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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

What is `BufferedReader`?

BufferedReader is used in Java to efficiently read text streams, and its core advantage is that the buffering mechanism reduces the number of I/O operations. 1. Reason for use: improve reading speed, provide readLine() method for easy line-by-line processing, and is compatible with other Reader classes. 2. Usage method: Use FileReader to create instances, read content by looping call readLine(), and it is recommended to use try-with-resources to manage resources. 3. Avoid scenarios: Not suitable for scenarios where binary data, small files require low performance, or complex parsing is required. In addition, it is not thread-safe and needs to be synchronized by itself in a multi-threaded environment.

Jul 01, 2025 am 01:00 AM

What is the Strategy pattern?

A policy pattern is a behavior design pattern, which is used interchangeably by encapsulating different algorithms or behaviors as independent classes. The core is to extract the changing parts so that the client can dynamically select specific policies without modifying the original logic when running. The policy pattern consists of three parts: the policy interface defines the public method; the specific policy class implements the interface; the context class holds policy references and provides a unified call portal. Applicable scenarios include: there are multiple implementation methods for the same behavior, no logic is wanted to be changed when replacing the behavior, and multiple similar types are only different in behavior. When using it, you should pay attention to avoid abuse, control the number of policies, keep the context and policy relationship clear, and recommend stateless policy classes to improve reusability.

Jul 01, 2025 am 12:57 AM

What is Javadoc?

Javadoc is a tool in Java for generating API documents, describing code elements through/annotations and tags such as @param and @return. Using javadoc commands or IDEs can quickly generate HTML documents, which helps team collaboration and improves code maintainability and professionalism. 1. Comments start with / and use tags to describe parameters, return values, etc.; 2. Use javadoc command or IDE to generate documents; 3. Mainly used for interface description, improve development efficiency, and publish as part of the SDK.

Jul 01, 2025 am 12:44 AM

javadoc Document generation

What is a `Statement` object?

AStatementobjectinJavaisusedtoexecuteSQLqueriesandcommandswithadatabaseviaJDBC.1.ItallowsexecutionofSELECT,INSERT,UPDATE,DELETE,andDDLstatements.2.ItiscreatedusingthecreateStatement()methodofaConnectionobject.3.ResultSethandlesqueryresultsbyiterating

Jul 01, 2025 am 12:37 AM

jdbc

What is the `import static` statement?

importstaticinJavaallowsaccessingstaticmemberswithoutclassqualification.1.ItsimplifiescodereadabilitywhenusingconstantslikeMathUtils.PIbecomingPI.2.Itappliestoutilitymethods,suchaslog(),reducingverbosity.3.SelectiveimportsusingasListinsteadof*prevent

Jul 01, 2025 am 12:14 AM

java

How to prevent deadlock?

The key to preventing deadlocks is to understand the cause of it and adopt reasonable strategies, which mainly includes four points: 1. Understand the four necessary conditions for deadlocks (mutex, holding and waiting, not preemptive, and loop waiting). Breaking one of them can prevent deadlocks; 2. Avoid loop waiting. The common practice is to unify the order of resource requests, such as all threads apply for resources in order A→B; 3. Set a timeout mechanism or use tryLock() and try to obtain resources to avoid unlimited waiting; 4. Use tools such as jstack, VisualVM, etc. to detect and monitor potential deadlocks, and combine stress testing and log analysis to find problems in advance.

Jul 01, 2025 am 12:11 AM

How to handle null with Optional?

Use Optional to avoid abuse and is suitable for return values ??rather than intermediate variable processing. 1. Do not use Optional for local variable judgment, such as userOpt.isPresent(), which increases overhead; 2. It is recommended to use it as a method return type to clearly express the "possibly resultless" semantics, such as findUserById returns Optional; 3. Use map and filter to simplify object chain calls, such as using chain map to avoid NullPointerException when obtaining user.getAddress().getCity(), and use orElse to provide default values; 4. Avoid wrapping basic types or collection classes because they

Jun 30, 2025 am 01:35 AM