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First Java Program

Beginner ~20 min read

Welcome to your first Java program! In this lesson, you'll write the classic "Hello, World!" program and learn the fundamental structure of a Java application. You'll understand how to compile and execute Java code, and get familiar with the main method - the entry point of every Java application.

Hello, World!

The traditional first program in any programming language is "Hello, World!" - a simple program that displays text. In Java, even this simple program requires understanding a few key concepts.

Output 
Click Run to execute your code
Key Components: Every Java program must have:
  • A class declaration (Java is object-oriented)
  • A main method (entry point for execution)
  • Code statements inside the method

Understanding the Structure

Let's break down the Hello World program line by line:

Output 
Click Run to execute your code

Line-by-Line Explanation

Line Explanation
public class HelloWorld Declares a public class named HelloWorld. The class name must match the filename (HelloWorld.java)
{ Opening brace starts the class body
public static void main(String[] args) The main method - entry point. public makes it accessible, static allows calling without creating an object, void means it returns nothing
System.out.println("Hello, World!"); Prints "Hello, World!" to the console and moves to a new line
} Closing braces end the method and class
Naming Convention: Class names in Java use PascalCase (each word starts with a capital letter). The class name must exactly match the filename (case-sensitive).

The main Method

The main method is special in Java - it's the entry point where program execution begins. The JVM looks for this exact method signature to start running your program.

main Method Signature: 
public static void main(String[] args)
  • public - Accessible from anywhere (required by JVM)
  • static - Can be called without creating a class instance
  • void - Doesn't return any value
  • main - Method name (must be exactly "main")
  • String[] args - Command-line arguments (array of strings)
Important: The main method signature is fixed. Changing any part of it (like removing static or changing the parameter type) will prevent your program from running!

System.out.println

System.out.println is used to display output on the console. Let's understand each part:

  • System - A built-in class in Java
  • out - A static member of System class (represents standard output stream)
  • println - Method that prints the argument and adds a newline

println vs print

Java provides two methods for output:

Output 
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Method Behavior Example
System.out.print() Prints without newline System.out.print("Hello"); System.out.print("World"); → "HelloWorld"
System.out.println() Prints with newline System.out.println("Hello"); System.out.println("World"); → "Hello\nWorld"

Compilation and Execution

Java programs require a two-step process: compilation and execution.

Java Compilation Process

Step 1: Compile

Use the javac command to compile your Java source file:

javac HelloWorld.java

This creates a HelloWorld.class file containing bytecode.

Step 2: Execute

Use the java command to run the compiled program:

java HelloWorld

Note: Don't include the .class extension when running!

Quick Test: The interactive compiler above handles compilation and execution automatically. Try modifying the code and clicking "Run" to see the output instantly!

Class Name Must Match Filename

In Java, the public class name must exactly match the filename (without the .java extension). This is a Java requirement.

Filename Class Name Valid?
HelloWorld.java public class HelloWorld ✓ Yes
HelloWorld.java public class Hello ✗ No - Compile error
hello.java public class HelloWorld ✗ No - Compile error
Case Sensitivity: Java is case-sensitive! HelloWorld, helloworld, and HELLOWORLD are all different. The class name and filename must match exactly, including case.

Common Mistakes

1. Missing main method

public class HelloWorld {
    // Missing main method!
    System.out.println("Hello");
}

Error: Error: Main method not found in class HelloWorld

Solution: Every program needs a main method

2. Wrong main method signature

// Wrong - missing static
public void main(String[] args) { ... }

// Wrong - wrong return type
public static int main(String[] args) { ... }

// Correct
public static void main(String[] args) { ... }

3. Class name doesn't match filename

// File: HelloWorld.java
public class Hello {  // Error! Class name must be HelloWorld
    ...
}

Error: class Hello is public, should be declared in a file named Hello.java

4. Missing semicolon

// Wrong
System.out.println("Hello")

// Correct
System.out.println("Hello");

Error: ';' expected

5. Missing quotes around strings

// Wrong
System.out.println(Hello World);

// Correct
System.out.println("Hello World");

Error: cannot find symbol: variable Hello

Exercise: Personalize Your Program

Task: Modify the Hello World program to introduce yourself!

Requirements:

  • Print your name
  • Print your favorite programming language
  • Print a greeting message
  • Use both print and println methods
Output 
Click Run to execute your code
💡 Hint

Try using multiple System.out.println() statements, or use System.out.print() for the name and System.out.println() for the language.

Summary

  • Every Java program must have a public class with a matching filename
  • The main method is the entry point: public static void main(String[] args)
  • System.out.println() prints output with a newline; System.out.print() prints without a newline
  • Compile with javac filename.java to create a .class file
  • Execute with java ClassName (no .class extension)
  • Java is case-sensitive - class names and filenames must match exactly
  • Statements must end with a semicolon (;)

What's Next?

Congratulations! You've written your first Java program. Now that you understand the basic structure, let's learn about Java syntax - statements, blocks, comments, and naming conventions. These fundamentals will help you write clean, readable Java code.

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Next Syntax Basics →