Getting Started with Java

JDK, JRE, and JVM

Before writing your first Java program, it's important to understand the three core components of the Java platform.

Installing the JDK

Follow these steps to install the JDK on your machine:

• Visit oracle.com/java/technologies/downloads and download the latest JDK (Java 21 LTS recommended).
• Run the installer and follow the on-screen instructions.
• Set the JAVA_HOME environment variable to the JDK installation directory (e.g., C:\Program Files\Java\jdk-21).
• Add %JAVA_HOME%\bin to your system PATH.
• Verify the installation by opening a terminal and running java -version.

Your First Java Program

Every Java program starts with a class. The entry point is the main method. Let's break down the classic Hello World program line by line.

// HelloWorld.java
public class HelloWorld {                          // 1. Class declaration
    public static void main(String[] args) {       // 2. Entry point
        System.out.println("Hello, World!");       // 3. Print to console
    }
}

• public class HelloWorld — declares a public class. The filename must match the class name exactly: HelloWorld.java.
• public static void main(String[] args) — the JVM calls this method to start the program.
• System.out.println() — prints a line to standard output.

Compile and Run

Java is a compiled language. You first compile the source file to bytecode, then run the bytecode on the JVM.

Reading User Input with Scanner

The java.util.Scanner class lets you read input from the keyboard (standard input).

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.print("Enter your age: ");
        int age = scanner.nextInt();

        System.out.println("Hello, " + name + "! You are " + age + " years old.");

        scanner.close(); // always close the scanner
    }
}