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What is a JAR file?

AJARfileisapackagedbundleofJavafilesusedforeasierdistribution.Itcontainscompiledclassfiles,amanifestfilewithmetadata,andotherresources.Themanifestdefineskeydetailslikethemainclasstorun.DevelopersuseJARsfororganization,portability,securitythroughsigni

Jul 03, 2025 am 02:19 AM

What is the `toString` method?

The toString method is used to return the string representation of the object, which is easy to debug and display. The default implementation information is limited, so developers often rewrite this method to provide more meaningful information. For example, in Java, return "Person{name='John',age=30}" by overwriting toString; define the toString method in JavaScript to achieve similar effects. Application scenarios include debugging, logging, user output and collection display. It is recommended to add a toString method for custom classes to keep the output concise and not throw exceptions.

Jul 03, 2025 am 02:19 AM

What is method overloading?

MethodOverloading means that multiple methods of the same name can be defined in the same class, but the parameters of these methods must be different. The core is "the method name is the same, the parameters are different", which is manifested as different number, type or order of parameters; the return value type cannot be used as the basis for overloading. For example, multiple add methods can be defined in Java to handle different types and quantities of inputs. The main purpose of using method overloading is to improve the readability and reusability of the code, so that the caller does not need to remember multiple method names. Common application scenarios include constructor overloading, tool-like method adaptation, etc. Notes include: Avoid excessive overloading, do not distinguish methods based on return values ??alone, and pay attention to problems that may be caused by automatic type conversion. Support method

Jul 03, 2025 am 02:18 AM

What are common built-in annotations?

Common built-in annotations in Java are mainly divided into three categories: annotations used by the compiler, annotations used to help the tool process, and annotations available at runtime. 1. @Override is used to rewrite the parent class method. If the parent class method is not really overwritten, the compiler will report an error. It is suitable for scenarios where the method is rewrite in inheritance; 2. @Deprecated marks the element is outdated, prompting developers to avoid using it and may be removed in the future. It is usually used with the @deprecated of Javadoc; 3. @SuppressWarnings suppresses compiler warnings, suitable for situations where specific "security" warnings are ignored, but they should be used with caution to prevent potential problems; 4. Meta annotations include @Retention and @Ta

Jul 03, 2025 am 02:18 AM

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

java

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