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Difference between soft weak and phantom references?

The difference between soft citations, weak citations and virtual citations in Java is their life cycle and purpose. 1. Soft References are used for cache and will be recycled when there is insufficient memory; 2. Weak References are recycled in the next GC and are suitable for short-term object tracking; 3. PhantomReferences cannot be retrieved and are only used for cleaning operations before object recycling. They are suitable for memory management requirements in different scenarios.

Jul 02, 2025 am 01:30 AM

What is a multidimensional array?

A multi-dimensional array is essentially an array of arrays that can store data from multiple dimensions. 1. It is suitable for representing complex data structures such as tables and matrices; 2. Two-dimensional arrays are common and shaped like grids, such as using rows to represent students and columns to represent subject scores; 3. Practical applications include game development, image processing, scientific computing and other fields; 4. When using them, you need to pay attention to the differences in index range and language implementation.

Jul 02, 2025 am 01:29 AM

What is the Decorator pattern?

The decorator pattern is used to dynamically add new functionality to objects, and its core lies in flexible expansion through composition rather than inheritance. When you need to combine functions in different ways (such as encryption, compressed messages), avoid code confusion caused by factor explosions. The decorator mode implements function overlay by wrapping the original objects layer by layer, while maintaining a unified interface. The specific steps are: 1. Define the public interface or base class (such as IMessage); 2. Create basic components (such as TextMessage); 3. Build abstract decorator classes, hold component references and implement the same interface; 4. Implement specific decorators (such as EncryptedMessageDecorator, CompressedMessageDecorator)

Jul 02, 2025 am 01:29 AM

Design Patterns

What is the Observer pattern?

The Observer pattern is a design pattern used to establish one-to-many dependencies between objects. Its core is that when an object's state changes, all dependencies (Observers) will automatically receive notifications and updates. This pattern contains four key components: Subject maintains the observer list and sends notifications; Observer defines the interface to receive updates (such as the update() method); ConcreteSubject implements specific notification logic; ConcreteObserver implements specific response behavior. Common application scenarios include user interface updates, event processing systems, and service notifications in distributed systems. The implementation steps are: 1. Define Obs

Jul 02, 2025 am 01:28 AM

How to generate Javadoc?

The key to generating Javadoc is to write good comments and master the basic commands and formats. 1. First, use the standard Javadoc comment block (starting with /* and ending with /) in the code, place it in front of the class, method, and field, and include summary sentences; 2. Use common tags such as @param, @return, @throws to describe parameters, return values ??and exceptions; 3. Use javadoc commands to generate documents through the command line. Common parameters include -d specifying the output directory, -author displaying the author, and -version displaying the version; 4. You can use the IDE to automatically generate documents, which is more convenient to operate. As long as you comment on specifications and be familiar with commands, you can easily generate clear API documents.

Jul 02, 2025 am 01:28 AM

How are equals and hashCode related?

Equals and hashCode must be rewritten at the same time, because if the two objects equals, their hashCodes must be the same, otherwise there will be a problem that the object cannot be located in HashMap or HashSet. 1. Both need to be rewritten based on the same field; 2. HashCode determines the storage location, and equals determines whether it matches; 3. After modifying the object field, it may cause inconsistent hash values, so try to use immutable fields to calculate hash; 4. Using Objects.hash can simplify the implementation and avoid null pointer exceptions.

Jul 02, 2025 am 01:27 AM

What is annotation in Java?

AnannotationinJavaisaformofmetadatathatprovidesinformationaboutthecodewithoutdirectlyaffectingitsexecution.1)Annotationslike@Override,@Deprecated,and@SuppressWarningsgiveinstructionstocompilersorframeworks.2)Theyreduceboilerplateconfiguration,improve

Jul 02, 2025 am 01:26 AM

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