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Difference between `throw` and `throws`?

InJava,throwisusedtomanuallytriggeranexceptionwithinamethod,whilethrowsdeclaresexceptionsamethodmightpropagate.1.throwisusedinsideamethodbodytoexplicitlythrowasingleexception,stoppingfurtherexecutionuntilcaught.2.throwsappearsinthemethodsignaturetoli

Jun 30, 2025 am 01:24 AM

What is abstraction?

Abstraction is a way of thinking that simplifies complex things. It helps us understand and deal with problems by ignoring details and grasping core features. For example, "mobile phone" represents a type of equipment that can make phone calls, send messages, and surf the Internet, rather than a specific brand or model; the "fruit" classification in supermarkets covers apples, bananas, etc., reflecting the process of ignoring individual differences and extracting common points; when defining "cars" in programming, only key attributes such as color, brand, and speed are retained; pattern helps manage complex systems, such as driving, only mastering the throttle brake logic without understanding the principles of the engine; during the learning process, abstract ability is reflected in identifying the commonalities of language structures and summarizing the rules of English grammar; practicing abstraction can start from small things such as organizing items and summarizing commonalities, and gradually improve the refining of simple structures from complexity;

Jun 30, 2025 am 01:22 AM

What is a Map?

Amapisavisualtoolthatrepresentsgeographicareasandhelpspeopleunderstandspatialrelationshipsandnavigateeffectively.Itcomesinvarioustypes,includingtopographicmapsshowingelevation,politicalmapshighlightingborders,thematicmapsdisplayingspecificdatalikecli

Jun 30, 2025 am 01:22 AM

How to use `LocalTime`?

LocalTime instances can be created by obtaining the current time, specifying the specific time, or parsing the string, such as LocalTime.now(), LocalTime.of(14,30) and LocalTime.parse("15:45"); the operation time needs to be generated by plusHours(), minusMinutes() or with() to achieve addition and subtraction or adjustment of specific parts; it is suitable for representing the time points of daily repetition, but does not contain date and time zone information. The isBefore() or isAfter() method should be used when comparing.

Jun 30, 2025 am 01:16 AM

Difference between `wait` `notify` and `notifyAll`?

Thedifferencebetweenwait(),notify(),andnotifyAll()liesinhowtheycoordinatethreads.1.wait()makesathreadreleasethelockandpauseuntilnotified.2.notify()wakesuponewaitingthread,chosenbytheJVM.3.notifyAll()wakesupallwaitingthreads,lettingthemcompeteforthelo

Jun 30, 2025 am 01:12 AM

What is a EAR file?

EAR files are packaging formats used in JavaEE to deploy enterprise-level applications. The structure includes 1. One or more WAR files (Web applications), 2. One or more JAR files (such as EJB modules, general libraries), 3. An application.xml deployment description file; the difference from WAR/JAR is that WAR is dedicated to web applications, and JAR is more general, while EAR is a higher-level packaging method that integrates multiple modules; it is commonly found in traditional enterprise systems, especially in application servers such as IBMWebSphere and OracleWebLogic. It is suitable for scenarios where multi-module centralized deployment, unified management services, and inter-module isolation and resource sharing.

Jun 30, 2025 am 01:10 AM

What is a ConcurrentHashMap?

ConcurrentHashMap is a thread-safe hash table implementation, suitable for efficient concurrent reading and writing in multi-threaded environments. 1. It improves concurrency performance through segmented lock (JDK1.7) or CAS synchronized (JDK1.8) mechanism, allowing multiple threads to operate different parts of data without blocking; 2. The usage method is the same as HashMap, and common operations such as put, get, remove, etc. are thread-safe; 3. Note that putIfAbsent and other methods should replace non-atomic judgments and insert operations to ensure thread safety; 4. No exceptions were thrown during traversal, but the results may be non-real-time, and are suitable for scenarios such as cache, counters and shared state management.

Jun 30, 2025 am 01:08 AM

What is a thread pool?

Athreadpoolimprovesperformancebyreusingthreadstoexecutetasks.Insteadofcreatinganddestroyingthreadsforeachtask,apoolmaintainsidlethreadsthatpickuptasksfromaqueue,reducingoverheadandresourceconsumption.Keybenefitsincludeimprovedperformance,betterresour

Jun 30, 2025 am 01:08 AM

What are terminal stream operations?

Terminal operations are the last step in producing results or side effects in the JavaStream pipeline, which trigger the actual processing of the stream. Common terminal operations include: 1.forEach is used to perform operations on each element; 2.collect collects stream elements into a collection; 3. reduce merges elements into a single value; 4. findFirst/findAny returns the first or any element that matches; 5. allMatch/anyMatch/noneMatch checks whether the element meets the conditions; 6.count returns the number of elements in the stream; 7.toArray converts the stream into an array. Note when using: the stream can only be consumed once, some operations such as findFirst or

Jun 30, 2025 am 01:03 AM

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What is the `transient` keyword?

ThetransientkeywordinJavameansavariableshouldnotbeserialized.1.Itexcludesfieldsfromserialization,suchassensitiveortemporarydata.2.Duringdeserialization,transientfieldsareresettodefaultvalues.3.Useitforfieldsthatshouldn'tpersist,likepasswordsorruntime

Jun 30, 2025 am 01:03 AM

What is a WAR file?

AWARfileisapackagedwebapplicationforJavaservers,containingcode,libraries,andconfigurationinastructuredformat.ItincludesWEB-INF/withweb.xml(servletdefinitions),classes/,lib/,andpublicresourceslikeHTMLandJSPoutsideWEB-INF.Tocreateone,followthestandardp

Jun 30, 2025 am 01:02 AM

What is the `this` keyword?

This points to the context of function execution in JavaScript, depending on how it is called. 1. When called as an object method, this points to the object; 2. When called independently, it points to the global object in non-strict mode, which is undefined in strict mode; 3. This inherits the outer context in the arrow function. Common problems include this pointing errors in event handling and callbacks, which can be solved by binding, saving references, or using arrow functions. call, apply, and bind can be used to explicitly specify this, where call and apply call immediately call the function, and bind generates the bound function. Mastering this behavior is crucial to writing effective code.

Jun 30, 2025 am 01:00 AM

How to create an array?

The key to creating an array is to select the appropriate method according to the programming language. The basic idea is to define a variable that can store multiple data items. 1. The way to declare arrays is slightly different in common languages: JavaScript uses letarr=[1,2,3], Python uses arr=[1,2,3], Java uses int[]arr={1,2,3}, C uses intarr[]={1,2,3}, and beginners are advised to start with Python or JavaScript. 2. The basic operations of an array include accessing elements (such as arr[0]), adding elements (such as arr.push or append), and deleting elements (such as arr.pop or splice). Note

Jun 30, 2025 am 12:59 AM

What is class loading in Java?

Java's class loading is a mechanism for JVM to load classes dynamically at runtime. Its core works by three class loaders at hierarchy and delegate model. 1. BootstrapClassLoader is responsible for loading the core class library; 2. ExtensionClassLoader is used to load the extension library; 3. ApplicationClassLoader loads classes in the application classpath. Class loading adopts on-demand loading strategies, which helps reduce memory overhead and improve startup efficiency. It is particularly critical in large applications (such as Spring, OSGi), web servers and plug-in systems, and can realize dynamic loading and isolation of classes. If the configuration is not correct, ClassNotFoundEx may be triggered

Jun 30, 2025 am 12:31 AM

java class loading