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What is the `super` keyword?

The super keyword is used to call the parent class's methods or access the parent class attributes. Common uses include calling the parent class methods, initializing the parent class constructor, and handling method override and multiple inheritance. 1. When calling the parent class method, you can use super.method() in the subclass to preserve the original behavior; 2. When initializing the constructor, you need to use super() to ensure that the parent class is initialized correctly; 3. In multiple inheritance, super() calls the next parent class method in the order of method parsing, making the inheritance chain clearer and easier to maintain. Use super rationally to improve code maintainability, but excessive use can cause logical confusion.

Jun 29, 2025 am 01:31 AM

oop super keywords

What is a HashSet?

HashSet is a collection based on a hash table, with elements that are non-repeatable and highly query efficient. Its core features include: 1. Unordered, elements have no fixed order and cannot be accessed through the index; 2. Uniqueness, adding duplicate values ??will be automatically ignored; 3. Efficient query, the time complexity of insertion, deletion and search is close to O(1). Compared with List, List is ordered, allows repetition and has lower query efficiency. Common operations include add(), remove() removal, contains() judgment existence, and size() obtaining quantity. When using it, you need to pay attention to overriding the equals() and hashCode() methods for custom classes. Suitable for deduplication, quick search and collection operation scenarios.

Jun 29, 2025 am 01:30 AM

data structure hashset

What is the Java Collections Framework?

The Java Collection Framework (JCF) is a set of classes and interfaces for storing and manipulating data collections, providing a unified and efficient way to process core data. It mainly includes three core interfaces: 1.Collection interface, which derives List, Set and Queue. List is an ordered and repeatable collection. Common implementations include ArrayList and LinkedList; 2.Set is a collection of non-repeated elements, such as HashSet and TreeSet; 3.Map is used to store key-value pairs, and common implementations include HashMap and TreeMap. Implementation classes are selected according to different scenarios, such as frequent access to ArrayList, insert and delete multiple uses LinkedList, go to

Jun 29, 2025 am 01:29 AM

java collection

How to refactor code?

The core of refactoring code is continuous improvement rather than rewriting. First, it is necessary to clarify the goals, such as solving problems such as confusion in code, excessive function, and difficulty in scaling; secondly, there must be test guarantees to ensure that the changes do not destroy the original functions; then iterate in small steps, only one change is made at a time and run the test to confirm that it is correct; finally, you can use tools to improve efficiency, such as using the built-in refactoring function and static analysis tools of IDE.

Jun 29, 2025 am 01:27 AM

What are buffers in NIO?

Buffers are used in NIO to efficiently manage data input and output operations. Its core mechanisms include: 1. Use buffers to temporarily store data to improve read and write efficiency; 2. The buffer has three key attributes: capacity, location and limitation, and switches read and write modes through the flip() method; 3. Provides multiple types of buffers such as ByteBuffer, CharBuffer, etc. to adapt to different data types; 4. Direct buffers can improve performance but consume more resources; 5. Suitable for non-blocking I/O, high-performance network servers, and large file processing scenarios.

Jun 29, 2025 am 01:19 AM

What is a for loop?

Aforloopisusedtorepeatcodeaspecificnumberoftimesoriteratethroughacollection.1.Itworksbyinitializingacounter,definingacondition,andupdatingthecounteraftereachiteration.2.Commonusesincludeprocessinglists,repeatingactions,iteratingthroughstrings,andperf

Jun 29, 2025 am 01:18 AM

How to use Maven build tool?

Maven is a widely used build tool in Java projects. Its core value lies in simplifying dependency management and unified construction process. 1. Before installing Maven, make sure that JDK is installed. After downloading, configure the environment variables MAVEN_HOME and PATH, set %MAVEN_HOME%\bin by Windows users, modify the .bash_profile or .zshrc file by Mac/Linux users, and customize the local repository path through .m2/settings.xml. 2. Create a Maven project by executing mvnarchetype:generate to generate a standard structure, or create a new Maven project in the IDE to automatically complete it. 3

Jun 29, 2025 am 01:17 AM

Build tools maven

What is the stack space?

Stackspaceisafastbutlimitedmemoryareausedfortemporaryvariablesinfunctions.Itstoresparameters,localvariables,andreturnaddresses.Eachfunctioncalladdsaframetothestack,whichisremovedwhenthefunctionfinishes.Stackspacemattersbecauseitisessentialyetlimited,

Jun 29, 2025 am 01:10 AM

What is an instance variable?

An instance variable is a variable defined in a class or outside a method, and belongs to an instance of a class. 1. They are different from class variables, and each instance has an independent copy; 2. They are often declared on the top of the class or before the constructor, and are used to store the data unique to the object; 3. Allocate memory when creating the object and accessed through self.variable_name; 4. Common errors include forgetting to use self, not initializing it in __init__, and misuse of mutable objects to cause data sharing.

Jun 29, 2025 am 01:09 AM

How does a while loop work?

Awhileloopexecutescoderepeatedlyaslongasaconditionremainstrue.1.Itcheckstheconditionbeforeeachiterationandstopswhentheconditionbecomesfalse.2.Itisusefulwhenthenumberofiterationsisunknown,suchaswaitingforuserinputormonitoringstatuschanges.3.Commonpitf

Jun 29, 2025 am 01:07 AM

cycle

What is inheritance?

Inheritance is the process of the transfer of assets, property or rights between generations, usually after someone dies. Its core content includes: 1. Inheritable items are not limited to money and land, but also personal belongings, titles, debts and digital assets; 2. If there is a will, the property shall be distributed according to the will, but the laws of some regions stipulate that some property must be left to immediate relatives; 3. When there is no will, the property shall be distributed to spouses, children or parents according to local laws; 4. During the inheritance process, tax, debts and family disputes should be paid attention to, and legal help is recommended to seek legal help to ensure smooth execution.

Jun 29, 2025 am 01:05 AM

What is Metaspace?

Metaspace is a local memory area introduced by Java 8 for storing class metadata, replacing PermGen to solve memory limitation problems; 1. Metaspace uses local memory instead of heap memory; 2. Supports automatic expansion and no default upper limit; 3. Class metadata is recycled during FullGC or metadata GC; 4. It can be tuned through -XX:MaxMetaspaceSize and -XX:MetaspaceSize; 5. Common monitoring tools include jstat, VisualVM, etc.; 6. Common problems include insufficient memory and metadata leakage, which need to be checked in combination with log and analysis tools.

Jun 29, 2025 am 01:02 AM

What is the NIO package?

NIO is an efficient I/OAPI introduced by Java 1.4. The core advantages are non-blocking IO and buffer-oriented operations; its key components are Buffers (such as ByteBuffer, CharBuffer) and Channel (such as FileChannel, SocketChannel); the non-blocking mode realizes multi-connection processing through Selector to improve system throughput; it is suitable for scenarios such as high concurrent servers and large file transfers; it is more efficient to copy files using FileChannel; when operating Buffers, you need to pay attention to capacity, position, limit status, flip(), clear() and other methods; NIO is not in

Jun 29, 2025 am 01:01 AM

java nio

What is a switch statement?

A switch statement is a control structure that executes different code blocks based on the value of a variable or expression. It works by evaluating the expression and matching multiple case values, and if no match is done, the default branch is executed. Common usages include handling user input, menu selection, and status codes. When using it, you should pay attention to adding break to prevent "through", use default reasonably, and avoid complex conditions. In JavaScript or C, its basic structure is: switch(expression){casevalue1:...break;casevalue2:...break;default:...}. For example, when processing operators in a calculator, Swi can be used

Jun 29, 2025 am 01:00 AM