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What is garbage collection?

Garbagecollection(GC)isanautomaticmemorymanagementsystemthatreclaimsunusedmemoryinprograms.Itworksbyidentifyingunreachableobjectsthroughmethodslikereachabilityanalysis,mark-and-sweep,andgenerationalcollection.GCrunsautomaticallywhenmemorypressureincr

Jul 03, 2025 am 02:17 AM

What is a LinkedHashSet?

LinkedHashSet combines HashSet and linked list features in Java, which not only ensures the uniqueness of elements but also maintains the insertion order. It records the order of elements added by the linked list, so that the traversal results are consistent with the order of insertion. It is suitable for scenarios where deduplication and order preservation is required, such as processing user operation logs or reading file deduplication. The checking is repeated based on the equals() and hashCode() methods, and custom objects need to rewrite these two methods correctly. In terms of performance, add/remove/contains operation is O(1), slightly inferior to HashSet but not much difference. Suitable for scenarios without index access and thread safety, not for memory sensitive or sequential maintenance.

Jul 03, 2025 am 02:16 AM

Difference between byte streams and character streams?

Bytestreamshandlerawbinarydata,whilecharacterstreamsprocesstextwithencoding.Bytestreamsareusedfornon-textualdatalikeimagesornetworkprotocols,usingclasseslikeInputStreamandOutputStream.Characterstreams,suchasReaderandWriterinJava,managetextfilesandaut

Jul 03, 2025 am 02:15 AM

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How does HashMap handle collisions?

WhenaHashMapinJavaencounterscollisions,ituseschainingtohandlethem.Eachbucketcanstoremultipleentriesinalinkedlistorbalancedtree.Iftwokeyshashtothesameindex,theyarestoredtogetherinthatbucket,andtheequals()methodisusedduringretrievaltofindthecorrectkey.

Jul 03, 2025 am 02:14 AM

hashmap Collision handling

What are common JVM flags?

JVMflags is used to configure Java virtual machine behavior, mainly covering memory settings, garbage collector selection and debugging diagnosis. In terms of memory, -Xms sets the initial heap size, -Xmx sets the maximum heap size, -Xmn sets the young generation size, and reasonable configuration can avoid frequent GC or OOM. In terms of garbage collectors, UseSerialGC is suitable for small applications, UseParallelGC is suitable for throughput priority services, and UseG1GC is suitable for modern low-latency scenarios. Debugging related parameters such as PrintGCDetails prints GC logs, Xloggc outputs logs to file, HeapDumpOnOutOfMemoryError generates heapdump when OOM is generated

Jul 03, 2025 am 02:12 AM

What is a LinkedList?

Linked lists are suitable for frequent insertion and deletion scenarios. Because the linked list node contains data domain and pointer domain, memory does not need to be continuous, and nodes are connected through pointers, so inserting and deleting only requires adjustment of pointers, which is efficient; but access requires sequential search, which is inefficient. Common types include one-way, two-way, and circular linked lists. Application scenarios include implementing stack queues, browser history, operating system memory management, file system, etc. The core advantage is flexibility and efficiency, and the disadvantage is that random access is slow and takes up extra space.

Jul 03, 2025 am 02:12 AM

Difference between checked and unchecked exceptions?

In Java, exceptions are divided into two types: check type and non-check type. Check-type exceptions are problems that programs should foresee. If the file is not found or the database errors, they must be caught with try-catch or thrown with throws declaration; non-checked exceptions are usually programming errors, such as null pointers or arrays that are out of bounds, and there is no need to force capture or declaration. During design, if it can be restored and is an external problem, check type exception is used; if it is a logical error or cannot be restored, non-check type exception is used.

Jul 03, 2025 am 02:07 AM

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

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