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Java

forthebadge forthebadge

Here is a little playground for Java development.

Some Java insight

The java language developed by Oracle is:

  • Portable, robust and dynamic
  • Platform independent
  • Runs on the JVM (Java Virtual Machine) which is system dependant

With the JVM, you can write .java files, that get compiled into Java byte codes into .class file. The .class files can be executed on the Java Virtual Machine.

Environment set up

MacOS

Get the open JDK (open source) with brew

brew install java8
brew install java11

then switch between multiple version using an alias to the Home of your java:

java11='export JAVA_HOME=/Library/Java/JavaVirtualMachines/adoptopenjdk-11.jdk/Contents/Home'
java8='export JAVA_HOME=/Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home'

Windows

SDK for Software Development Kit also called JDK - Java Development Kit. You can get the Oracle Java SDK here. Choose the one for your system, here for windows.
There's the Oracle JDK (Commercial, Stable with proprietary code from Oracle) and the Open JDK (Open source, maintained by Oracle).

The JDK includes, it can be called inside a command prompt:

  • The Java compiler javac - javac
  • The Java Archiving Tool - jar
  • the Java Debugging Tool - jdb

When installing the JDK, don't forget to add the JAVA_HOME (path to the JDK and JRE - Java Runtime Environment) as an environment variable.

Environment variables in windows are accessed via the start up menu (windows key) then:

  • right click on computer > properties > advance system settings
  • Click on the environment variable button

I have added setPathJava.bat script to update the JAVA_HOME. The path variable is only changed for the current cmd session. (Need to update the key registry to make it definitive).

Get an IDE

IDE for Integrated Development Environments.
I'll be trying IntelliJ: Community version by Jet Brain which has pro feature. If you click on the link you should be able to download it (the community version is free).

Maven

What is maven

Maven is a framework developped by Apache that add standards in Java projects. By having the same hierarchy it helps keep a consistent project, manage dependencies and falicitate the build. It is a good way to share information and JAR across multiple projects.

"Maven, a Yiddish word meaning accumulator of knowledge"

Installation

Here are the steps to install Maven on Windows:

  • Make sure you've intalled java and set JAVA_HOME
  • Download maven at maven.apache.org
  • Unzip it in C:\Program Files\Apache\maven
  • Add this location to the M3_HOME and MAVEN_HOME environment variable
  • Add %M3_HOME%\bin to the PATH environment variable
    • ⚠️ no space between the variable path separator ; and %M3_HOME%\bin

Maven commands

First make sure maven is installed by running:

mvn -version

Maven can now be used to build the project:

  • mvn compile to run the test, compile the project, install the dependencies, create the library package.
  • mvn package to create the library package (such as a JAR file for example)
  • mvn test to use the maven "surefire" plugin to run unit test in the src/test/jave folder with a matching *Testname
  • mvn install to add your project's JAR file to your local repository (like a compile but making it ready as a dependency to be referenced by another project
  • mvn clean install to copy the libraries if the first one fails.

Getting started

Here is a getting started from the Apache Maven website:

The pom.xml file

When using Maven a pom.xml file is created to manage the dependencies of the project. Here is a basic example of what it could look like:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>groupId</groupId>
    <artifactId>Java_hero</artifactId>
	<packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>

  <build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>22.0</version>
        </dependency>

        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.6</version>
        </dependency>
    </dependencies>

</project>

Here are some details on all of the shown tags:

  • <modelVersion>: POM model version (always 4.0.0).
  • <groupId>: Group or organization that the project belongs to. Often expressed as an inverted domain name.
  • <artifactId>: Name to be given to the project’s library artifact (for example, the name of its JAR or WAR file).
  • <version>: Version of the project that is being built.
  • <packaging>: How the project should be packaged (Optional tag). Defaults to "jar" for JAR file packaging. Use "war" for WAR file packaging.
  • <dependencies>: Dependencies to be installed or loaded with the project
  • <build>: Define what will be built and how.

Gradle

Gradle is an open source build automation system that builds upon the concepts of Apache Ant and Apache Maven and introduces a Groovy-based domain-specific language (DSL) instead of the XML form used by Apache Maven for declaring the project configuration

Installation

How to use

IntelliJ

IntelliJ is an IDE developed by JetBrain, the community version is free and comes with a lot of pro features (that might be overkill in some cases).

Getting started with IntelliJ

First you have to create a Java project: File > New > Project. When a screen pops up, you need to specify the link to your project SDK.

  • You should find the path to the SDK here : C:\Program Files (x86)\Java with a name such as jdk1.8.1_02
  • Or you can use the installed JDK of IntelliJ (Clicking the down arrow and selecting it)

Then you can start your project, creating a class and doing some coding. To compile the code, you will need to hit the build button (top right) or ctrl + F9.

To Run it, will have to edit configurations (next to the build button). Click on the arrow and select Edit Configurations.
A window appear, on the left side there's a dropdown menu (called Defaults). Click on it and select Application. On the right panel, add a name and the main class you've just created. Press ok to validate.

Now that you have that set up you should see a green arrow next to that field (next to the build button). If you click on it, it will run your project and you'll see on the bottom a split screen with the output (like Netbeans, yeah!).

Maven on IntelliJ

So to use Maven with IntelliJ, you first need to right click on a project and select Add framework support ....

A new page will show up and you can select Maven, it will automatically reformat your code structure to adapt to Maven. A pom.xml will be created.

⚠️ Sometime IntelliJ push the compiler to v1.5 #2. There is two way to specify the right language for the compiler in the pom.xml file. Either add or modify the already existing <build> tag in the <project> tag :

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>

The above solution worked great on IntelliJ. You can also can try this solution from Apache Maven.

Setting up dependencies, add external libraries

You can add external libraries to your Java project. Here are two ways of doing in with IntelliJ.

with JAR

You can download the JAR file of the external library and add it to your project. For that you can do go on File > Project Structure (or [ALT] + [ctrl] + [shift] + [S]).

Once there you can click on the green plus and add the .jar file. (Save the Jar in your place for libraries such as lib/directory).

Now you should be all set, you can now import the library:

//Importing a library
import main.java.com.library;

with Maven

To add an external library to your project, just copy and past the <dependency> into <dependencies> inside <project> of your pom.xml

Here for example for the Guava library from Google:

<project>
	<!-- information about your project -->
	<dependencies>
	        <dependency>
	            <groupId>com.google.guava</groupId>
	            <artifactId>guava</artifactId>
	            <version>22.0</version>
	        </dependency>
	<dependencies>
</project>