringBuilder,

问题 I am in big trouble, created a hashMap and inserted two values with same key using StringBuilder as a key of map. Now, while trying to retrieve the data using StringBuilder object is working fine, but in other case its fails to return any value. I have listed three cases in the below given code, ` class MainClass { public static void main(String[] args) { MainClass m = new MainClass(); StringBuilder sb = new StringBuilder("sb"); StringBuilder sb1 = new StringBuilder("sb"); Map<StringBuilder,

问题 I am in big trouble, created a hashMap and inserted two values with same key using StringBuilder as a key of map. Now, while trying to retrieve the data using StringBuilder object is working fine, but in other case its fails to return any value. I have listed three cases in the below given code, ` class MainClass { public static void main(String[] args) { MainClass m = new MainClass(); StringBuilder sb = new StringBuilder("sb"); StringBuilder sb1 = new StringBuilder("sb"); Map<StringBuilder,

问题 How can I read File 1 and File2 from the below diagram into class objects and write all the instances of the class into a new File containing all the attributes of both File 1 and File 2? I am taking all attributes as an instance of student class as shown below. How to read both file 1 and file2 into one file either as file 3, file 4, or file 5. which format is more feasible to store those attribute, ArrayList or hashmap, and how 来源： https://stackoverflow.com/questions/64858171/how-to-read

问题 How can I read File 1 and File2 from the below diagram into class objects and write all the instances of the class into a new File containing all the attributes of both File 1 and File 2? I am taking all attributes as an instance of student class as shown below. How to read both file 1 and file2 into one file either as file 3, file 4, or file 5. which format is more feasible to store those attribute, ArrayList or hashmap, and how 来源： https://stackoverflow.com/questions/64858171/how-to-read

问题 In my database, I have 3 columns: Date(PrimaryKey, TEXT) , FlightNumber(PrimaryKey, INTEGER) , and LoadEstimate(INTEGER) . What I would like to do is put all the values from database into a hashmap. I have to make sure that all the datatypes are correct in order to load them into it and would like to filter the LoadEstimate data by user input (a date and the flight number) and then return the predicted number for LoadEstimate and if none return -1 . Here is my database: 回答1: Text Combine your

问题 MapKey mapKey = new MapKey(reqId, name, status); LinkedHashMap<Object, List<Dashboard>> map = new LinkedHashMap<>(); Mapkey Class: public class MapKey { private Integer id; private String name; private Integer status; public MapKey(Integer id, String name,Integer status) { this.id = id; this.name = name; this.status=status; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } //All getters and setters POJO CLASS: public class Dashboard { int requestId;