Java introduction: what Java is, where it's used, the JDK and JVM, your first program, syntax rules and input/output

Main.java  --javac (compiler)-->  Main.class (bytecode)  --JVM-->  runs on Windows/Linux/macOS

javac Main.java     # creates Main.class
java Main           # runs it
java Main.java      # newer JDKs can compile and run a single file in one step

class Main {
    public static void main(String[] args) {
        System.out.println("Habari, Kenya!");
        System.out.println("Learning Java on the Marzley learning hub.");
    }
}

class Main {
    public static void main(String[] args) {
        int students = 45;               // whole numbers
        long population = 53_000_000L;   // big whole numbers (underscores for readability)
        double fee = 12500.50;           // decimals
        float discount = 0.1f;           // smaller decimals (f suffix)
        char grade = 'A';                // one character
        boolean paid = true;             // true/false
        String school = "Kenya High";    // text (a class, not a primitive)

        System.out.println(school + ": " + students + " students, fee KSh " + fee);
        System.out.println("Grade " + grade + ", paid: " + paid + ", population " + population);

        final double VAT_RATE = 0.16;    // final = constant
        var town = "Kisumu";             // var infers the type (String)
        System.out.println(town + " VAT rate " + VAT_RATE + ", discount " + discount);
    }
}

class Main {
    public static void main(String[] args) {
        int a = 17, b = 5;
        System.out.println(a + b);          // 22
        System.out.println(a / b);          // 3: int / int drops the decimal!
        System.out.println(a % b);          // 2: remainder
        System.out.println((double) a / b); // 3.4: cast to double first

        int count = 10;
        count++;                            // 11
        count += 5;                         // 16
        System.out.println(count);

        double price = 1999.99;
        int whole = (int) price;            // narrowing cast: 1999 (decimal dropped)
        System.out.println(whole);

        int bigSum = Integer.MAX_VALUE;
        System.out.println(bigSum + 1);     // overflow wraps to a negative number!
        System.out.println(Math.round(12.567 * 100) / 100.0);   // 12.57
    }
}

class Main {
    public static void main(String[] args) {
        String name = "Amina";
        double balance = 15250.5;
        int items = 3;
        System.out.printf("Hello %s, you have %d items.%n", name, items);
        System.out.printf("Balance: KSh %,.2f%n", balance);
        System.out.println(String.format("%-10s|%6d|", "Chapati", 30));
    }
}

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = sc.nextLine();
        System.out.print("Enter your age: ");
        int age = sc.nextInt();
        System.out.println("Hello " + name + ", next year you'll be " + (age + 1));
        sc.close();
    }
}

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/** Javadoc comment: documents classes and methods */