Let us write our first program in Java. We’ll print “Hello World” on the console.

1. Write Hello World program in Java

Write in following peace of code and save as file HelloWorldApplication and extension as java in your favorite code editor.

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

Output

Hello World

2. javac – Compile java program

javac HelloWorldApplication.java

The code which you have written will not work until and unless it is compiled and run. So, first we have to compile the code.

javac command is used to compile the Java code. The syntax of javac is:

javac [options] [source files]

Note: Both options and source files are optional in the javac command. You can provide multiple entries for both options and source files.

There are many options which you can provide with javac command.

When we compile the java file, .class file is generated if there is no compilation error.

2.1 Get help about javac command

javac -help

2.2 Compiling multiple files

You can provide multiple files with javac command separated by spaces.

javac FirstFile.java SecondFile.java

3. Run Java application

java HelloWorldApplication

java command is used to launch JVM which then executes the program. The basic syntax of java command is:

java [options] class [arguments]

Here options and arguments are optional and both can have multiple values.

Note: You need not to provide any extension with the class name. It is the .class file which will be executed. You can not use .java file with java command to run the program.

3.1 Passing arguments to java command

java HelloWorldApplication 1 99

In this command 1 and 99 are arguments provided to the java command. These will be received in the args parameter in our code:

public static void main(String[] args)

args[] is an array. If you want to access these arguments, you can access these arguments as args[0] and args[1].

public class HelloWorldApplication {
	public static void main(String[] args) {
		System.out.println(args[0]);
		System.out.println(args[1]);
	}
}

Output

1
99

4. Compiling Java file with package

When your class file is defined in a package, you need to provide some extra information to compile and run Java program.

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

Note that we have defined here that HelloWorldApplication is in com.learnitweb package.

4.1 How to compile a class defined in a package

javac -d . HelloWorldApplication.java

• Here -d defines the directory where generated files should be placed.
• . (dot) means place the generated class files in the current directory.
• HelloWorldApplication.java is the java program to be compiled.

On successful compilation, one folder will be created for every level of package. So our HelloWorldApplication.class file will be placed in learnitweb folder inside com folder.

4.2 How to run the Java program defined in a package

Since we have defined our java program inside a package, and class is generated inside the com\learnitweb folder. We have to provide fully classified name to run the program.

java com.learnitweb.HelloWorldApplication

5. Let’s discuss our Hello World Application

Let us understand the program we have written.

1. public class HelloWorldApplication

We are declaring a class here with name HelloWorldApplication. Class name should be preceded with word class. public here is the access modifier which defines class as public which means the class can be accessed within the package and outside the package.

2. curly braces ({ })

Curly braces define the starting and ending point of the block of code.

3. public static void main(String[] args)

Here we are defining a method with name main.

void means the method does not return any value.

The main method accepts one argument args which is of type String array. This is public and static method.

static means this method is associated with the class and not the object. We have declared main method as static because we run it without an object.

4. System.out.println(“Hello World”);

Prints Hello World in new line.

System is a class. out is a member variable in class System of type PrintStream. println is a method in class PrintStream.