Day 1: Welcome to Java

What You'll Learn Today

• What Java is and why it matters
• Setting up your development environment
• Writing your first program (Hello World)
• Understanding program structure
• Writing comments

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What is Java?

Java is an object-oriented programming language developed by Sun Microsystems (now Oracle) in 1995. It was designed around the concept of "Write Once, Run Anywhere" -- meaning code written on one platform can run on any other platform that has a Java Virtual Machine.

flowchart LR
    subgraph Java["Key Features of Java"]
        OOP["Object-Oriented"]
        Platform["Platform Independent"]
        GC["Automatic Memory Management"]
        Typed["Statically Typed"]
    end
    style OOP fill:#3b82f6,color:#fff
    style Platform fill:#22c55e,color:#fff
    style GC fill:#f59e0b,color:#fff
    style Typed fill:#8b5cf6,color:#fff

How Java Works

Java source code is not compiled directly into machine code. Instead, it is compiled into an intermediate form called bytecode. This bytecode is then executed by the JVM (Java Virtual Machine).

flowchart LR
    Source[".java file<br>Source Code"] -->|"javac<br>Compile"| Bytecode[".class file<br>Bytecode"]
    Bytecode -->|"java<br>Execute"| JVM["JVM<br>Java Virtual Machine"]
    JVM --> OS["OS<br>Windows / macOS / Linux"]
    style Source fill:#3b82f6,color:#fff
    style Bytecode fill:#8b5cf6,color:#fff
    style JVM fill:#22c55e,color:#fff
    style OS fill:#f59e0b,color:#fff

Term	Description
JDK	Java Development Kit. The full set of tools needed for development
JRE	Java Runtime Environment. The environment needed to run Java programs
JVM	Java Virtual Machine. Executes bytecode
javac	The Java compiler
java	The Java execution command

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Setting Up Your Development Environment

Installing the JDK

Install the latest LTS (Long Term Support) version:

# macOS (Homebrew)
brew install openjdk@21

# Ubuntu / Debian
sudo apt install openjdk-21-jdk

# Windows (winget)
winget install EclipseAdoptium.Temurin.21.JDK

Verifying the Installation

java --version
javac --version

Expected output:

openjdk 21.0.2 2024-01-16
OpenJDK Runtime Environment (build 21.0.2+13)
OpenJDK 64-Bit Server VM (build 21.0.2+13, mixed mode)

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Your First Program

Hello World

Create a file named HelloWorld.java:

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

Compiling and Running

# Compile
javac HelloWorld.java

# Run
java HelloWorld

Output:

Hello, World!

Important: The file name must exactly match the class name. A HelloWorld class must be saved as HelloWorld.java.

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Program Structure

// 1. Class declaration
public class HelloWorld {

    // 2. main method (entry point)
    public static void main(String[] args) {

        // 3. Statement
        System.out.println("Hello, World!");
    }
}

flowchart TB
    subgraph Structure["Java Program Structure"]
        Class["Class<br>public class MyApp"]
        Method["main Method<br>public static void main(String[] args)"]
        Statement["Statement<br>System.out.println(...)"]
    end
    Class --> Method --> Statement
    style Class fill:#3b82f6,color:#fff
    style Method fill:#22c55e,color:#fff
    style Statement fill:#f59e0b,color:#fff

Element	Description
public	Access modifier (accessible from anywhere)
class	Declares a class
static	Can be called without creating an instance
void	No return value
main	The program's entry point
String[] args	Command-line arguments

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Output Methods

public class OutputDemo {
    public static void main(String[] args) {
        // With newline
        System.out.println("Hello");
        System.out.println("World");

        // Without newline
        System.out.print("Hello ");
        System.out.print("World");
        System.out.println(); // Newline only

        // Formatted output
        String name = "Java";
        int version = 21;
        System.out.printf("Welcome to %s %d!%n", name, version);
    }
}

Method	Newline	Use Case
println()	Yes	Standard output
print()	No	Continuous output
printf()	No	Formatted output

printf Format Specifiers

Specifier	Type	Example
%s	String	"Hello"
%d	Integer	42
%f	Floating point	3.14
%.2f	2 decimal places	3.14
%n	Newline	-

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Comments

public class CommentDemo {
    public static void main(String[] args) {
        // Single-line comment

        /*
         * Multi-line comment
         * Second line
         */

        /**
         * Javadoc comment
         * Used for generating documentation
         * @param args command-line arguments
         */

        System.out.println("Comment demo");
    }
}

Comment Type	Syntax	Purpose
Single-line	// ...	Brief explanation
Multi-line	/* ... */	Longer explanation
Javadoc	/** ... */	API documentation

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Handy Features in Java 21+

JShell (Interactive REPL)

$ jshell
|  Welcome to JShell -- Version 21
jshell> System.out.println("Hello!")
Hello!

jshell> 2 + 3
$1 ==> 5

jshell> /exit

Tip: JShell lets you try out Java code without creating a file. It is extremely useful while learning.

Running Source Files Directly (Java 11+)

# Run directly without compiling first
java HelloWorld.java

Since Java 11, you can run a single source file directly without an explicit javac step.

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Practice: Self-Introduction Program

public class SelfIntroduction {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 25;
        String hobby = "programming";

        System.out.println("=== Self Introduction ===");
        System.out.printf("Name: %s%n", name);
        System.out.printf("Age: %d%n", age);
        System.out.printf("Hobby: %s%n", hobby);
        System.out.println("=========================");
    }
}

Output:

=== Self Introduction ===
Name: Alice
Age: 25
Hobby: programming
=========================

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Summary

Concept	Description
Java	A platform-independent, object-oriented language
JDK	Development toolkit (includes the compiler)
JVM	Virtual machine that executes bytecode
javac	Compiles source code
java	Runs a program
main method	The program's entry point
System.out.println()	Prints to the console

Key Takeaways

1. Java is a compiled language (source -> bytecode -> JVM execution)
2. The file name must match the class name
3. The main method is the entry point of every program
4. Use jshell to interactively experiment with code

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Exercises

Exercise 1: Basic

Write a program that displays your name, age, and favorite food.

Exercise 2: Applied

Using printf, write a program that displays a product name and its price in a formatted way, such as "Product: $19.99".

Exercise 3: Challenge

Using command-line arguments (args), write a program so that running java Greeting Alice prints "Hello, Alice!".

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References

• Oracle - Java Documentation
• OpenJDK
• JShell User's Guide

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Next up: On Day 2, we'll explore Variables and Data Types. You'll learn about Java's type system and how to store and manipulate data.