Install Java

You can download Java in two main forms.

The JRE - Java Runtime Environment (Java Virtual Machine JVM) is needed to run a pre-compiled Java application.

The JDK - Java Development Kit (Java Compiler javac) includes the compiler that converts source files with .java extension to compiled files with .class extension. It also include the Runtime Environment.

For develpment you'll need the JDK. The users of your code will only need the JRE.

Command line

In these article we will use the command line and not any IDEs so let's look at the command line tools we get when we install the JDK:

Once you have downloaded and installed the JDK you have two command-line tools:

javac is the Java compiler. If you run javac -version it will display

$ javac -version
javac 1.8.0_60

java s the runtime environment or Java Virtual Machine.

$ java -version
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

Hello World

Now that we have the tools ready we can creat the following file in any text editor (e.g.even Notepad if you are really hard-core)

examples/java/HelloWorld.java

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

The we switch to the terminal (or command prompt in Windows) cd to the directory where we saved the .java file and run:

$ javac HelloWorld.java

this fill create a file called HelloWorld.class but won't print anything to the screen.

Then we can run our application using:

$ java HelloWorld
Hello World

Troubleshooting

A couple of issues I've encountered while making these first steps:

I've tried to run javac only giving the name of the file without the extension:

$ javac HelloWorld
error: Class names, 'HelloWorld', are only accepted if annotation processing is explicitly requested
1 error

Passed the filename including the extension to the java program which only needs the name of the file without the extension:

$ java HelloWorld.class
Error: Could not find or load main class HelloWorld.class

$ java HelloWorld.java
Error: Could not find or load main class HelloWorld.java