Your First Java Program

This lesson takes you through writing your first Java program and understanding how Java programs are structured and executed.

Hello World

The first step for every programmer learning a new language is writing Hello World.

Example: Hello World

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

Output:

Hello, World!

Code Breakdown

Compile and Run Process

Java programs must be compiled before they can run.

Step 1: Write the Source Code

Create a file named HelloWorld.java (the filename must match the class name).

Step 2: Compile

javac HelloWorld.java

A successful compile generates a HelloWorld.class bytecode file.

Step 3: Run

java HelloWorld

Complete Process

# Compile
javac HelloWorld.java

# Run
java HelloWorld

# Output: Hello, World!

Program Structure Explained

Class Definition

public class HelloWorld {
    // Class body
}

• public: Access modifier, means publicly accessible
• class: Keyword, indicates a class definition
• HelloWorld: Class name, must match the filename
• { }: Class body, contains methods and attributes

Main Method

public static void main(String[] args) {
    // Method body
}

• public: Publicly accessible
• static: Can be called without creating an object
• void: No return value
• main: Method name, fixed as the program entry point
• String[] args: String array parameter

Output Statements

// Print with newline
System.out.println("Hello");

// Print without newline
System.out.print("Hello");

// Formatted output
System.out.printf("Name: %s, Age: %d", "Alice", 25);

JShell Interactive Learning

JShell is Java's interactive tool. It lets you execute code snippets directly without writing a complete class.

// Direct calculation
jshell> 5 * 3
$1 ==> 15

// Define a variable
jshell> String name = "Alice"
name ==> "Alice"

// Print output
jshell> System.out.println("Hello, " + name)
Hello, Alice

// Define a method
jshell> int add(int a, int b) {
   ...>     return a + b;
   ...> }
| created method add(int,int)

// Call the method
jshell> add(3, 4)
$2 ==> 7

Comments

Comments are for developers to read. The compiler ignores them.

Single-line Comments

// This is a single-line comment
int age = 25;  // End-of-line comment

Multi-line Comments

/*
 * This is a multi-line comment
 * You can write many lines
 */

Documentation Comments

/**
 * Calculates the sum of two numbers
 * @param a The first number
 * @param b The second number
 * @return The sum of a and b
 */
public int add(int a, int b) {
    return a + b;
}

❓ Frequently Asked Questions

📖 Summary

• Java programs are made up of classes, and the main method is the program entry point
• Use javac to compile and java to run
• The filename must match the public class name
• JShell lets you execute code snippets interactively
• There are three types of comments: single-line, multi-line, and documentation

📝 Exercises

1. Hello World: Write and run your first Java program that outputs "Hello, Java!"
2. Personal info: Write a program that prints your name, age, and city (using multiple println statements)
3. JShell practice: Use JShell to calculate: If you save 1000 per month, how much can you save in a year?

Next Lesson

In the next lesson, we'll learn about Variables and Data Types — understanding how data is stored in Java.