Java Basics - First Java Application

Java is a general-purpose computer programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible.

• Wikipedia

Setup

So what do we need to program anything in Java?

• Java Development Kit
• Editor / IDE

How to install this depends on the system you are using. You can get the JDK from oracle.

Once installed, you should be able to call java -version and javac -version, which would print out the version you’ve installed.

Then you need an editor. Source files are plain text-files, so you could use the standard notepad in windows, but there are Integrated Development Environments (IDE) available that help you write your code. Usually you would choose from Eclipse, NetBeans, and IntelliJ IDEA, but you also can use Visual Studio Code, Sublime, emacs, vim or any other editor that can edit text-files.

Hello world

The first program in every language is usually a hello world-application. It just prints out the message hello world. So, let’s start wil the simpliest hello world in java. Create a file with the name HelloWorld.java, and put the following contents in there (using any editor).

class HelloWorld {

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

}

So this is the source-code for the application. This first needs to be compiled and then it can be executed.
Java does not compile a standalone program, that would run on any machine, Java uses a runtime that interprets the compiled Program. So here’s what you need to call:

javac HelloWorld.java
java HelloWorld

This is how the outcome should look like:

So what has really happened here?
We compiled a java source file (HelloWorld.java) with the java compiler (javac). The java compiler created a java class file (HelloWorld.class) that contains the compiled sources. Now java can run this compiled class file.

The Source-Code

So, what did the source-code of the HelloWorld contain?
The first line says something about a class (class HelloWorld). This does tell the java compiler that now a class definition starts. The curly bracket defines the beginning of the class, and the last curly brackets define the end of the class. Brackets always need to be opened and closed in pairs. The java compiler would complain, if the opening and closing brackets do not match.

What is a class?

Java is an object-oriented language. The java language uses the concept of objects. These objects are representation of entities. An entity can be a fruit, a vehicle or just simply a list of characters or a number. To distinguish between different objects, there are classes that define how objects behave. In this example we define the class of HelloWorld. The class of HelloWorld defines an application that can write hello world to the console.

Inside the class definition, there’s now another context with curly braches. It starts with public static void main(String[] args), then comes a block again, as for the HelloWorld class. This declaration is called a method.

What is a method?

Every code that is executed (mostly) within methods. Methods are some kind of funktionality that can be called. If java tries to run a class, it is defined that it tries to call a static method with the name main.
Nut besides main, there are several other things in that line, they all have a meaning. let’s start with public.

What does public mean?

Every method that is defined with public can be called by anyone else. It’s like a button on a traffic light. anybody can press that button.

What does static mean?

static is means that the method can be called directly from the class. Don’t bother too much about this, once we get deeper into objects, you will understand this.

What does void mean?

Every method must return something. It’s like a result. Usually a method can return a value. Like if you calculate the sum of two values, the method would return the sum of those values. As this method does not return anything, java uses the token void to identify that nothing is returned by the method.

What’s the String[] args for?

If java runs a class like we did (java Helloworld), you can provide further arguments in the commandline. Those will be available in a array of Strings. String is the class of a list of characters and [] indicates that the variable args is a array of Strings.

What is a property?

Another thing is a property. When we take a look at the command, we see this part: System.out. System is a class that has a property with the name out. We just need this to understand what actually is happening.

How is the message written to the console?

Within the main-method, there’s the command System.out.println("Hello World");. First of all, every command always ends with a semicolon. There can be as many whitespaces including newlines as wanted. The semicolon marks the end of each command. So here, the command is System.out.println("Hello World"). System is a class that is defined by java. it contains many possibillities to communicate with the system. System.out for example asks for a out property of the System class. This is defined by java, that this property exists. The property contains an Object that can be used to write something to the console. The class of this Object is PrintStreamm and provides the method println. This method then takes one argument ("Hello World") and writes it to the console.