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What is the `final` keyword for methods?

In Java, methods use final keyword to prevent methods from being overwritten. 1. Methods declared as final cannot be overwritten by subclasses to ensure that the behavior of key methods remains consistent throughout the inheritance level; 2. It can be used for performance optimization, allowing the compiler or JVM to make optimizations such as inline calls; 3. It is suitable for building immutable classes or security-sensitive code to ensure that specific logic is not modified; 4. It can be used in combination with other keywords such as private, but final is not necessary for each method, and should be selected reasonably according to the needs.

Jul 03, 2025 am 01:59 AM

How does NIO differ from classic IO?

The biggest difference between NIO and traditional IO is the difference in data processing methods: traditional IO is based on streams and can only read and write in one direction at a time, while NIO is based on buffers and channels, supporting more flexible data control. ① Traditional IO is stream-oriented, with low read and write efficiency by byte-byte; NIO is buffer-oriented, and batch operations improve performance. ② Traditional IO is blocking, and threads need to wait for data to be ready; NIO supports non-blocking mode to improve concurrency capabilities. ③NIO provides a Selector mechanism to realize multiplexing. One thread can manage multiple connections, significantly reducing resource consumption. In terms of development suggestions, traditional IO can be used for simple file operations, while high-performance network services recommend NIO. Especially after Java7, NIO.2 can also be used to enhance performance.

Jul 03, 2025 am 01:56 AM

How to create an object in Java?

There are mainly the following ways to create objects in Java: 1. Use the new keyword to call the constructor, which is the most common method, which is suitable for directly instantiating the class and passing in the corresponding parameters; 2. Use Class.newInstance() (outdated), which is commonly used in old versions, but is not recommended after Java9, because it only supports non-argument construction and is complicated to handle exceptions; 3. Use Constructor.newInstance() to support more flexibly parameter construction, suitable for reflection scenarios; 4. Cloning and deserialization, used for specific needs such as deep copying or object restoration. Different methods are suitable for different scenarios, new and Constructor.newInstance() are most commonly used.

Jul 03, 2025 am 01:49 AM

What are code smells?

Code odor refers to the structure or writing method in the code that does not cause errors but imply a potential problem. The most common ones include: 1. Repeat code, the public logic should be extracted, encapsulated into functions or reused using inheritance combination; 2. The method is too long or the function is complex, and it needs to be split into small functions with a single responsibility and use guardclause to reduce nesting; 3. The responsibilities of the class or module should be unclear, and the principle of single responsibility should be followed and decoupled through splitting. These odors affect readability and maintenance and need to be reconstructed and optimized regularly.

Jul 03, 2025 am 01:45 AM

What is the `clone` method?

The cloning method is used to create a separate copy of an object, usually implemented by either a shallow copy or a deep copy. 1. Shallow copy copy the object itself and the basic type values, but the reference type is shared; 2. Deep copy recursively copy all nested objects to ensure complete independence. Most languages ??provide shallow copies by default, such as Java requires manual deep copies. Use clones are suitable for when you need to keep the original object, build the undo system, or create a variant. When implementing, you should clarify the copy type and pay attention to handling variable fields to avoid shared state problems.

Jul 03, 2025 am 01:32 AM

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

java

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