Writing Your First "Hello World" Program in Java

Welcome to the fourth tutorial of the "How to Code in Java" tutorial series. In the previous tutorial, I guided you on how to setup Java and an IDE. In this tutorial, I'll be guiding you on how to write your first "Hello world" program in Java and running it using Netbeans IDE.

A "Hello World" program is a simple program that will prints out a "Hello World" text. It consists of the basic structure that every single program needs for a programming language, therefore it's the best example to be used. Since on the last tutorial, I explained how to set up Netbeans, a Java IDE, I'll be demonstrating the examples with the help of Netbeans. If you're asking what is an IDE and why we need to use one, read this article to know about it. 

Index for this tutorial:

1. Creating a new project in Netbeans
2. Creating a new file in Netbeans
3. Code walkthrough
4. Running the source code file

Creating A New Project Folder

First, let us create a new folder for our project. On the toolbar section, click in the "file" section and click on the "New Project" button to open the menu for creating a new project. There are few ways to open the menu, one of it is as mentioned just now. Another way is to click on the "plus folder" logo near the toolbar section, if you want a more convenient way, on your keyboard press "Shift+CTRL+N" simultaneously.

Creating a new folder from the navigation menu

Creating a new folder using the folder logo

After the menu is opened, you'll notice that there are many options available to create a new project for Java application. We'll be creating a project folder with Maven included. Maven is a build automation tool. A build tool helps us to develop an executable application (.class file) from the source code (.java file) of the application. While automating builds reduce the manual, annoying and time-consuming work need to be done when programmers want to build executables. 

After you choose "Java with Maven" in the left menu, now in the right list, select the "Java application" option and click next. In this section, you can specify the name and location to save the project folder. I'll name mine "HelloWorld". For the packaging field, leave it as it is. Finally, you can click finish.

Creating a new file

When the project folder is created, there won't be any files created beforehand for you, so you'll need to create one yourself. First, you'll need to open the menu for file creation, and there are couples of ways to this. You can do this by either press "CTRL+N" or click the "file" icon at the toolbar section. The third way will require longer time but knowing it might come in handy one day. Now, navigate to the project file manager at the left-hand side of the Netbeans window, navigate to your project folder and search for the folder that has the same name as the project folder. Now, right-click the folder and choose.

After you opened up the menu, choose "Java" in the left panel and "Java class" in the right panel, click next afterwards. Now give the file a name, any name will be just fine, I'll name mine as "HelloWorld". Lastly, click finish, and we have now added a new file to our project. 

The Code

Code walkthrough

Every programming language has its syntax when compiling a programmer's code. A programming language syntax is a set of rules that defines the correct symbol's sequence that's used to form a structured program correctly. It's just like English. It has its own unique grammar rules as well as other languages in our world. For us to communicate in English, we will need to follow those rules so that other people can understand what we're saying. Now, let's walk through the code together.

    package org.helloworld; 

Packages are a way for Java developer to organize and group relate Java classes in multiple folders in a directory while providing them to write maintainable code. When the size of an application becomes bigger, programmers will sometimes forget the class names that they have used, which would often lead to class naming conflict. 

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In Java, we cannot have two classes with the same name in the same package. It'll cause an error during compilation. Therefore, by using packages, a class with the same name but are from different packages would not cause an error.

The package is the keyword to indicate that this file belongs to a group/folder, while org.helloworld is the name for the package. Keywords or reserved words are important in Java and any other programming language. We cannot use any of them as a variable or object name as they're reserved by Java.

public class HelloWorld {

This line of code declares a class called HelloWorld. The public before the class keyword is an access modifier, and access modifier specifies the visibility of a class towards other class. So by making a class public means that the class is visible to all other classes. I'll provide more information regarding access modifier in another article. Every code in Java must be inside of a class, except for things like package declaration and libraries importing. This means that every file in Java will need at least one class in it.

The next keyword class is a general term or a blueprint for an object. You can imagine an object as the things that are present in our world, a Toyota car, for instance. A class or you can say a blueprint for an object can be used to make many other different objects, while every single one of them has their own unique properties.

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For a longer explanation, let's use cars as our example. A car (represent a class) is the general term for all cars. All cars use the very same basic blueprint for their structure and have the same functionality, and that's transportation. A Mercedes car (represent an object or an instance of a class) is a car of a luxury's vehicle brand. It uses the blueprint of a car for its structure. This also goes to the other car of the same brand and model. All of them are the same thing (built using the same blueprint) except they come with different design of their own (colour, spoiler, plate number and many more). Making every single one of them have their unique properties and different from each other. 

I hope that this example shows you the concept of class and object. If you're still confused about this, please leave a message in the comment section below and I'll try to re-explain it based on your understanding.

Now, for the open curly bracket, {, means the beginning of a new block or a new scope. Whenever we create a new class, method, conditional statement and loop, we will use a curly bracket to specify the scope of the code and variable that belongs to block. While, the closing curly bracket, }, indicate the end of a block. In our example, it's used to define a new scope for our HelloWorld class. I will write an article regarding scope soon, so don't worry.

public static void main (String[] args) {

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Every executable Java program must have a main method. When the Java Interpreter executes an application, it'll start by calling the class's main method. Then the other class that is required will be called afterwards.

Here, public means the same as the explanation I gave earlier, it's visible by all other classes, including the Java runtime. If we provide the main method with a non-public access modifier, there will be some restriction to the accessibility of the class and will cause an error during compilation. 

Static

Next, static is another common keyword in Java. Explaining this would need another article, as there's a lot of things that we need to know about the use of static in Java. But, in our case, the static main method can be accessed without the presence of an object. Since no object is present during the runtime, therefore, the main method has to be static so that the JVM can load the class into the main memory and execute it.

Void 

Java requires that all of its methods will need to provide a return type. Our main method doesn't need to return anything; therefore, its return type is a void type. More details regarding the return type will be explained in the upcoming tutorial. 

main

This is the name for the Java main method. It's fixed and cannot be changed to any other name. Changing the name will cause an error to occur.

A method

You must have been wondering what does a method means, as I have mentioned it in the previous sections. A method is a block/section of reusable code. We can use back the block of code as many times we need and whenever we need it. More information regarding method can be found here.

(String[] args)

This is the parameter for our main method, with String as its type. As it's the parameter for the main method, the value passed to it is called the command line arguments. The arguments are taken when the programmers specify some data during the compilation process. I'll write a tutorial on how to include arguments during Java program compilation in Window's Command-Line Interface (CLI). 

In short, Arguments is the values that we want to pass to a method and parameters are the variable defined in the method definition. If you don't know what's a variable, don't panic as I'll cover it on the next tutorial. For simple explanation, think of it as aa temporary way to store data when the program is running. Regarding why we need parameters for a method will be explained in another article. Also, another important thing is that whenever we specify a parameter for a method, we need to enclose them in parentheses, (). 

System.out.println("Hello World");

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Now for the final line, The System is the name of a class that handles standard input, output, and error output stream. While, out is one of the fields (class variable) in the System class, it is defined as with the datatype PrintStream. While println is a method inside of the PrintStream class. This method passes the arguments specified in it to the standard console and printing out the arguments to the console while adding a new line at the end of the output. In our case, the console will print out a text "Hello World". There will be an article that will cover on outputting things to the console.

Lastly, the semicolon (;), indicates the end of a statement. Every single statement that you've written in Java will need to have a semicolon at their end. We need to do it add it because it's to separate the current statement with the next statement in the source code parsing phase during the compilation process. 

Running the file.

Now that we've gone through the code, it's time for us to run our program. Since we're using Netbeans running the Java file is easy, just press "shift+F6" and Java will start to compile the source code, and you will see the result of the code in the console that popped up. To compile a Java file through the command line, please refer to this tutorial.

That's it for this tutorial, and I hope you now understand how the basic structure of a Java program works. If you find this tutorial helpful remember to share it with other people that might be interested in this topic. As always, if you have any suggestion, critics, question, or mistakes regarding this tutorial, feel free to leave a comment below. I'll try my best to reply to your comments. The next tutorial will be "What is a variable in Java", so stay tuned.