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How does HashSet handle duplicates?

HashSet handles duplicates through hashCode() and equals() methods. When an object is added to a HashSet, its hashCode() determines the storage location. If a hash conflict occurs, equals() will be used to further determine whether it is equal; if the object already exists, it will not be added repeatedly. To make the custom object recognize duplicates correctly, you must ① rewrite hashCode() to ensure that the same content returns the same hash value; ② rewrite equals() to define the logical equality of the object; ③ maintain consistency between the two and use the same fields. Common errors include forgetting to rewrite two methods at the same time, modifying objects causes hash values ??to change, or logical inconsistency between the two.

Jul 03, 2025 am 01:16 AM

How to use `LocalDateTime`?

How to deal with dates and times in Java? Use the LocalDateTime class to create, format, parse, add, subtract and compare times. The creation methods include: 1. LocalDateTime.now() gets the current time; 2. LocalDateTime.of() manually specify the time; 3. LocalDate and LocalTime are combined to build. Format and parsing, DateTimeFormatter is required, such as ISO format or custom format to convert strings. The addition and subtraction operations support plus and minus methods, such as adding days, decreasing hours, etc., and can be called in chain. Notes include: without time zone information, not suitable for cross-time zone scenarios; conversion with old version of Date

Jul 03, 2025 am 01:04 AM

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What is the Liskov substitution principle?

LiskovSubstitutionPrinciple(LSP)statesthatsubclassesshouldnotaltertheexpectedbehavioroftheirparentclasses.1.LSPensuresthatobjectsofaparentclasscanbereplacedwithobjectsofasubclasswithoutbreakingtheprogram.2.Violationsoccurwhensubclasseschangemethodbeh

Jul 03, 2025 am 12:57 AM

Explain the new Date Time API?

Java8's new Date-Time API solves problems such as insecure threads and chaotic designs. It has the advantages of clear structure, powerful functions and intuitive use. 1. Get the current date and time with LocalDate (year, month, day), LocalTime (hour, minute, second), LocalDateTime (year, month, day, time, without time zone), and the object is immutable and suitable for multi-threading; 2. Get the ZonedDateTime by processing time with time zone, and supports the current time zone time and conversion to other time zones by ZoneId; 3. Use DateTimeFormatter to format and parse dates, which is thread-safe and clear, and supports ISO and custom formats; 4. Support chained tune

Jul 03, 2025 am 12:20 AM

What is a `PreparedStatement` object?

PreparedStatement is used to execute precompiled SQL statements in Java. Its core advantages include: 1. Prevent SQL injection through parameterized queries to improve security; 2. Improve performance when repeatedly executing SQL statements; 3. Simplify code and reduce errors. When using it, you must first obtain the database connection, call the prepareStatement method and set the placeholder parameters, then assign values ??through the setXxx method and execute executeQuery or executeUpdate. For example, the insertion operation can use "INSERTINTOusers(name,email)VALUES(?,?)" as the template. But discomfort

Jul 03, 2025 am 12:20 AM

What are logical operators?

Logicaloperatorsarefundamentaltoolsinprogrammingandlogicusedtoevaluateorcombineconditions,returningabooleanresult.TheyincludeAND(&&),whichreturnstrueonlyifbothconditionsaretrue;OR(||),whichreturnstrueifatleastoneconditionistrue;andNOT(!),whic

Jul 02, 2025 am 01:33 AM

Can a class have multiple constructors?

Yes, classes can have multiple constructors. Through constructor overloading, the class can define multiple constructors with different parameter lists, so that it can be flexibly initialized according to available information when creating an object; for example, the Person class can contain constructors with no arguments, name only, and name and age parameters; the benefits of using multi-constructors include flexibility, default value settings, and code clarity; to avoid duplicate code, other constructors can be called through this() and keep the logic concise.

Jul 02, 2025 am 01:33 AM

Constructor kind

What is serialization?

Serialization is the process of converting complex data structures or objects into formats that can be stored, transferred, or subsequently reconstructed. It is crucial when saving data to a file, sending data through the API, storing web session data, and cacheing objects. Common formats include JSON (suitable for web applications), XML (more structured), Pickle (specific but not secure for Python), MessagePack (efficient and compact), and ProtocolBuffers/Thrift (high performance services). When using it, you should only serialize the necessary data, choose the appropriate format, and pay attention to security, such as avoiding deserializing untrusted PythonPickle data.

Jul 02, 2025 am 01:32 AM

How to convert primitive to wrapper object?

In Java, the basic type to wrapping class is mainly achieved through automatic boxing and manual conversion. 1. Automatic boxing (such as Integerinteger=10) is automatically completed by the compiler, suitable for all basic types, but may affect performance; 2. Manual conversion uses valueOf method (such as Integer.valueOf(age)), which is more clear and often used for collection operations; 3. Pay attention to the cache mechanism, such as Integer caches objects from -128 to 127. .equals() should be used instead of == when comparing to avoid reference judgment errors. These mechanisms ensure that primitive types can be used in scenarios where objects are needed, especially in generics.

Jul 02, 2025 am 01:32 AM

What is a `CountDownLatch`?

CountDownLatch is used for thread synchronization in Java, and its core mechanism is to control thread execution through counters. It sets the count when initializing, calls countDown() to count down, and calls await() to make the thread wait until the count returns to zero. 1. Applicable to multi-threaded coordination task completion, unified startup signal and service readiness check; 2. Once the count is zeroed, it cannot be reset, and only a single time is valid; 3. In the example, two threads trigger countDown() after executing the task, and the main thread calls await() to wait for the two to complete to ensure that the task order is synchronized.

Jul 02, 2025 am 01:32 AM

What is a constructor?

Aconstructorisaspecialmethodusedtoinitializeobjectswhentheyarecreated.Itensuresthatnecessarysetuporpropertyassignmentshappenautomatically,preventingincompleteorinvalidstates.Keypointsinclude:1)Constructorshavethesamenameastheclassandnoreturntype.2)Th

Jul 02, 2025 am 01:32 AM

Constructor

What is the `enum` type in Java?

Enums in Java are special classes that represent fixed number of constant values. 1. Use the enum keyword definition; 2. Each enum value is a public static final instance of the enum type; 3. It can include fields, constructors and methods to add behavior to each constant; 4. It can be used in switch statements, supports direct comparison, and provides built-in methods such as name(), ordinal(), values() and valueOf(); 5. Enumeration can improve the type safety, readability and flexibility of the code, and is suitable for limited collection scenarios such as status codes, colors or week.

Jul 02, 2025 am 01:31 AM

java enum

What is a `CyclicBarrier`?

CyclicBarrierinJavaisasynchronizationtoolthatmakesmultiplethreadswaitatacommonpointuntilallhavearrived,thenreleasesthemtogether.1)Itworksbyinitializingwithanumberofthreads(parties),eachcallingawait()toblockuntilallreachthebarrier.2)Onceallarrive,they

Jul 02, 2025 am 01:31 AM

What is a TreeSet?

TreeSet is a collection class in Java, which realizes automatic sorting and deduplication of elements through red and black trees. Its internal is based on a self-balancing binary search tree (red and black tree), and supports insertion, deletion and search operations of O(logn) time complexity; automatically sorting according to natural order or custom comparator when adding elements; suitable for scenarios where elements are always ordered, quickly retrieve extreme values ??and avoid duplication, such as ranking lists or priority queues; basic operations include add, first, last, floor, ceiling, etc.; limitations include performance overhead greater than HashSet, non-thread-safe, and no null value (unless processed by custom comparator); and only apply to Java language.

Jul 02, 2025 am 01:31 AM