hashmap | 易学教程

Retrieving Key from the Hash Map

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问题 I'm new to Java. I implemented a Hash Map as shown below. In a particular scenario I wanted to retrieve the key using values of the key. For Eg: If user enters "Dravid", then I need to retrieve "2" from the Hash Map. And I want to use only one hash map to implement this. Can anyone help me with this? HashMap<String,String> streetno=new HashMap<String,String>(); streetno.put("1", "Sachin"); streetno.put("2", "Dravid"); streetno.put("3","Sehwag"); streetno.put("4", "Laxman"); streetno.put("5",

Java Hashmap: Swap two values?

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问题 Can I swap the keys of two values of a Hashmap, or do I need to do something clever? Something that would look something like this: Map.Entry<Integer, String> prev = null; for (Map.Entry<Integer, String> entry: collection.entrySet()) { if (prev != null) { if (entry.isBefore(prev)) { entry.swapWith(prev) } } prev = entry; } 回答1: Well, if you're just after a Map where the keys are ordered, use a SortedMap instead. SortedMap<Integer, String> map = new TreeMap<Integer, String>(); You can rely on

Inheritance: Proper initialization to store values into a HashMap

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问题 I am working with separate programs, one will read in a .txt file and then that same program will store the read in values into another program containing a HashMap . The program that contains the HashMap , is named Hero.java and is an abstract class . I have a program that extends that program called Cop.java . With that in mind, I would like to know the proper way to transfer the files read in from my GameDriver.java over to the HashMap . To my knowledge, one way of doing this is

How to generically implement calling methods stored in a HashMap?

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问题 I want to route certain chars to methods, so that when the char is typed in the command-line the method is then executed. Based on the answer How to call a method stored in a HashMap, I'm mapping these chars to methods by using the "Command" design-pattern. However I want to generically implement this, so it seems that I need to implement reflection in order to use the Method class as a parameter. My attempt is getting a NullPointerException on the field private Method method in my anonymous

Hashmap is put into alphabetical order?

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问题 I have a HashMap defined as follow: private final Map<String, DataTable> reports = new HashMap(); When I put new entries into this HashMap they end up in Alphabetical order based on the key. Why is it doing this? How do I put them in the order I added them to the HashMap? 回答1: A HashMap does explicitly define NO order of the elements you add. Like in a HashSet the elements are ordery by their hashcode and this is more or less random. If you want to preserve the order in the Map in the order

Java，该学什么？

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本人大学学的是生物技术专业，毕业后入坑Java。最近有人问我是如何转行的，需要学一些什么。我在网上看到一篇帖子，觉得写得很全。如果是我来写，可能还写不了这么全的。在此分享给网友。 2019秋招几个月累积的知识点，东西太多，懒得重整理做索引，尽量用(*)和加粗标注出高频知识点, 都是面试问过的或笔试考过的 Java基础知识(*) https://blog.csdn.net/qq_16633405/article/details/79211002 Spring Boot 启动流程(*) https://juejin.im/post/5b679fbc5188251aad213110#heading-0 Spring 一些面试题(*) https://www.ctolib.com/topics-35589.html 匿名内部类编译class(*) https://blog.csdn.net/lazyer_dog/article/details/50669473 为什么集合类没有实现Cloneable和Serializable接口？ https://www.nowcoder.com/questionTerminal/2a4902f67d5b49b6b4c05f9d7e422caf 自动装箱原理 https://www.jianshu.com/p/0ce2279c5691

Checking for and Removing elements in Java HashMap

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问题 I am trying to check for and remove elements, using a HashMap in Java. Its keys are a type I created called ClusterKey, and its values are a type I created called ClusterValue. Here is the code that is causing issues: ClusterKey ck = new ClusterKey(Long.parseLong(split[0].split("=")[1]), Integer.parseInt(split[1].split("ey")[1])); if (arg == 0) keys.put(ck, new ClusterValue(index, false)); if (arg == 1) { if (keys.containsKey(ck)) { index = keys.get(ck).messageNo; keys.remove(ck); } keys.put

springboot编程之全局异常捕获

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springboot编程之全局异常捕获 1、创建GlobalExceptionHandler.java，在类上注解@ControllerAdvice，在方法上注解@ExceptionHandler(value = Exception.class)，Exception.class表示拦截所有的异常信息 package com.imooc.web.controller; import com.imooc.exception.UserNotExistException; import org.springframework.http.HttpStatus; import org.springframework.web.bind.annotation.ControllerAdvice; import org.springframework.web.bind.annotation.ExceptionHandler; import org.springframework.web.bind.annotation.ResponseBody; import org.springframework.web.bind.annotation.ResponseStatus; import java.util.HashMap; import java.util.Map; @ControllerAdvice

Using multiple values for the same param in a multipart's PartMap request with Retrofit 2

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问题 I want to send multiple values of the same param in a multipart query. Here is my code: Interface: @Multipart @POST("user") Observable<Void> updateUser(@PartMap() Map<String, RequestBody> partMap, @Part MultipartBody.Part photo); This request allow me to update a user with a new picture and some parameters. In the parameters I can specify the user's skills with a parameter named "skills[]". To specify the parameters that can vary in number I use a HashMap; however with a HashMap I cannot

Why does Rails' `HashWithIndifferentAccess` store keys as strings and not symbols?

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问题 I am using enum to map integers in my database to semantic values in my ruby code, however I noticed that the keys that it uses are strings. When I checked the type of the hash, I discovered that it was an ActiveSupport::HashWithIndifferentAccess , not a standard Hash , which makes sense, but lead to the question of why Rails chose to store and compare the values as strings, not symbols, internally. The documentation states: Internally symbols are mapped to strings when used as keys in the