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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

java Static blocks

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

gather math

What is an if statement?

Anifstatementisaprogrammingcontrolstructurethatexecutescodebasedonaconditionbeingtrue.1.Itallowsprogramstomakedecisions,suchasgrantingaccessifapasswordiscorrectorconvertinganegativenumbertopositive.2.Thebasicsyntaxchecksacondition,andiftrue,runstheas

Jul 01, 2025 am 01:15 AM

What is the method area?

The method area is part of JVM memory and is used to store class-level data. It contains class structure, static variables, method and field signatures, and a runtime constant pool for each class. Unlike the heap, the method area mainly stores class information rather than object instances. Since Java 8, the traditional method area (PermGen) has been removed, replaced by a Metaspace stored in local memory, whose size can automatically grow and can be restricted by -XX:MaxMetaspaceSize. The method area also performs garbage collection, but is less frequent, and usually occurs when the class is unloaded. Common triggering scenarios include web application deployment, dynamic scripting language usage, and dynamic generation applications.

Jul 01, 2025 am 01:12 AM

What is the `main` method signature?

The entry point of a Java program must be the main method of the standard signature. 1. It must be public so that the JVM can access externally; 2. It must be static because the JVM calls it before creating the class instance; 3. The return type must be void; 4. The parameter must be String[] or String.... If one of these elements is missed or changed, the program will fail to run with an error. Although there can be multiple overloaded main methods, only methods that meet the signature will be recognized by the JVM as the main entry point.

Jul 01, 2025 am 01:10 AM

java main method

What is a `ResultSet`?

ResultSet is the result set returned after executing a database query. Taking JDBC as an example, its workflow includes: 1. When initializing the ResultSet, pointing to the first row; 2. Calling .next() to move to the first row; 3. Use .getString(), .getInt() and other methods to obtain column data; common errors include not calling .next(), accessing non-existent columns, and not closing the ResultSet; when using it, you should prioritize column names rather than indexes, pay attention to large data processing, check whether the result set is empty, and ensure resource release through try-with-resources or finally blocks.

Jul 01, 2025 am 01:08 AM

How to import packages?

If you want Python programs to use external functions, the most direct way is to import packages. The core methods include: 1. Use the import module name to import the entire module, such as importmath; 2. Use the import function from the module to import specific content, such as frommathimportsqrt; 3. Use alias to simplify the code, such as importnumpyasnp; 4. If the module is not recognized, check the running environment, restart the IDE or manually add the path; 5. Customize the module to ensure the reasonable file structure and pay attention to avoid loop import problems.

Jul 01, 2025 am 01:06 AM

What is a method reference?

Method reference is a concise syntax in Java, used to directly refer to methods without calling them, and is often used in functional programming scenarios such as stream operations or Lambda expressions. The core of it is to use the :: operator, such as System.out::println instead of item->System.out.println(item). There are four main types: 1. Reference static methods (such as Integer::valueOf); 2. Reference instance methods of specific objects (such as System.out::println); 3. Reference instance methods of any object (such as String::length); 4. Reference constructors (such as ArrayList:

Jul 01, 2025 am 01:03 AM

java Method reference

What is the hashCode method contract?

Whenoverridingtheequals()methodinJava,youmustalsooverridehashCode()tomaintainthecontractthatequalobjectsmusthaveequalhashcodes.Thisensuresproperbehaviorinhash-basedcollectionslikeHashMapandHashSet.Therulesincludeconsistencyofhashcodesforunmodifiedobj

Jul 01, 2025 am 01:00 AM