Welcome!

Related Topics: Java IoT

Java IoT: Article

Your First Java Program

Lesson 1: Hello World

Getting Started

The Java Development Kit (JDK) could be downloaded from the Sun Microsystems' Internet site at http://java.sun.com/j2se/1.4/ .

The installation process is pretty simple - just run the downloaded executable file and it'll install it on your disk (the default directory for Java under Microsoft Windows is  c:\j2sdk1.4).

To start writing a Java program you could use any plain text editor. In Windows, it could be an editor called Notepad. In UNIX, it could be the vi editor. The files with Java programs must be saved in a plain text format and must have names ending in .java.  For example, if you want to write a program called HelloWorld, enter its code in Notepad and save it in a class named HelloWorld.java.

Keep in mind that Java is a case sensitive language, which means that if you named the program HelloWorld with a capital H and a capital W, do not try to start the program helloworld.

Here is the infamous program that prints the words Hello World on the screen:

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


Now you need to compile this program. We'll be using the  javac compiler, which is a part of JDK.
 
Let's say you've saved your program in the directory called  c:\practice. Open a command window, change the current directory to c:\practice and compile the program:

c:\>cd \practice

c:\practice>javac HelloWorld.java

If your environment is set properly and your program does not have syntax errors, it will create a new file called HelloWorld.class in the same directory.

If an error message is displayed  saying something  like "javac  is not found", or "bad command/file name" make sure that the directory  c:\j2sdk1.4\bin  is  included to the  search path of your environment.    

- If you are using Windows 98, open the file c:\autoexec.bat
        and add  the directory where your JDK is installed to the environment
        variable PATH, for example 
 
        c:\j2sdk1.4\bin;   

-  In Windows 2000 or XP set the PATH using the menu Settings |
         Control  Panel | System | Environment Variables. 

- In Unix - add it to the shell's PATH environment variable.

You  won't see any confirmation of a successful compilation, just type dir in Windows or ls in Unix, and a new file named HelloWorld.class has to be there. This  proves that your program has been successfully compiled.

If the program has some syntax errors, the compiler will print error messages. In this case you'd need to fix the errors, and recompile the program again. You may need to do it more than once until the file HelloWorld.class is created.

Now let's run the program -  enter the following command:

c:\practice> java HelloWorld

Please note that we do not start  javac, but java , which is called the Java run-time environment or the Java Virtual Machine (JVM).

This time the error message may say that the HelloWorld.class is not found.   Even though you  have the .class file in the same directory as your .java file, JVM is not going to look for it in the current directory unless the current directory is listed in the so-called CLASSPATH variable. Don't confuse this with the variable  PATH, that's been discussed  earlier. 

The variable  CLASSPATH variable is used by the JVM to find compiled classes.  Let's do a procedure similar to what you've done with the PATH.

For example, in Windows 98, open the file autoexec.bat and add the following line to it:

set CLASSPATH=.;

The dot above represents the current directory. If you already had the CLASSPATH variables set in your machine, just add the dot and semicolon to the end of its value.

Give  your Java class and its file the same name.  There could be exceptions to this rule, but not in this simple program.

While writing Java programs, you create classes which represent objects from real life. You'll learn more about classes in the lesson called "Introduction to Object-Oriented Programming in Java".

Our HelloWorld program is also a class and it contains a  method main(). Methods in Java classes represent actions that the class could perform.  The method main() calls the method println() to display the text "Hello World" on the screen.

Here is the method signature of the method main():

public static void main(String[] args)

The method signature includes the access level - public, instructions on usage - static, return value type - void, name of the method - main, and the argument list -  String[] args.

The keyword public means that the method main() could be accessed by any other Java class. The keyword static means that you don't have to create an instance of  this class to use this method. The keyword void says that the method main() doesn't return any value to the calling program.

The keyword Stirng[] args  tells us that this method will receive an array of Strings as the argument (some values could be passed to this method from a command line).

The main() method is the starting point of your program. You can have a program that consists of more that one class, but at least one of them usually has the method main(), otherwise the program will not start. A Java class can have more than one method. For example, a class Employee can have the methods  updateAddress(), raiseSalary(),changeName(), etc.

The body of the method  main()contains the following  line :

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

The println() is a method that is used to print data on the system console (command window). Java's method names are always followed by parentheses.

System and out are not methods, but names that represent other Java classes.

System.out means that the  variable out is defined inside the class System.

The out.println() tells us that there is an object represented by a variable called  out  and it has  a method called println().

We will be using this so-called dot notation to access class methods or variables. Say you have a class Employee that has a method changeAddress().  Here is an example:

Employee.changeAddress("25 Broadway")

Let's review the steps you would perform to create and run the HelloWorld program:

Step 1. Set the  values for the PATH and CLASSPATH system variables.

Step 2. Create a new directory called practice.

Step 3. Using a text editor, enter the code of the class 
             HelloWorld  and save it in the file  
             c:\practice\HelloWorld.java. 

Step 4.  Compile and run the program:
 
             c:\practice> javac HelloWorld.java
             c:\practice> java HelloWorld


Assignment. Write a program to print your address using  more than one statement println().

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (4) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Peter Sweet 04/23/04 11:53:44 AM EDT

This is good stuff. The explanation of what PATH and CLASSPATH do was useful.

Andrey Postoyanets 11/19/03 11:42:21 AM EST

As a Java programmer in a college IT department, I''ve worked with quite a few college interns (and also have been approached by fellow co-workers who want to try Java.) They would usually take a college-level Java course or try self-study, but are quite scared to start on their own. Thus, the usual question is "What should I do first?" Many of them expect some magic IDE and are quite surprised with an answer that a pure Notepad would be enough :)
This lesson (and the series as a whole) would provide an invaluable help to the students and guide them step-by-step into the Java world. It contains examples which are easy to follow and understand. Such useful approach can make more people say "Hey, I can do this! Let me try further..." and attract new developers into our community.

Yakov FaAin 11/18/03 04:25:54 PM EST

Hi J.D,

I agree with you, object-oriented nature is important. But if I start with explaining OOP to people with different backgrounds, I''ll lose a half of my students right away. Guess what? I usually do this on the lesson #2 :)

Regards,
Yakov Fain

J.D. Baker 11/18/03 11:53:06 AM EST

What''s here is ok, but it ignores the object-oriented nature of the Java programming language. IMHO, the proper approach to Hello World in Java is a version published by Shengyang Shong (I forget where). This version of Hello World had a class with a main called SayHello that instantiated a Mouth that had a say() method that printed "hello". This example can be expanded, introducing constructors for the Mouth and overloaded say() methods.

J.D.

Latest Stories
Despite being the market leader, we recognized the need to transform and reinvent our business at Dynatrace, before someone else disrupted the market. Over the course of three years, we changed everything - our technology, our culture and our brand image. In this session we'll discuss how we navigated through our own innovator's dilemma, and share takeaways from our experience that you can apply to your own organization.
Blockchain has numerous revolutionary ambitions, but technology hasn’t evolved enough to make them practical. What is the best use of blockchain technology today? How are asset owners and managers looking at blockchain to transform ownership structures? How will blockchain technology allow global investors to access new markets? What kinds of companies will take advantage of blockchain technology as a more efficient way to raise capital?
This session describes how Professional Services organisations can deliver within Technology-as-a-Service (IaaS) constructs, in private and public enterprise cloud scenarios. See how professional services can be packaged and funded by IaaS cash flows, based upon consumption of technology services. Learn how significant, IT infrastructure transformations can be funded through OPEX spending models with multi-year As-a-Services based contracts. Understand how the automation of repeatable services c...
Since their inception, Total Uptime has been delivering on their promise of unprecedented uptime to major brands as well as Fortune 500 corporations in the United States and around the world. Their Cloud network spans 17 countries and delivers the unique ability to seamlessly route Internet traffic from one site to the next when our customers need us to. It is built on the best SSAE 16 SOC 2 Type 2 and CSAE 3416 Datacenters and boasts an impressive line-up of Network and Transit Provider Redunda...
​Blockchain, also known as Distributed Ledger Technology (DLT), can be used for more than just crypto currencies. This presentation discusses how blockchain and the Internet of Things (IOT) can be used for several supply chain management and logistics use cases. Real world examples that utilize the open source project Hyperledger will be discussed.
Gym Solutions is a software as a service (SaaS) solution purpose-built to service the fitness industry, with over 1000 clients servicing over 2 million gym members across 40 countries making Perfect Gym Solutions one of the largest and fastest growing SaaS applications in the fitness industry. Their platform is a comprehensive package of modern modular products for the management of fitness clubs, sports facilities and gyms an end- to end solution, revolutionising the way that gyms are managed. ...
Enterprises are universally struggling to understand where the new tools and methodologies of DevOps fit into their organizations, and are universally making the same mistakes. These mistakes are not unavoidable, and in fact, avoiding them gifts an organization with sustained competitive advantage, just like it did for Japanese Manufacturing Post WWII.
Eric Taylor, a former hacker, reveals what he's learned about cybersecurity. Taylor's life as a hacker began when he was just 12 years old and playing video games at home. Russian hackers are notorious for their hacking skills, but one American says he hacked a Russian cyber gang at just 15 years old. The government eventually caught up with Taylor and he pleaded guilty to posting the personal information on the internet, among other charges. Eric Taylor, who went by the nickname Cosmo...
There's no doubt that blockchain technology is a powerful tool for the enterprise, but bringing it mainstream has not been without challenges. As VP of Technology at 8base, Andrei is working to make developing a blockchain application accessible to anyone. With better tools, entrepreneurs and developers can work together to quickly and effectively launch applications that integrate smart contracts and blockchain technology. This will ultimately accelerate blockchain adoption on a global scale.
As the fourth industrial revolution continues to march forward, key questions remain related to the protection of software, cloud, AI, and automation intellectual property. Recent developments in Supreme Court and lower court case law will be reviewed to explain the intricacies of what inventions are eligible for patent protection, how copyright law may be used to protect application programming interfaces (APIs), and the extent to which trademark and trade secret law may have expanded relev...
The Blockchain Benchmark asks and answers the questions many people want to know about the state of Blockchain: What are the biggest barriers? What was your motivation to get involved? When will it mainstream? Who are the true influencers? What are its top use cases? Who will win over the next 5 years? How will the future unfold? And 20+ other valuable questions.
Ivo Lukas is the Founder/CEO for 24Notion. 24Notion is the first integrated marketing/digital PR & lifestyle agency with special emphasis on giving back to the global communities. With a broad understanding the art of non- traditional marketing, new media, communications and social influence. 24Notion ranked #12 in Corporate Philanthropy nominated by Portland Business Journal Book of List.
SUSE is a German-based, multinational, open-source software company that develops and sells Linux products to business customers. Founded in 1992, it was the first company to market Linux for the enterprise. Founded in 1992, SUSE is the world’s first provider of an Enterprise Linux distribution. Today, thousands of businesses worldwide rely on SUSE for their mission-critical computing and IT management needs.
Provide an overview of the capabilities of Azure Stack allowing you or your customers to adopt truly consistent Hybrid Cloud capabilities to deliver greater productivity in your cloud world. Ultan Kinahan is on a member of the Global Black Belt team at Microsoft with a focus on Azure Stack Hybrid Cloud. Ultan has been in the Azure team since the beginning, Has held roles in Engineering, Sales and now consults with both small to medium size business and the worlds largest organizations on how to ...
SAP is the world leader in enterprise applications in terms of software and software-related service revenue. Based on market capitalization, we are the world's third largest independent software manufacturer. Harness the power of your data and accelerate trusted outcome-driven innovation by developing intelligent and live solutions for real-time decisions and actions on a single data copy. Support next-generation transactional and analytical processing with a broad set of advanced analytics - r...