There are three main types of collections in Java: set (set), list (list) and map (mapping).
1. List (ordered, repeatable)
The objects stored in List are ordered and repeatable. List focuses on indexes. It has a series of index-related methods and the query speed is fast. Because when inserting or deleting data into the list collection, it will be accompanied by the movement of subsequent data, all insertion and deletion of data are slow.
2. Set (unordered, cannot be repeated)
The objects stored in Set are unordered and cannot be repeated. The objects in the set are not in a specific way. Sorting simply adds objects to a collection.
3. Map (key-value pairs, unique keys, non-unique values)
The Map collection stores key-value pairs. Keys cannot be repeated, but values ??can be repeated. . Obtain the value according to the key. When traversing the map collection, first obtain the set collection of the key, traverse the set collection, and obtain the corresponding value.
For more java knowledge, please pay attention to java basic tutorial.
The above is the detailed content of What collections are there in java. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics











Use len() to count the total number of elements in the list, such as len([1,2,3,4,5]) to return 5; 2. Use count() to count the number of occurrences of specific elements, such as ['apple','banana','apple'].count('apple') to return 3; 3. Use collections.Counter to count the frequency of each element, such as Counter(['a','b','a']) to output Counter({'a':3,'b':2,'c':1}); 4. Use dictionary to manually count the traversal and get methods to achieve the same effect, such as loop accumulation to obtain {'a':3,'b':2,'c':1}.

Reading CSV files is commonly implemented in Python using pandas library or csv module. 1. Use pandas to read through pd.read_csv(), return DataFrame, supports specifying parameters such as sep, header, index_col, encoding, na_values, etc., suitable for data analysis; 2. Use the csv module to read line by line through csv.reader or csv.DictReader, the former returns a list, and the latter returns a dictionary, suitable for lightweight or no dependencies of third-party libraries; 3. Frequently asked questions: Use a complete path to avoid path errors, set encoding='gbk' or 'utf-8' to solve Chinese

In Go, range is used to iterate over data types and return corresponding values: 1. For slices and arrays, range returns index and element copy; 2. Unwanted indexes or values can be ignored using _; 3. For maps, range returns keys and values, but the iteration order is not fixed; 4. For strings, range returns rune index and characters (rune type), supporting Unicode; 5. For channels, range continues to read values until the channel is closed, and only a single element is returned. Using range can avoid manually managing indexes, making iteratives simpler and safer.

UseMavenorGradleconsistentlywithcentralizedversionmanagementandBOMsforcompatibility.2.Inspectandexcludetransitivedependenciestopreventconflictsandvulnerabilities.3.EnforceversionconsistencyusingtoolslikeMavenEnforcerPluginandautomateupdateswithDepend

init is a method used in Python to initialize object properties. 1. When creating an instance of the class, __init__ is automatically executed, which is used to set the initial state of the object, such as binding the parameter to the instance through self.name=name. 2. You can set default values for parameters, such as breed="Unknown" and age=1 in the Dog class, making initialization more flexible. 3. Logical verification can be added to init, such as the BankAccount class checks whether balance is negative, improving data security. 4. Note that init is an initialization method rather than a constructor. The object already exists before the method is executed and must be spelled correctly and cannot be written as int or ini.

Kotlinoffersthebestbalanceofbrevityandreadability,Javaisverbosebutpredictable,andScalaisexpressivebutcomplex.2.Scalaexcelsinfunctionalprogrammingwithfullsupportforimmutabilityandadvancedconstructs,KotlinprovidespracticalfunctionalfeatureswithinanOOPf

MySQL's REPLACE is a mechanism that combines "delete insert" to replace old data when unique constraint conflicts. When there is a primary key or unique index conflict, REPLACE will first delete the old record and then insert the new record, which is atomic. 1. There must be a primary key or a unique index to trigger the replacement; 2. The old data is deleted during conflict and the new data is inserted; 3. Unlike INSERTIGNORE, the latter ignores conflicts and does not insert them and does not report errors; 4. Pay attention to data loss, self-increasing ID changes, performance overhead and multiple triggering problems of triggers; 5. It is recommended to use INSERT...ONDUPLICATEKEYUPDATE to update some fields instead of full replacement.

ProjectReactor is a Java library based on responsive stream specifications used to handle asynchronous data flows. Its core types are Mono and Flux. 1. Use operators such as map, flatMap, and filter for data flow conversion and processing. 2. Control data flow rate through backpressure mechanisms such as onBackpressureBuffer and onBackpressureDrop. 3. Use onErrorResume, onErrorReturn, and retry for error processing. 4. Use subscribeOn and publishOn to implement thread scheduling. 5. It is widely used in microservice asynchronous calls, event-driven architecture, real-time
