Lesson 1.7: Writing Your First Java Program

Introduction

Now that your Java development environment is ready, it’s time to write your first Java program.

Every programmer starts with a simple program called “Hello, World!”. Although it is a small program, it introduces the basic structure of every Java application.

In this lesson, you will write, compile, and run your first Java program while understanding every line of code.

Learning Objectives

After completing this lesson, you will be able to:

• Write your first Java program.

• Understand the structure of a Java program.

• Explain the purpose of each line of code.

• Compile and run a Java program.

• Follow Java naming conventions.

Your First Java Program

Create a file named Hello.java and type the following code:

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

Save the file.

Output

Hello, World!

Understanding the Program Line by Line

Line 1

public class Hello

Explanation

• public → An access modifier that makes the class accessible from anywhere.

• class → A keyword used to define a class.

• Hello → The name of the class.

Note: The file name must be Hello.java because the class name is Hello.

Line 2

{

The opening curly brace { marks the beginning of the class.

Line 3

public static void main(String[] args)

This is called the main method.

It is the starting point of every Java application.

Whenever a Java program runs, the JVM first looks for the main() method.

Understanding Each Keyword

public

Allows the JVM to access the method.

static

Allows the method to run without creating an object of the class.

void

Means the method does not return any value.

main

The predefined method name where program execution begins.

String[] args

Stores command-line arguments passed to the program.

Line 4

{

The opening brace of the main() method.

Everything inside this block will be executed.

Line 5

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

This statement prints text on the screen.

Understanding Each Part

System

A predefined Java class.

out

Represents the standard output stream (the console).

println()

Prints text and moves the cursor to the next line.

“Hello, World!”

This is a String Literal.

Everything inside double quotation marks is treated as text.

Line 6

}

Closes the main() method.

Line 7

}

Closes the class.

Program Flow

When you run the program:

1. JVM loads the class.

2. JVM finds the main() method.

3. The statements inside main() execute from top to bottom.

4. The text Hello, World! is displayed.

5. The program ends.

Using Different Output Statements

Example 1

System.out.println("Welcome to Java");

Output

Welcome to Java

Example 2

System.out.println("My Name is Rahul"); System.out.println("I am learning Java.");

Output

My Name is Rahul
I am learning Java.

Example 3

System.out.print("Hello "); System.out.print("Students");

Output

Hello Students

Difference

print()

• Prints output on the same line.

println()

• Prints output and moves to the next line.

Escape Characters

Java provides special characters called escape sequences.

Example

public class EscapeExample { public static void main(String[] args) { System.out.println("Java\nProgramming"); } }

Output

Java
Programming

Common Mistakes Made by Beginners

Incorrect File Name

hello.java

Class

public class Hello

This causes a compilation error because the file name and class name must match.

Missing Semicolon

Incorrect

System.out.println("Hello")

Correct

System.out.println("Hello");

Missing Curly Braces

Always close every opening { with a closing }.

Wrong Capitalization

Incorrect

system.out.println("Hello");

Correct

System.out.println("Hello");

Java is case-sensitive.

Best Practices

• Use meaningful class names.

• Keep proper indentation.

• Always end statements with a semicolon (;).

• Save the file before running it.

• Match the file name with the public class name.

Key Points to Remember

• Every Java program starts from the main() method.

• Java is case-sensitive.

• The file name and public class name must be the same.

• System.out.println() prints text on the console.

• Statements end with a semicolon (;).

Lesson Summary

In this lesson, you learned:

• How to write your first Java program.

• The structure of a Java program.

• The purpose of the main() method.

• How System.out.println() works.

• The difference between print() and println().

• Common programming mistakes and how to avoid them.

You have now successfully written and understood your first Java program.

Practice Questions

1. What is the purpose of the main() method?

2. What does the public keyword mean?

3. Why is the static keyword used in the main() method?

4. What is the difference between print() and println()?

5. Why must the file name match the public class name?

6. What is an escape sequence?

Programming Exercise

Write Java programs to:

1. Print your name.

2. Print your college name.

3. Print your address.

4. Print three lines using println().

5. Print two words on the same line using print().

6. Use \n and \t in a program.

Assignment

Create a Java program that displays the following:

Name      : Your Name
College   : Your College Name
Department: Computer Science
Course    : Core Java Programming

Use proper formatting with println() and escape characters.