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
HyperConvergence came to market with the objective of being simple, flexible and to help drive down operating expenses. It reduced the footprint by bundling the compute/storage/network into one box. This brought a new set of challenges as the HyperConverged vendors are very focused on their own proprietary building blocks. If you want to scale in a certain way, let's say you identified a need for more storage and want to add a device that is not sold by the HyperConverged vendor, forget about it...
In his keynote at 19th Cloud Expo, Sheng Liang, co-founder and CEO of Rancher Labs, discussed the technological advances and new business opportunities created by the rapid adoption of containers. With the success of Amazon Web Services (AWS) and various open source technologies used to build private clouds, cloud computing has become an essential component of IT strategy. However, users continue to face challenges in implementing clouds, as older technologies evolve and newer ones like Docker c...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
@DevOpsSummit at Cloud Expo, taking place November 12-13 in New York City, NY, is co-located with 22nd international CloudEXPO | first international DXWorldEXPO and will feature technical sessions from a rock star conference faculty and the leading industry players in the world.
The next XaaS is CICDaaS. Why? Because CICD saves developers a huge amount of time. CD is an especially great option for projects that require multiple and frequent contributions to be integrated. But… securing CICD best practices is an emerging, essential, yet little understood practice for DevOps teams and their Cloud Service Providers. The only way to get CICD to work in a highly secure environment takes collaboration, patience and persistence. Building CICD in the cloud requires rigorous ar...
"We're focused on how to get some of the attributes that you would expect from an Amazon, Azure, Google, and doing that on-prem. We believe today that you can actually get those types of things done with certain architectures available in the market today," explained Steve Conner, VP of Sales at Cloudistics, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Sanjeev Sharma Joins November 11-13, 2018 @DevOpsSummit at @CloudEXPO New York Faculty. Sanjeev Sharma is an internationally known DevOps and Cloud Transformation thought leader, technology executive, and author. Sanjeev's industry experience includes tenures as CTO, Technical Sales leader, and Cloud Architect leader. As an IBM Distinguished Engineer, Sanjeev is recognized at the highest levels of IBM's core of technical leaders.
JETRO showcased Japan Digital Transformation Pavilion at SYS-CON's 21st International Cloud Expo® at the Santa Clara Convention Center in Santa Clara, CA. The Japan External Trade Organization (JETRO) is a non-profit organization that provides business support services to companies expanding to Japan. With the support of JETRO's dedicated staff, clients can incorporate their business; receive visa, immigration, and HR support; find dedicated office space; identify local government subsidies; get...
As Cybric's Chief Technology Officer, Mike D. Kail is responsible for the strategic vision and technical direction of the platform. Prior to founding Cybric, Mike was Yahoo's CIO and SVP of Infrastructure, where he led the IT and Data Center functions for the company. He has more than 24 years of IT Operations experience with a focus on highly-scalable architectures.
Dion Hinchcliffe is an internationally recognized digital expert, bestselling book author, frequent keynote speaker, analyst, futurist, and transformation expert based in Washington, DC. He is currently Chief Strategy Officer at the industry-leading digital strategy and online community solutions firm, 7Summits.
All organizations that did not originate this moment have a pre-existing culture as well as legacy technology and processes that can be more or less amenable to DevOps implementation. That organizational culture is influenced by the personalities and management styles of Executive Management, the wider culture in which the organization is situated, and the personalities of key team members at all levels of the organization. This culture and entrenched interests usually throw a wrench in the work...
Bill Schmarzo, author of "Big Data: Understanding How Data Powers Big Business" and "Big Data MBA: Driving Business Strategies with Data Science," is responsible for setting the strategy and defining the Big Data service offerings and capabilities for EMC Global Services Big Data Practice. As the CTO for the Big Data Practice, he is responsible for working with organizations to help them identify where and how to start their big data journeys. He's written several white papers, is an avid blogge...
DXWorldEXPO LLC announced today that Dez Blanchfield joined the faculty of CloudEXPO's "10-Year Anniversary Event" which will take place on November 11-13, 2018 in New York City. Dez is a strategic leader in business and digital transformation with 25 years of experience in the IT and telecommunications industries developing strategies and implementing business initiatives. He has a breadth of expertise spanning technologies such as cloud computing, big data and analytics, cognitive computing, m...
As DevOps methodologies expand their reach across the enterprise, organizations face the daunting challenge of adapting related cloud strategies to ensure optimal alignment, from managing complexity to ensuring proper governance. How can culture, automation, legacy apps and even budget be reexamined to enable this ongoing shift within the modern software factory?