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Why use the `Serializable` interface?

ImplementingtheSerializableinterfaceinJavaallowsaclasstobeconvertedintoabytestreamforstorageortransmission.Asamarkerinterfacewithnomethods,itsignalsthattheclassisreadyforserialization,enablingmechanismslikeObjectOutputStreamtoprocessit.Failingtoimple

Jun 26, 2025 am 01:02 AM

java

How to connect to a database using JDBC?

The key to using JDBC to connect to databases is to correctly configure the driver and URL. 1. Prepare the JDBC driver: Download the corresponding driver package according to the database type, add dependencies through pom.xml for the Maven project, and manually add .jar files to buildpath for ordinary projects; for example, MySQL uses mysql-connector-java. 2. Use the correct URL format: such as jdbc:mysql://localhost:3306/mydatabase?useSSL=false&serverTimezone=UTC, pay attention to the host address, port and parameter settings. 3. Load the driver and establish a connection: it can be passed through Cl

Jun 26, 2025 am 01:01 AM

What is the Singleton pattern?

Singleton pattern is used to ensure that a class has only one instance and provides a global access point. 1. Prevent external instance creation through private constructors. 2. Create a static private instance inside the class. 3. Provide a public static method to obtain the instance. Implementation needs to pay attention to thread safety, such as using double check locking or static internal classes. Advantages include resource saving and unified management, while disadvantages are high coupling, hiding dependence, and complex multi-threading processing.

Jun 26, 2025 am 01:01 AM

Design Patterns

What is TestNG?

TestNG is a Java-based test framework, mainly used for automated testing. It is more powerful and flexible than JUnit, and is suitable for various scenarios such as unit testing, integration testing, etc. Its core features include: 1. Supports multiple test types; 2. Powerful annotation system; 3. Supports concurrent execution; 4. Parameterized testing; 5. Test grouping and dependency management; 6. Produced report generation function. Compared with JUnit, TestNG has a more flexible annotation mechanism, naturally supports dependency testing, and is more suitable for automated testing projects. To start using TestNG, you can follow these steps: 1. Add Maven dependencies; 2. Write test classes with annotations; 3. Run tests through the IDE or command line; 4. View the generated HT

Jun 26, 2025 am 12:59 AM

What is the equals method contract?

In Java, five rules must be followed when overriding the equals() method: reflexivity, symmetry, transitiveness, consistency and non-emptiness. 1. Reflexivity requires that the object compares with itself returns true; 2. Symmetry ensures that the results of x.equals(y) and y.equals(x) are consistent; 3. Transmission requirements: If x.equals(y) and y.equals(z) are true, then x.equals(z) should also be true; 4. Consistency ensures that the result of the same object calls equals() multiple times unchanged; 5. Non-empty stipulates that the object compares with null should return false. In addition, hashCode() must be overwritten at the same time when overwriting equals() to ensure that the phase

Jun 26, 2025 am 12:59 AM

What is a weak reference?

Weak references are a method that points to an object without increasing its reference count, which is mainly used to avoid memory leaks caused by circular references or unnecessary object retention. It is suitable for situations where you want to attach information to objects that have no control over their life cycle, such as cache systems, event listener registrations, and mapping additional data without having to own object ownership. In Python, weak references can be created through the weakref module, for example, using weakref.ref(obj); when the original object is deleted, the weak reference will return None. Not all objects support weak references. Some built-in types such as list or dict need to be explicitly enabled, while primitive types such as integers or strings may show different behaviors due to the internal caching mechanism. Pay attention to when using:

Jun 26, 2025 am 12:57 AM

What are intermediate stream operations?

IntermediatestreamoperationsinJavaaremethodsthattransformorfilterdatawithoutproducingafinalresult.Theseoperations,suchas1.filter(),2.map(),3.sorted(),4.limit(),5.skip(),and6.distinct(),returnanewStream,enablingmethodchaining.Theyarelazyandonlyexecute

Jun 26, 2025 am 12:56 AM

How to read a file in Java?

There are three common ways to read files in Java. First, use BufferedReader to read line by line, which is suitable for large files. The steps include creating a FileReader, wrapping it into a BufferedReader, reading and closing the stream with readLine(); Second, use Files.readAllLines() to read the content of the small file into the list at one time; Third, use Scanner to read and parse the data as needed, which is suitable for structured text. In addition, pay attention to issues such as path setting, resource closing and encoding specification. The selection method should be determined based on specific needs such as file size and processing methods.

Jun 26, 2025 am 12:48 AM

What is tight coupling vs loose coupling?

Tight coupling refers to the existence of a strong dependency between modules, such as the class directly instantiates another concrete class, resulting in multiple adjustments required to modify one place; loose coupling refers to reducing dependencies through interfaces, abstract classes, etc., improving flexibility and maintainability. 1. The phenomenon of tight coupling includes directly instantiating specific classes, calling dependency specific implementations, and altering involves multiple modules; 2. The loose coupling implementation methods include using interfaces or abstract classes, dependency injection, event-driven communication, and API calls to replace direct references; 3. Selection based scenarios: tight coupling is suitable for small projects, performance sensitive, and module stability, and loose coupling is suitable for complex systems, team collaboration, and scenarios that require flexible expansion.

Jun 26, 2025 am 12:43 AM

Why use generics?

The core role of generics is to solve the problems of code reuse and type safety. 1. Improve code reusability: Only once can you write logic through generics to adapt to multiple types, such as function identity(value:T):T can handle any type input; 2. Ensure type safety: Compared with any type, generics ensure that the incoming and return type are consistent, such as function firstElement(arr:T[]):T|undefined can accurately deduce array element types; 3. Better development experience: generics combine with IDE to provide automatic completion and type prompts, such as encapsulating request function fetchData(url:string):Promise can clearly return structures, improving development efficiency.

Jun 25, 2025 pm 06:22 PM

Generics Generics

How to monitor JVM performance?

Monitoring JVM performance requires paying attention to core indicators such as memory, GC, threading and class loading. 1. Use built-in tools such as jstat, jmap, jstack to quickly troubleshoot problems, such as jstat-gc1000 to view GC frequency and time consumption in real time; 2. Use VisualVM, JConsole or Prometheus Grafana to perform visual monitoring, centrally display key indicators and facilitate team collaboration; 3. Set an alarm mechanism to pay attention to heap memory usage, GC pause time, thread status and class loading number, and promptly detect exceptions; 4. Combine GC log analysis and heapdump, thread snapshot and other information to locate performance bottlenecks and gradually optimize.

Jun 25, 2025 pm 06:05 PM

monitor jvm performance

What is a thread in Java?

AthreadinJavaisthesmallestunitofexecutionthatallowsconcurrentoperations,enhancingapplicationresponsivenessandefficiency.Itrunscodeindependentlyfromthemainprogram,enablingtaskslikebackgroundprocessingandmultitasking.Javaprogramsstartwithamainthread,bu

Jun 25, 2025 pm 06:03 PM

Multithreading java thread

Why use the `synchronized` keyword?

The synchronized keyword is used to ensure that the access to shared resources in a multi-threaded environment is thread-safe. 1. It prevents multiple threads from executing the same piece of code at the same time by locking objects or classes, thereby avoiding the problems of race conditions and data inconsistency; 2. It prevents memory consistency errors, forces threads to read and write data from main memory, and ensures visibility between threads; 3. It automatically manages the acquisition and release of locks, simplifies lock control in concurrent programming, and is suitable for most simple mutually exclusive scenarios.

Jun 25, 2025 pm 05:14 PM

Thread safety

What are the different wrapper classes?

The wrapper class in Java encapsulates the basic data type into an object, so that the basic type has object characteristics. Its core uses include: 1. Used for collection frameworks (such as ArrayList, HashMap storage objects); 2. Provide practical methods (such as Integer.parseInt); 3. Support null values ??to represent "no value" state; 4. Used in generics. Java 5 supports automatic boxing and unboxing, but attention should be paid to null pointer exceptions and performance overhead. Common methods include string conversion, obtaining maximum/minimum value, converting to strings and comparing operations, etc., which are commonly found in set operations, generic programming and potentially empty data processing scenarios.

Jun 25, 2025 pm 05:02 PM

java Packaging