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What is the wrapper class concept?

Awrapperclassencapsulatesaprimitivedatatypetoprovideadditionalfunctionality.Itallowsprimitivestobetreatedasobjects,enablesuseofusefulmethodsforconversionandmanipulation,supportsnullvalues,andisessentialwhenworkingwithcollectionsorgenerictypes.However

Jun 27, 2025 am 01:24 AM

Difference between while and do-while loops?

Themaindifferencebetweenwhileanddo-whileloopsisthetimingoftheconditioncheck.1.Awhileloopcheckstheconditionbeforeexecutingthebody,potentiallynotrunningatalliffalseinitially;2.Ado-whilelooprunsthebodyfirst,ensuringatleastoneexecutionregardlessofthecond

Jun 27, 2025 am 01:24 AM

What is an inner class?

InnerclassesinJavaareusedtologicallygroupclasses,improveencapsulation,andsimplifycodestructure.Theyallowbetterorganizationwhenaclassisonlyrelevanttoanotherclass,enableaccesstoouterclassmembers,andareusefulineventhandling.Typesincludenon-staticnestedc

Jun 27, 2025 am 01:21 AM

What is the Optional class?

Optional is a container class introduced by Java 8 to wrap objects that may be null to avoid null pointer exceptions. 1. Creation methods include: of() (non-null value), ofNullable() (enable null) and empty() (empty Optional). 2. The methods for getting the value are get(), isPresent(), ifPresent(), orElse(), orElseGet() and orElseThrow(). 3. It can improve code security and readability by forcibly processing null values, and supports secure chain calls. 4. Not suitable for collection elements, method parameters or overuse. Use Optiona reasonably

Jun 27, 2025 am 01:20 AM

What is a phantom reference?

PhantomreferencesinJavaareusedtotrackwhenanobjecthasbeenfinalizedandreclaimed,enablingsafeandpredictablecleanup.1.Theydonotallowretrievingtheobjectviaget(),alwaysreturningnull.2.Theyareenqueuedafterfinalizationandcollection,idealforpost-mortemcleanup

Jun 27, 2025 am 01:16 AM

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How to throw an exception?

The core of throwing exceptions is to discover problems, create exception objects and throw them. The syntax is similar in different languages ??but the keywords are slightly different. 1. The basic syntax requires the use of throw or raise keywords followed by exception objects, and the exact exception type is selected; 2. Exceptions should be thrown when the parameters are illegal, the status is inconsistent, the resources are unavailable, or there should be no state, and not all errors are applicable; 3. Custom exception classes can improve the meaning of errors, which is convenient for debugging and targeted processing; 4. Exceptions should be thrown in combination with document descriptions, reasonably captured and necessary logs to avoid hidden dangers caused by empty catch.

Jun 27, 2025 am 01:15 AM

What is the `synchronized` block?

SynchronizedblocksinJavacontrolaccesstocriticalcodesections,allowingonlyonethreadatatime.1.Theyprovidefine-grainedlocking,improvingperformanceoversynchronizingentiremethods.2.Threadsacquirealockonaspecifiedobject,blockingothersuntilreleased.3.Commonu

Jun 27, 2025 am 01:10 AM

What is `Duration` and `Period`?

The core difference between Duration and Period is that Duration represents a fixed time length, measured in seconds, minutes, hours, etc., and is not affected by calendar rules; Period represents a date difference based on the calendar, taking into account month and year changes. For example, a day in Duration is always 24 hours, while a day in Period may be adjusted to 23 or 25 hours due to daylight saving time. Duration is suitable for precise time measurements, such as calculating function run time or processing UTC timestamps; Period is suitable for scenarios involving calendar logic, such as calculating age or scheduling tasks by month. In code, Java's Duration is suitable for Instant, providing nanosecond precision, while Per

Jun 27, 2025 am 01:03 AM

What is an Iterator?

An iterator is an object that allows access to collection elements one by one, and its core mechanism includes getting values ??and judging the end. In Python, 1. Iterable objects such as lists generate iterators through iter(); 2. Call next() to get elements one by one until StopIteration exception is thrown; 3. The for loop automatically handles the iteration process; 4. Directly use the iterator to control large files, data flows and other scenarios; 5. Custom iterators need to implement the __iter__() and __next__() methods.

Jun 27, 2025 am 01:01 AM

What are selectors in NIO?

SelectorsinNIOsolvetheproblemofefficientlymanagingmultipleI/Ochannelswithoutdedicatingathreadtoeachone.1)Theyenableasinglethreadtomonitormanychannelsforreadinessinreading,writing,orotheroperations.2)Thisreducesmemoryandperformanceoverheadcomparedtool

Jun 27, 2025 am 12:53 AM

Difference between JAR WAR and EAR files?

JAR, WAR, and EAR are packaging formats for different purposes in Java. JAR (JavaARchive) is used to encapsulate Java classes, resources and metadata, and is suitable for distribution libraries or independent applications; WAR (WebApplicationARchive) is specially designed for JavaWeb applications, including Servlets, JSPs, etc., and is deployed in Web containers; EAR (EnterpriseApplicationaRchive) is used for complex enterprise applications, and can contain multiple JAR and WAR modules and be deployed in a complete JavaEE server.

Jun 27, 2025 am 12:52 AM

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What is a TreeMap?

ATreeMapvisualizeshierarchicaldatausingnestedrectangleswhereeachrectangle’sarearepresentsavalue.Itisusefulforshowingdatastructureandcomponentproportions,especiallyforlargedatasets.1.Theentiredatasetstartsasonelargerectangle.2.Itsplitsintosmallerrecta

Jun 27, 2025 am 12:50 AM

What is the `==` operator for strings?

In JavaScript, the == operator compares whether the contents of the string value are the same through type casting. When both operands are strings, directly compare the character sequence; if the types are different, convert the type first and then compare. For example, when the string "100" is compared with the number 100, it will be converted to the value 100 for comparison, and the result is true. However, using == may cause unexpected results, such as string objects returning false due to different memory addresses, or case sensitive issues. Therefore, it is recommended to use === strict comparison, requiring that the values ??and types are consistent, and avoid unpredictability caused by type conversion. Common errors include confusing original strings with string objects, ignoring case, and not checking data types. Summary: 1

Jun 27, 2025 am 12:49 AM

What is the JVM?

JVM (Java virtual machine) is the core component of the Java platform. It enables Java programs to run on any device or operating system, realizing "write once, run everywhere". As the running engine of Java applications, JVM loads and converts compiled bytecode into machine code execution; its main components include class loaders, runtime data areas and execution engines; in addition, JVM also supports languages ??such as Kotlin and Scala, and manages memory through automatic garbage collection; common JVM options include setting heap size, printing GC details and viewing versions. Understanding JVM helps to better develop, debug and optimize Java programs.

Jun 27, 2025 am 12:45 AM