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How to sort a Map by value in Java

To sort the values of the Map, you need to convert the entries into a list or stream and sort them using a comparator. The specific steps are: 1. Use entrySet() to obtain the entries collection; 2. Sort by value through List.sort() or Stream.sorted() and Map.Entry.comparingByValue(); 3. Optionally collect the LinkedHashMap with Collectors.toMap() to maintain the order; if descending order is required, use reversed(). When processing equal values, you can add thenComparing() key to sort quadratic order, and finally get a value-sorted and ordered Map.

Aug 07, 2025 pm 12:41 PM

java Map排序

How to implement a binary search in Java

BinarysearchrequiresasortedarrayandhasO(logn)timecomplexity.2.Theiterativeimplementationusesleftandrightpointerswithmid=left (right-left)/2topreventoverflowandreturnstheindexiffound,else-1.3.TherecursiveversionfollowsthesamelogicbutusesO(logn)stacksp

Aug 07, 2025 am 11:16 AM

java binary search

Solution to access the login page of localhost after Spring Boot application is started

This article aims to solve the confusion of Spring Boot newbies encountering login page when accessing localhost:8080 after launching the application. When Spring Boot app enables Spring Security by default, a user name (usually user) and temporary password are automatically generated and printed on the console. Users need to use these credentials to log in to access application content. The article will explain this mechanism in detail and provide methods to obtain credentials and disable default security configurations to help developers develop smoothly.

Aug 07, 2025 am 10:33 AM

Implement a rating system using HashMap

This article describes how to implement a simple rating system using LinkedHashMap in Java. The system finds the corresponding level from the predefined grade level mapping relationship based on the scores entered by the user and outputs the results. The article will explain the code implementation in detail and provide optimization suggestions to help readers understand the use of HashMap and apply it to actual scenarios.

Aug 07, 2025 am 10:30 AM

Integrating JPA metamodel generator in Gradle project: FAQs and version management strategies

This tutorial is designed to guide developers on how to properly integrate JPA metamodel generators in Gradle projects, especially for building failures encountered when using Spring Boot dependency management. The core solution is to avoid explicitly specifying the version for the hibernate-jpamodelgen annotation processor, but to rely on Spring Boot's dependency management mechanism to automatically select compatible versions, thereby resolving compilation errors caused by version conflicts or incompatibility, ensuring that the metamodel can be generated smoothly, and improving the type safety of JPA queries.

Aug 07, 2025 am 10:24 AM

Solve the problem that the PDF file cannot be opened after Android PDFView restart

This article provides a detailed solution to the problem that SecurityException occurs after restarting the phone when using the PDFView library to open a PDF file in Android applications, causing the file to be unable to be opened. The main reason is the persistence of URI permissions. By removing unnecessary flags and correctly using the takePersistableUriPermissions() method in onActivityResult(), you can ensure that the application can still access the authorized PDF file after restart.

Aug 07, 2025 am 10:18 AM

Security analysis and best practices based on one-time password (OTP) verification

This article explores in-depth potential security vulnerabilities in the mailbox verification mechanism based on one-time password (OTP), especially the risk of OTP collisions that may arise when multiple users register at the same time. The article analyzes the probability of this risk and provides a variety of strategies to enhance the security of OTP systems, including limiting the validity period of OTP, preventing OTP reuse, and using deterministic encryption algorithms to generate OTP, thereby ensuring the security of user accounts.

Aug 07, 2025 am 10:06 AM

HashMap iterative implementation of scoring system: Java tutorial

This article aims to guide developers how to build a simple scoring system in Java using HashMap. The function of automatically judging the grade level and the corresponding score threshold is achieved through HashMap, and combined with user input. This article will provide detailed code examples and explanations to help readers understand the iterative methods of HashMap and its application in actual scenarios.

Aug 07, 2025 am 09:51 AM

How to find the most expensive and cheapest specific type object in polymorphic array in Java

This tutorial details how to process arrays containing polymorphic objects in Java to accurately identify the most expensive and cheapest instances in a specific subtype, such as UAV. The article deeply analyzes common programming errors, namely obfuscating object prices and array indexes, and provides a clear structure and rigorous logical solution. It ensures correctness by separating price and index variables, and comes with complete code examples and precautions, aiming to improve developers' ability to find extreme values when dealing with complex object collections.

Aug 07, 2025 am 09:45 AM

Common reasons and solutions for not writing logback log files

This article discusses in-depth common problems and solutions for Logback log system when it is configured correctly but log files are not written. The core problem is usually that in the Logback configuration file, the logger (Logger) and the file appender are improperly associated with the file appender (Appender), especially the root logger (Root Logger) does not reference the file appender correctly. The article will analyze typical configuration cases, provide correct configuration examples, and provide practical debugging techniques to ensure that the logs can be written to files as expected.

Aug 07, 2025 am 09:36 AM

How to efficiently find the most expensive and cheapest objects in Java arrays

This article details how to iterate through an array of objects in Java and find the most expensive and cheapest objects that meet the criteria (such as UAV type) based on specific properties (such as price). The article will correct normal implementation errors, guide readers to correctly separate price and index tracking, and provide clear sample code and best practices to ensure the robustness and accuracy of the code.

Aug 07, 2025 am 09:30 AM

The Role of `final` and `immutability` in Java

Final is a keyword used in Java to restrict variables, methods and class modification, but it is not equivalent to immutability; 2. Final variables cannot be reassigned after initialization, but the content of the referenced object can still be changed; 3. Final method cannot be rewritten by subclasses, and final class cannot be inherited; 4. immutability refers to the object state immutable after creation, and needs to be implemented by declaring the class as final, the field as privatefinal, and no modification method is provided; 5. Final helps to implement immutability, but only when the referenced object itself is immutable, the entire object is truly immutable; 6. Immutable object is thread-safe,

Aug 07, 2025 am 09:24 AM

java final

Solve the problem of not writing Logback file logs: Configuration and Troubleshooting Guide

This article aims to solve the problem that the Logback log framework cannot write logs to files. The core reason is usually because the Appender and Logger are improperly associated with the Logback configuration file, especially the Root Logger does not configure the file output correctly. The article will thoroughly analyze common configuration misunderstandings, provide correct Logback configuration examples, and elaborate on multiple troubleshooting methods including dependency management, log level, file permissions, etc. to ensure that Logback logs can be output to the specified file as expected.

Aug 07, 2025 am 09:21 AM

Integrated JPA metamodel generator in Gradle project: Configuration and FAQ Solving

This article details how to configure the Hibernate JPA metamodel generator in a Gradle project to implement type-safe JPA queries. The content covers dependency management, source directory configuration, compiler parameter settings, and focuses on how to avoid version conflicts through Spring Boot's dependency management mechanism, ensure the smooth progress of the meta-model generation process, thereby effectively solving the problem of building failure.

Aug 07, 2025 am 09:18 AM