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What is an object?

Objects have different meanings in different contexts: 1. In daily language, refer to perceived objects such as chairs and mobile phones; 2. In programming, structures containing data and functions, such as Car class examples in Python; 3. In grammar, it is the action bearer such as the ball in "kicking the ball"; 4. In philosophy or science, it is the research object such as cells or time.

Jul 02, 2025 am 01:24 AM

What is the interface segregation principle?

Interface Isolation Principle (ISP) requires that clients not rely on unused interfaces. The core is to replace large and complete interfaces with multiple small and refined interfaces. Violations of this principle include: an unimplemented exception was thrown when the class implements an interface, a large number of invalid methods are implemented, and irrelevant functions are forcibly classified into the same interface. Application methods include: dividing interfaces according to common methods, using split interfaces according to clients, and using combinations instead of multi-interface implementations if necessary. For example, split the Machine interfaces containing printing, scanning, and fax methods into Printer, Scanner, and FaxMachine. Rules can be relaxed appropriately when using all methods on small projects or all clients.

Jul 02, 2025 am 01:24 AM

Difference between extending Thread and implementing Runnable?

There are two ways to create threads in Java: inheriting the Thread class and implementing the Runnable interface. Their differences are mainly reflected in the following three points. 1. Whether multiple inheritance is supported: Using Runnable can avoid single inheritance restrictions, so that classes can still inherit other classes; 2. Resource sharing and collaboration: Runnable facilitates multiple threads to share the same task object, while inheriting Thread is difficult to implement this function; 3. Separation of responsibilities: Runnable better realizes the decoupling of tasks and execution, improves the scalability and testability of code, and is suitable for the needs of modern concurrent programming.

Jul 02, 2025 am 01:20 AM

runnable thread

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