⚙️ Setup & Hello World

Installing the JDK

Before you can write and run Java applications, you need to install the Java Development Kit (JDK). Here are the standard steps:

1. Download the latest JDK from the official Oracle website or adopt an OpenJDK distribution like Eclipse Temurin.
2. Run the installer and follow the on-screen instructions.
3. Note the installation path (e.g., C:\Program Files\Java\jdk-17).

Setting the PATH Variable

To compile and run Java programs from any command line window, you must add the JDK's bin folder to your system's PATH variable:

• Windows: Open System Properties > Advanced > Environment Variables. Edit the Path variable and add the path to your JDK bin directory.
• macOS/Linux: Add export PATH=$PATH:/path/to/jdk/bin to your ~/.bashrc or ~/.zshrc file.

Anatomy of a Java Program

Every Java application requires at least one class and a main method. The main method acts as the entry point for the JVM.

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

Compilation and Execution Commands

To run the program above manually from a command line:

1. Compile the code: javac HelloWorld.java. This creates a file named HelloWorld.class containing the bytecode.
2. Run the code: java HelloWorld. This commands the JVM to execute the main method inside the compiled class.

Naming Conventions

• Classes: PascalCase (e.g., HelloWorld)
• Methods & Variables: camelCase (e.g., main, myVariable)
• Constants: UPPER_SNAKE_CASE (e.g., MAX_VALUE)
• Packages: lowercase (e.g., java.util)

Interactive Programs using Scanner

Let's make our programs interactive by reading user input using the Scanner class.

import java.util.Scanner;

public class UserInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        
        System.out.println("Welcome to Java, " + name + "!");
        scanner.close();
    }
}
            

Building a Simple Calculator

import java.util.Scanner;

public class SimpleCalculator {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        
        System.out.print("Enter first number: ");
        int num1 = scan.nextInt();
        
        System.out.print("Enter second number: ");
        int num2 = scan.nextInt();
        
        int sum = num1 + num2;
        System.out.println("The sum is: " + sum);
        
        scan.close();
    }
}