Pure Programmer
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Getting Started with Java

Every programming language has some boilerplate material that we use to start a programming project. Mostly it is code to describe where the program should start and what additional libraries should be used to augment the programming language itself. Thus we have, in every language, this smallest program that we can write. We will ignore some of the explanation as to exactly what these minimal programs are doing until later. It is sufficient for now that we treat this as boilerplate code that forms the foundation of our programs. We can then expand upon these boilerplate programs as we explore the various features of the language.

 * Simple first program.
 * Copyright © 2020 Richard Lesh.  All rights reserved.

public class HelloWorld {

	public static void main(String[] args) {
		System.out.println("Hello, world!");

$ javac -Xlint HelloWorld.java $ java -ea HelloWorld Hello, world!

This approach to introducing a programming language by writing a simple program that outputs one phrase, typically "Hello, world!" was first seen in Brian Kernighan and Dennis Ritchie's book "The C Programming Language" in the 1970s.

Language Execution Models

In order to get our programs as they are written in our favorite language to run on a computer, it is necessary to convert from the English-like text of our source code to something that the computer CPU can understand. Depending on the language, this either involves a compilation or interpretation phase. Some modern languages require both phases.

Compilation involves taking our source code and then translating it into another language, typically the language of the CPU known as machine language. Compilation is typically performed by a program called a compiler and it is a required step when writing in a compiled language. Once our program has been compiled into machine code, we can then run it directly on the CPU. Languages such as C, C++, Objective C, Swift, FORTRAN and Cobol are all compiled languages. These languages are typically used in the development of operating systems and commercially distributed code.

Compile Execution Model
Compiled Language Execution Model

Interpretation involves taking our source code and then line by line decomposing the source into steps that are immediately carried out by the CPU. Interpreted languages require a program called an interpreter to perform this function. There is no compilation phase, we just get to the business of running our program straight from the source code each time we want it to run. Compared to compiled languages, it can often be easier to develop and debug interpreted programs. Interpreted languages, often called scripting languages, include JavaScript, Python and Ruby.

Interpret Execution Model
Interpreted Language Execution Model

Some languages, most notable are Java and JavaScript, use both approaches. In the case of Java, the source code is compiled to the machine code for a fictional CPU known as the virtual machine (VM). Because your CPU doesn't typically use the same instruction set as the virtual machine, the compiled program must then be interpreted by a program that in essence pretends to be the virtual machine. So languages that use a VM have both a compiled and an interpreted phase. Notable VM languages are Java and C#.

VM Execution Model
VM Language Execution Model

Tool Setup

Because Java uses the VM Language Execution Model we need to have the following software tools available to us: programming text editor to create/edit our source files, compiler to translate our source files into VM object files (.class files), a Java VM to run our compiled object files and a command-line console to launch the VM and handle the input and output from our program.

Command-Line Console

The Command-Line Console is used to invoke the JavaScript interpreter and is also used by the programs we write for text input and output. Linux, Mac OS X and Windows all come with a Command-Line Console program though many are not accustomed to using it.

On this site we will use the terms console, terminal and command-line prompt interchangeably. Illustrations of terminal command sequences will be presented in a box with a black background. User input will appear in white while the command prompts ($) and program output will appear in green.

$ user input program output

Programming Text Editor

Programming Text Editors can be as simple as a plain text editor or as complicated as a text editor built-in to an Integrated Development Environment (IDE). For our purposes a middle-of-the-road programming editor will suffice. These types of text editors offer some niceties beyond the basic ability to edit text files. The most common of these programming oriented features is syntax highlighting which colors the parts of the programming text to indicate its role in the programming language's syntax. Other features include auto-completion, auto-balancing of brackets and auto-indenting of source code.

Programming Text Editors are not the same types of programs as Word Processing software. Word Processing software is designed to format and layout complex text documents such as manuals, brochures or flyers. The documents that they create are stored in special word processing formats that are not compatible with our programming tools. Programming Text Editors not only have features aimed at helping write programs, but more importantly they save the source files in a plain text format ([[ASCII]] or [[UTF-8]]) required by our programming tools.

While bare-bones text editors that come with your OS like TextEdit (Mac OS X), LeafEdit (Raspbian) or Notepad (Windows) will work to edit our source code files, you will be more productive using a programming text editor. Based on your operating system you might try some of the programming text editors listed below.

Most Linux distributions based on the Gnome desktop will come with GEdit already installed. Some, however, like the distribution for the Raspberry Pi do not. If your distribution does not have GEdit installed it is easy to install from the command-line terminal using apt.
$ sudo apt install gedit

Raspberry Pi OS comes with Geany already installed. If your Linux distribution does not have Geany installed it is easy to install from the command-line terminal using apt.
$ sudo apt install geany

Visual Studio Code can be installed on Linux distributions including Raspberry Pi OS from the command-line terminal using apt.
$ sudo apt install code

Compiler and Virtual Machine

To see if your system already has the Java tool chain installed, use the terminal to print the version of your installed compiler and VM.

$ javac -version javac 11.0.9 $ java --version java 11.0.9 2020-10-20 LTS Java(TM) SE Runtime Environment 18.9 (build 11.0.9+7-LTS) Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.9+7-LTS, mixed mode)

The Java compiler program is javac while the Java VM program is java. They both should display the same version number.

The Java compiler, Java VM, Java standard class library and many other development tools come in a single package known as the Java Development Kit (JDK). The JDK is needed to perform Java software development. Just the Java VM and standard class library also come in a package known as the Java Runtime Environment (JRE). The JRE is used when you only want to run Java programs but not develop them. The JRE comes as a component in the JDK.

Linux users can download the [[Oracle JDK]] in RPM or tarball format. Alternately you can install the open source version of the JDK from [[Amazon Corretto]], [[Azul Zulu]] or [[Eclipse Temurin]].

You might also be able to install the OpenJDK using apt.

$ sudo apt update $ sudo apt install openjdk-17-jdk

MacOS X users can download the [[Oracle JDK]] directly from Oracle. After you download the Standard Edition JDK double-click the image file (.dmg) to mount the image on your desktop. Open the mounted image and double-click the installer to install Java. Alternately you can install the open source version of the JDK from [[Amazon Corretto]], [[Azul Zulu]] or [[Eclipse Temurin]].

The Raspbian Pi users can install the latest JDK using apt.

$ sudo apt update $ sudo apt install default-jdk

Windows users can download the [[Oracle JDK]] directly from Oracle. You can also download an open source version of the JDK from [[Amazon Corretto]], [[Azul Zulu]] or [[Eclipse Temurin]]. After you download the installer, double-click it to install the JDK.