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What are the three class loaders?

The three main class loaders in Java are BootstrapClassLoader, ExtensionClassLoader and ApplicationClassLoader, which form the parent delegation model. 1.BootstrapClassLoader is the top-level class loader, implemented by C/C, responsible for loading the Java core class library (such as rt.jar), located in the jre/lib directory, which cannot be directly accessed by users; 2.ExtensionClassLoader is its subclass loader, responsible for loading the extended class library under the jre/lib/ext path, which can be used through ClassLoader.getS

Jul 02, 2025 am 01:07 AM

How to use arithmetic operators?

Arithmetic operators include addition, subtraction, multiplication, division and modulus, and are used for basic mathematical calculations. 1. Addition, subtraction, multiplication and division are represented by , -, and /, but different languages ??handle division results in different ways. For example, Python returns floating point numbers, C or Java returns integers; 2. The modulus operation is represented by % and returns remainder, which can be used to judge parity or loop control; 3. Compound assignment operators such as = and = can simplify code and improve readability; 4. The operation sequence follows priority rules, and the logical order can be adjusted by brackets to improve clarity and maintenance.

Jul 02, 2025 am 01:06 AM

How to declare a String?

Declaring strings differ slightly in different programming languages, but the core idea is to wrap text in quotes and assign values ??to variables. For example: 1. Java needs to explicitly declare types, such as Stringname="Hello"; 2. Python and JavaScript do not require type declarations, and write them as name="Hello" and letname="Hello" respectively; 3. Most languages ??allow single or double quotes, but only double quotes can be used in Java and C#; 4. When strings contain quotes, they can be escaped by backslashes or alternately single and double quotes to improve readability; 5. Common errors include forgetting to add quotes and mixing

Jul 02, 2025 am 01:05 AM

How to implement Singleton?

The core of singleton pattern is to ensure that a class has only one instance and provide a global access point. 1. The basic lazy style may lead to multiple instance creation in a multi-threaded environment, which is not suitable for concurrent scenarios; 2. Although adding a synchronous lock ensures thread safety, locking affects performance every call; 3. Double check locking is optimized through pre-locking judgment and volatile keyword optimization, which is the current recommended method; 4. The Hungry style initializes instances during class loading, which is simple and reliable, but does not support delayed loading. In addition, it is necessary to pay attention to the privatization of constructors, prevent reflection attacks and serialization issues. Choose appropriate implementation methods in different scenarios. In most cases, double check locking is enough.

Jul 01, 2025 am 01:31 AM

What is aspect-oriented programming AOP concept?

AOP(Aspect-OrientedProgramming)increasesmodularitybyseparatingcross-cuttingconcernslikelogging,security,andtransactionmanagementfrombusinesslogic.1.Itkeepsmaincodecleanbycentralizingrepetitivebehaviors.2.Aspectsencapsulatereusablelogicthatwrapsaround

Jul 01, 2025 am 01:31 AM

What is `ZonedDateTime`?

ZonedDateTime is used in Java to handle dates and times with time zones, and supports scenarios such as "January 1, 2025 at 3 pm Tokyo time". 1. An instance of the current system time and default time zone can be obtained through ZonedDateTime.now(); 2. Use ZonedDateTime.of() to create an instance of the specified date, time and time zone; 3. Use LocalDateTime.atZone() to convert LocalDateTime without time zones to ZonedDateTime with time zones. It can automatically handle time zone differences such as daylight saving time, such as using the withZoneSameInstant() method to implement it

Jul 01, 2025 am 01:29 AM

How to achieve loose coupling?

The key to achieving loose coupling is to reduce direct dependence between modules and improve the maintainability, scalability and testability of the system. 1. Use interfaces or abstract classes to define dependencies, so that the caller only relies on abstraction rather than concrete implementation, so that the implementation is replaced without affecting the calling logic; 2. Introduce event-driven or message mechanisms to enable modules to communicate through events or messages, reducing synchronous dependencies; 3. Control the direction of dependency and follow the principle of dependency inversion. Both high-level and low-level modules rely on abstraction to maintain the stability of core logic; 4. Reasonably divide the boundaries of modules and divide modules according to business capabilities to avoid confusion of responsibilities and ensure independent changes of modules.

Jul 01, 2025 am 01:28 AM

What are comparison operators?

Comparisonoperatorsevaluaterelationshipsbetweentwovalues,returningtrueorfalse.Theyareusedtocheckequality(==),inequality(!=),greaterthan(>),lessthan(=,

Jul 01, 2025 am 01:27 AM

What is a class in Java?

In Java, classes are blueprints or templates of objects that define the behavior and properties of objects. 1. The class contains variables (fields) to store data; 2. The method defines the object behavior; 3. The constructor is used to initialize the object; 4. The access modifier controls the access method of members. For example, the Car class may contain color and speed fields, accelerate method, and constructor. Create an instance of the class through the new keyword, such as CarmyCar=newCar(30);, each instance runs independently to implement the encapsulation and reuse of data and logic.

Jul 01, 2025 am 01:24 AM

What is a thread local variable?

Thread local variables are used to provide independent data copies for each thread, avoiding data competition and synchronization problems between multiple threads. Its core uses include: 1. Avoid synchronization overhead in multi-threaded programs; 2. Storing user session information in web applications; 3. Passing context information without using method parameters. Its underlying implementation relies on a mapping table maintained by each thread. The key is ThreadLocal instance and the value is thread-specific data. Note when using: 1. Call remove() manually after use to prevent memory leakage; 2. Legacy state may be left in the thread pool environment, and 3. It is not advisable to use it excessively to avoid increasing code complexity and testing difficulty.

Jul 01, 2025 am 01:24 AM

How to perform unit testing in Java?

Unit testing is crucial to Java code quality. Use JUnit5 as the mainstream framework to introduce dependencies through Maven or Gradle and write test cases; tests should cover normal processes, boundary values, error input and exception handling; use Mockito to simulate external dependencies to avoid real calls; follow clear naming specifications and organizational structures, such as "Method Name_Scenario_Expected Behavior", and place the test class under src/test/java to maintain the package structure consistent with the tested class; insist on writing tests simultaneously in development to improve efficiency.

Jul 01, 2025 am 01:21 AM

What is JVM tuning?

JVMtuning improves Java application performance and stability by adjusting JVM parameters. The core lies in optimizing memory, reducing GC frequency and pauses. 1. Set reasonable heap memory (-Xms and -Xmx are equal) to avoid OOM and frequent capacity expansion; 2. Select GC algorithms according to the business, such as G1 or ZGC; 3. Fine-tune GC parameters and combine monitoring tools to continuously optimize, while paying attention to resource limitations and logging.

Jul 01, 2025 am 01:18 AM

Difference between static and instance blocks?

In Java, the main difference between static blocks and instance blocks is execution timing and purpose. Static blocks run once when the class is first loaded into memory. They are used to initialize static variables or perform one-time setup tasks, and cannot access instance variables; instance blocks run every time a new instance of the class is created. They are located before the constructor. They can be used for the initialization logic shared by multiple constructors, and can access static and instance variables. In execution order, static blocks are executed before the instance block, and the instance block is before the constructor. Therefore, when multiple objects are created, the static blocks are executed only once, while the instance block and the constructor are executed with each object creation.

Jul 01, 2025 am 01:17 AM

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What is a Set?

Sets are the basic structures composed of different objects in mathematics, and their core characteristics are the disorder and uniqueness of elements. 1. The elements in the set have no order, and each element can only appear once; 2. The set is usually represented by uppercase letters, and the elements are represented by lowercase letters or other symbols. Common representation methods include enumeration and description; 3. Common set operations include union (merging and deduplication), intersection (common elements), difference (removing part of the elements) and subsets (including relationships); 4. Collections are widely used in practice, such as object classification, Set data structures in programming languages, and database query scenarios.

Jul 01, 2025 am 01:15 AM

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