How to understand Maven in Java

1. Description

Maven is a software project management tool based on the Project Object Model (POM) that can manage project construction, reports and documents through a short piece of description information.

2. Reasons for use

(1) A project is a project

If the project is very large, it is not suitable to use packages to divide modules. It is best if each module corresponds to a project, which facilitates division of labor and collaboration. With the help of maven, you can split a project into multiple projects

(2) When using jar packages in the project, you need to "copy" and "paste" the same jar in the project's lib

Packages appear repeatedly in different projects, and you need to do repetitive work of copying and pasting. With the help of maven, the jar package can be saved in the "warehouse", no matter which project it is in, just use the reference.

(3) Whenever the jar package is needed, you must prepare it yourself or download it from the official website

With the help of maven, we can download the jar package in a unified and standardized way, specification

(4) Risk of jar package version inconsistency

When different projects use jar packages, the jar package versions of each project may be inconsistent, resulting in non-execution errors. With the help of maven, all jar packages are placed in the "warehouse", and all projects use a copy of the jar package in the warehouse.

(5) A jar package that depends on other jar packages needs to be added to the project manually

FileUpload component->IO component, commons-fileupload-1.3.jar depends on commons- io-2.0.1.jar

greatly wastes our time and cost in importing packages, and also greatly increases the learning cost. With the help of maven, it will automatically import the dependent jar packages.

3. Usage

(1) Add third-party jar package

(2) Dependencies between jar packages: Maven can do it for us Automatically import all other jar packages that the current jar package depends on

(3) Obtain third-party jar packages: Maven provides a completely unified and standardized jar package management system, which only needs to be added in the project with The coordinate method relies on a jar package, and Maven will automatically download it from the central warehouse to the local warehouse

(4) Split the project into multiple project modules

(5) Build the project( Packaging, compilation, etc.)

What collection classes are there in Java?

Collections in Java are mainly divided into four categories:

1. List: ordered, repeatable ;

2. Queue: ordered, repeatable;

3. Set: non-repeatable;

4. Map: unordered, unique key, The value is not unique.

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