Welcome!

Eclipse Authors: Pat Romanski, Elizabeth White, Liz McMillan, David H Deans, JP Morgenthal

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)

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.


IoT & Smart Cities Stories
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
AI and machine learning disruption for Enterprises started happening in the areas such as IT operations management (ITOPs) and Cloud management and SaaS apps. In 2019 CIOs will see disruptive solutions for Cloud & Devops, AI/ML driven IT Ops and Cloud Ops. Customers want AI-driven multi-cloud operations for monitoring, detection, prevention of disruptions. Disruptions cause revenue loss, unhappy users, impacts brand reputation etc.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
Atmosera delivers modern cloud services that maximize the advantages of cloud-based infrastructures. Offering private, hybrid, and public cloud solutions, Atmosera works closely with customers to engineer, deploy, and operate cloud architectures with advanced services that deliver strategic business outcomes. Atmosera's expertise simplifies the process of cloud transformation and our 20+ years of experience managing complex IT environments provides our customers with the confidence and trust tha...
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 tailored market studies; and more.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility. As they do so, IT professionals are also embr...
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility.
Today's workforce is trading their cubicles and corporate desktops in favor of an any-location, any-device work style. And as digital natives make up more and more of the modern workforce, the appetite for user-friendly, cloud-based services grows. The center of work is shifting to the user and to the cloud. But managing a proliferation of SaaS, web, and mobile apps running on any number of clouds and devices is unwieldy and increases security risks. Steve Wilson, Citrix Vice President of Cloud,...