Introduction

What is Java?

Java is a programming language developed by Sun Microsystems in 1995, which got later acquired by Oracle. It's now a full platform with lots of standard APIs, open source APIs, tools, huge developer community and is used to build the most trusted enterprise solutions by big and small companies alike. Android application development is done fully with Java and its ecosystem. To know more about Java, read this and this.

Version

The latest version is Java 1.8 or Java 8, which was released in 2014 with various improvements over the previous version, Java 7. This year, Oracle has a target to release Java 9. But for all intents and purposes, we will use Java 8 in this wiki for all tutorials.

Java is also divided into several "Editions" :

SE - Standard Edition - for desktop and standalone server applications
EE - Enterprise Edition - for developing and executing Java components that run embedded in a Java server
ME - Micro Edition - for developing and executing Java applications on mobile phones and embedded devices

Installation : JDK or JRE ?

Download the latest Java binaries from the official website. Here you may face a question, which one to download, JDK or JRE? JRE stands for Java Runtime Environment, which is the platform dependent Java Virtual Machine to run Java codes, and JDK stands for Java Development Kit, which consists of most of the development tools, most importantly the compiler javac, and also the JRE. So, for an average user JRE would be sufficient, but since we would be developing with Java, we would download the JDK.

Platform specific installation instructions

Windows

Download the relevant .msi file (x86 / i586 for 32bits, x64 for 64bits)
Run the .msi file. Its a self extracting executable file which will install Java in your system!

Linux

Download the relevant tar.gz file for your system and install :
```
$ tar zxvf jdk-8uversion-linux-x64.tar.gz
```
RPM based Linux platforms download the relevant .rpm file and install :
```
$ rpm -ivh jdk-8uversion-linux-x64.rpm
```
Users have the choice to install an open source version of Java, OpenJDK or the Oracle JDK. While OpenJDK is in active development and in sync with Oracle JDK, they just differ in licensing stuff. However few developers complain of the stability of Open JDK. Instructions for Ubuntu :

Open JDK installation :
```
sudo apt-get install openjdk-8-jdk
```
Oracle JDK installation :
```
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer
```

Mac

Either download Mac OSX .dmg executable from Oracle Downloads

Or use Homebrew to install :

brew tap caskroom/cask
brew install brew-cask
brew cask install java

Verify Installation

Verify Java has been properly installed in your system by opening Command Prompt (Windows) / Windows Powershell / Terminal (Mac OS and *Unix) and checking the versions of Java runtime and compiler :

$ java -version
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b17, mixed mode)

$ javac -version
javac 1.8.0_66

Tip : If you get an error such as "Command Not Found" on either java or javac or both, dont panic, its just your system PATH is not properly set. For Windows, see this StackOverflow answer or this article on how to do it. Also there are guides for Ubuntu and Mac as well. If you still can't figure it out, dont worry, just ask us in our Gitter room!

JVM

Ok now since we are done with the installations, let's begin to understand first the nitty gritty of the Java ecosystem. Java is an interpreted and compiled language, that is the code we write gets compiled to bytecode and interpreted to run . We write the code in .java files, Java compiles them into bytecodes which are run on a Java Virtual Machine or JVM for execution. These bytecodes typically has a .class extension.

Java is a pretty secure language as it doesn't let your program run directly on the machine. Instead, your program runs on a Virtual Machine called JVM. This Virtual Machine exposes several APIs for low level machine interactions you can make, but other than that you cannot play with machine instructions explicitely. This adds a huge bonus of security.

Also, once your bytecode is compiled it can run on any Java VM. This Virtual Machine is machine dependent, i.e it has different implementations for Windows, Linux and Mac. But your program is guranteed to run in any system thanks to this VM. This philosophy is called "Write Once, Run Anywhere".

Hello World!

Let's write a sample Hello World application. Open any editor / IDE of choice and create a file HelloWorld.java.

public class HelloWorld {

    public static void main(String[] args) {
        // Prints "Hello, World" to the terminal window.
        System.out.println("Hello, World");
    }

}

N.B. Keep in mind in Java file name should be the exact same name of the public class in order to compile!

Now open the terminal / Command Prompt, and compile the file :

$ javac HelloWorld.java

Now run the file using java command!

$ java HelloWorld
Hello, World

Congrats! Your first Java program has run successfully. Here we are just printing a string passing it to the API System.out.println. We will cover all the concepts in the code, but you are welcome to take a closer look! If you have any doubt or need additional help, feel free to contact us anytime in our Gitter Chatroom!

Documentation

Java is heavily documented, as it supports huge amounts of API's. If you are using any major IDE such as Eclipse or IntelliJ IDEA, you would find the Java Documentation included within.

Lets learn some Java Basics to start with!

FreeCodeCamp/wiki