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How to use abstract classes in Java

When using abstract classes, when you need to share code among multiple related subclasses, define non-public members or partially implement behaviors; 2. The abstract class is defined with the abstract keyword, which can contain abstract methods and concrete methods. The subclass must implement all abstract methods or declare them as abstract; 3. The abstract class cannot be instantiated directly, but can be used as a reference type to point to subclass instances; 4. The abstract class supports constructors, fields, and access modifiers, which are suitable for modeling "is-a" relationships with shared states; 5. Compared with interfaces, abstract classes are more suitable for code reuse and control access levels, while interfaces are used to define "what can be done" and multiple inheritance.

Aug 08, 2025 pm 01:54 PM

How to get the MAC address in Java?

Use the NetworkInterface.getHardwareAddress() method to obtain the MAC address of the network interface. You need to obtain the NetworkInterface instance through InetAddress or interface name; 2. You can list all MAC addresses through all network interfaces, but you need to deal with the situation where null is returned; 3. Pay attention to permissions, virtual interface without MAC addresses, IPv4/IPv6 mapping and cross-platform compatibility issues; 4. You can directly obtain the MAC address of a specific interface through the specified interface name through NetworkInterface.getByName(), which is suitable for the server environment; the final result must be formatted into hexadecimal words by formatting the byte array into hexadecimal words.

Aug 08, 2025 pm 01:33 PM

What is the explicit contract between the equals() and hashCode() methods in Java?

Iftwoobjectsareequalaccordingtoequals(),theirhashCode()valuesmustbeidentical;2.ThehashCode()valuemustremainconsistentduringasingleexecutioniftheobject’sstatedoesn’tchange;3.Equalobjectsmustproducethesamehashcode,asrequiredforproperfunctioninginhash-b

Aug 08, 2025 pm 01:20 PM

Building Microservices with Java, Spring Boot, and Docker

First, use SpringBoot to create microservices and define REST interfaces, then containerize them through Docker, and finally optimize them according to production-level best practices. 1. Use SpringInitializr to build SpringBoot projects, add Web, Actuator, JPA and other dependencies, write UserController to provide /users interface, run and verify services. 2. Execute mvncleanpackage to build JAR package, create Dockerfile in the project root directory, copy JAR file based on the openjdk:17-jdk-slim image, expose port 8080 and set the startup command, execute doc

Aug 08, 2025 pm 01:07 PM

java microservices

How to upload a file in a Java servlet?

To implement file upload in JavaServlet, you must first use the @MultipartConfig annotation to enable multipart request support, then submit multipart/form-data data through the HTML form, then process file writing in the doPost method, and finally pay attention to safety and exception handling; the specific steps are: 1. Use the @WebServlet and @MultipartConfig annotation to configure the servlet and set temporary directory, file size limit and other parameters; 2. Create an HTML form with enctype="multipart/form-data", including file input fields and text fields;

Aug 08, 2025 pm 12:47 PM

How to create a singleton pattern in Java

Usedouble-checkedlockingwithvolatileforthread-safelazyinitialization;2.Useeagerinitializationiftheinstanceisalwaysneeded;3.Preferstaticinnerclassforclean,thread-safe,andlazy-loadedsingleton;4.Useenumsingletontoensureserializationandreflectionsafety;a

Aug 08, 2025 pm 12:20 PM

How to convert a stack trace to a string in Java

Using StringWriter and PrintWriter is the standard way to convert Java exception stack traces to strings. 1. Capture the complete stack information of exceptions through StringWriter and PrintWriter; 2. Avoid using e.getMessage() or e.toString() because they do not contain stack trace details; 3. If the project uses ApacheCommonsLang, you can directly call ExceptionUtils.getStackTrace(e) to achieve the same effect; this method is suitable for any Throwable type and contains nested exception and cause information, but should only be used when necessary to avoid it.

Aug 08, 2025 pm 12:13 PM

A Guide to Reactive Streams in Java

ReactiveStreamsisaspecification,notalibrary,thatenablesnon-blocking,asynchronousstreamprocessingwithbackpressureinJava.1.Itdefinesfourcoreinterfaces:Publisher,Subscriber,Subscription,andProcessor,availableinJava9 underjava.util.concurrent.Flow.2.Thef

Aug 08, 2025 pm 12:06 PM

java

How to make an asynchronous POST request with HttpClient in Java

To use the HttpClient of Java11 to send an asynchronous POST request, you need to follow the following steps: 1. Create an HttpClient instance, such as HttpClientclient=HttpClient.newHttpClient(); 2. Build an HttpRequest containing URI, header information and request body, such as using POST (HttpRequest.BodyPublishers.ofString(json)) and set Content-Type; 3. Call client.sendAsync(request,HttpResponse.BodyHandle

Aug 08, 2025 am 11:58 AM

How to calculate a factorial in Java?

TocalculateafactorialinJava,usetheiterativeapproachforbetterperformancebyinitializingresultto1andmultiplyingitbyeachintegerfrom1toninaloop.2.Usetherecursiveapproachforclarity,wherethefunctioncallsitselfwithn*factorialRecursive(n-1)andbasecasesreturn1

Aug 08, 2025 am 10:46 AM

How to create and start a thread in Java

TocreateandstartathreadinJava,youcanextendtheThreadclassandoverrideitsrun()method,thencallstart()tobeginexecution;2.Alternatively,implementtheRunnableinterface,passaninstancetoaThreadconstructor,andcallstart();3.PreferimplementingRunnableorusinglambd

Aug 08, 2025 am 10:37 AM

Implementing CQRS and Event Sourcing in a Java Application

CQRS and event traceability can significantly improve the scalability, maintainability and auditing capabilities of Java applications by separating read and write models and storing state changes in event-driven mode; 1. Understand that CQRS separates commands from queries, and event traceability rebuilds state through event streams; 2. Use SpringBoot to integrate AxonFramework 4.6.0 and add H2 and JPA dependencies; 3. Define the aggregate root containing the commands, events and @Aggregate annotation of @TargetAggregateIdentifier; 4. Update the JPA read model in the projection component through @EventHandler; 5. Expose the REST endpoints of commands and queries respectively, and use Comma

Aug 08, 2025 am 10:18 AM

cqrs

Java: How to call non-static methods of an existing class without creating a new object? (Especially for GUI event handling)

This article explains in detail how to call a non-static method of an existing class in Java without creating a new object instance. Especially for graphical user interface (GUI) development, a common problem encountered when an event listener needs to interact with an existing JFrame instance. The article will use instance code to demonstrate how to obtain and operate target objects safely and effectively through constructor injection or using the Inner Class mechanism, avoid creating unnecessary duplicate instances, and ensure the correctness of program logic and effective utilization of resources.

Aug 08, 2025 am 10:12 AM

A Guide to Java Native Image (JNI) and Interoperability

JNIisusedforJavatointeractwithnativeC/C code,enablingintegrationwithexistinglibrariesorsystemAPIs;1.WriteJavaclasswithnativemethodsandloadlibrary;2.Generateheaderwithjavac-h;3.ImplementCfunctionusingJNIconventions;4.Compiletosharedlibrary;5.RunonJVM

Aug 08, 2025 am 10:11 AM