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What are sealed classes and interfaces in Java?

Sealing classes and interfaces enhances control over inheritance in Java by restricting which classes or interfaces can inherit or implement them. The problem is that before introducing sealed classes, any class can inherit non-final classes, and any class can implement interfaces, resulting in potential security and maintenance issues. When using sealed classes, developers must explicitly declare a list of subclasses that allow inheritance or implementation, such as publicsealedclassShapepermitsCircle, Rectangle, Triangle{}, ensuring that only the specified class can be extended or implemented. Key rules include: All allowed subclasses must be in the same module or package and must be explicitly declared. Sealing class is suitable for the need to restrict inheritance and construction.

Jul 12, 2025 am 02:43 AM

The role of the 'transient' keyword in Java

ThetransientkeywordinJavapreventsspecificfieldsfrombeingserialized.1.Itisusedtoexcludesensitivedata(e.g.,passwords),temporaryvalues,orfieldsthatcanberecomputedafterdeserialization.2.Fieldsmarkedastransientareskippedduringserializationandresettodefaul

Jul 12, 2025 am 02:41 AM

java

What is method hiding in Java?

MethodhidinginJavaoccurswhenasubclassdefinesastaticmethodwiththesamenameandparametersasastaticmethodinitssuperclass.1.Itonlyappliestostaticmethods,notinstancemethods.2.Themethodcallisdeterminedbythereferencetypeatcompiletime,nottheobjecttypeatruntime

Jul 12, 2025 am 02:33 AM

java Method hidden

Practical Use Cases for Java Reflection API

The core uses of the JavaReflection API include dynamic creation of objects and calling methods, implementing common frameworks and libraries, and testing private members and methods. 1. Dynamically create objects and call methods can be loaded through Class.forName(), getDeclaredConstructor().newInstance(), and invoke() call methods; 2. General frameworks such as Spring and Hibernate use reflection to implement dependency injection and database mapping; 3. Private members can be accessed through reflection in unit tests, but abuse should be avoided in production code. Rational use of reflection can improve code flexibility and scalability, but also pay attention to performance and security

Jul 12, 2025 am 02:32 AM

What are Java Streams?

JavaStreamsprovideadeclarativewaytoprocesscollections.1.Theyallowoperationslikefiltering,mapping,sorting,andreducing.2.Streamsdonotstoredatabutcarryelementsthroughapipelineofoperations.3.Intermediateoperationsreturnstreamsandcanbechained,whiletermina

Jul 12, 2025 am 02:24 AM

How does Java class loading work (Bootstrap, Extension, System ClassLoaders)?

Java's class loading mechanism is composed of Bootstrap, Extension and SystemClassLoaders, and uses a parent delegation model to ensure the security and unique loading of classes. BootstrapClassLoader is responsible for loading core class libraries such as rt.jar; ExtensionClassLoader loads extension libraries such as jar under jre/lib/ext; SystemClassLoader loads classes on the application classpath. When the class is loaded, the parent loader is given priority to find the class. If none of them are found, a ClassNotFoundException will be thrown. Understanding this mechanism can help troubleshoot conflicts, avoid security risks and improve

Jul 12, 2025 am 02:24 AM

Java ExecutorService example

How to create and use ExecutorService for Java? 1. Use the Executors factory class to create a thread pool, such as newFixedThreadPool(4) to create a fixed-size thread pool; 2. Submit a task without return value through execute(), or submit a task with return value and get the Future result; 3. When closing the thread pool, call shutdown() first, cooperate with awaitTermination() to wait for the task to complete, and call shutdownNow() to interrupt execution if necessary.

Jul 12, 2025 am 02:09 AM

How to profile CPU and memory usage of a Java application?

To understand the CPU and memory usage of Java applications, you can use the following methods: 1. Use VisualVM to view real-time performance data, including heap memory, GC situation and thread analysis; 2. Use jstat and jmap command line tools to diagnose GC behavior and generate heap snapshots; 3. Add monitoring logic to the code to estimate memory changes. These methods are applicable to graphical interface debugging, server environment inspection and specific logical observations, and can be flexibly selected according to the actual scenario.

Jul 12, 2025 am 02:08 AM

Java Performance Analysis CPU memory analysis

Difference between primitive and reference types?

JavaScript's data types are divided into primitive types and reference types, and the core difference is the storage method and assignment behavior. Primitive types include string, number, boolean, null, undefined, symbol, and bigint, which are immutable and passed by value, such as leta=10;letb=a; modifying b does not affect a. Reference types such as objects, arrays, and functions are mutable and passed by reference. For example, letobj1={name:"Tom"}; letobj2=obj1; Modifying obj2.name will affect obj1.name. Typeof can be used to determine the type, but please note that n

Jul 12, 2025 am 02:08 AM

What is an exception in Java?

AnexceptioninJavaisaneventthatdisruptsthenormalflowofaprogram,oftencausedbyprogrammingerrorsorexternalissues.1)ExceptionscanresultfrommistakeslikeArrayIndexOutOfBoundsExceptionorNullPointerException.2)Theycanalsostemfromexternalproblemssuchasmissingf

Jul 12, 2025 am 02:07 AM

What is garbage collection in Java?

GarbageCollection(GC)inJavaisanautomaticmemorymanagementprocessthatidentifiesandremovesunusedobjectstofreeupmemory.1)GCworksbydeterminingobjectreachabilityfromGCrootssuchasactivethreads,staticfields,andlocalvariables.2)Unreachableobjectsaremarkedforc

Jul 12, 2025 am 02:04 AM

Understanding the Java volatile Keyword Usage

The volatile keyword in Java often feels a bit abstract, especially for those who are new to concurrent programming. In fact, its function is very clear: to ensure the visibility of variables between multiple threads. That is to say, when one thread modifies the variable value modified by volatile, other threads can see the change immediately. It is not a master key to solve all concurrency problems, but it is very useful in some scenarios. Let’s take a look at how to use it and where it is suitable for use. When do you need to use volatile? The most typical application scenario is the status flag, such as controlling whether the thread continues to run: privatevolatilebooleanrunning=true;

Jul 12, 2025 am 01:50 AM

java volatile

What is the strictfp keyword in Java?

ThestrictfpkeywordinJavaensuresconsistentfloating-pointresultsacrossplatformsbyenforcingIEEE754compliance.1.Itappliestoclasses,interfaces,andmethods,restrictingintermediatecalculationstostandardprecision.2.Withoutstrictfp,JVMsmayusehigher-precisionre

Jul 12, 2025 am 01:44 AM

wait() vs sleep() in Java multithreading

The main difference between sleep() and wait() is the purpose and lock handling. 1.sleep() is a static method of the Thread class, used to pause thread for a period of time without releasing locks; it is suitable for simulation delays and other scenarios. 2. wait() is an instance method of the Object class. It must be used in synchronized. It will release the lock and wait for notifications from other threads; it is suitable for thread collaboration such as the producer-consumer model. 3.sleep() does not rely on synchronous blocks and does not require notify to wake up, while wait() must be waken by notify or notifyAll. 4. Both need to catch InterruptedException, but wait() needs to prevent virtuality

Jul 12, 2025 am 01:43 AM

java multithreading