// ① PACKAGE DECLARATION (optional — organises .java files into folders)
package com.university.java;

// ② IMPORT STATEMENTS (bring in classes from other packages)
import java.util.Scanner;
import java.util.Arrays;

// ③ CLASS DECLARATION (required — wraps ALL code)
public class MyProgram {

    // ④ FIELDS / CLASS VARIABLES (optional — data at class level)
    static int counter = 0;

    // ⑤ MAIN METHOD (required entry point — JVM starts here)
    public static void main(String[] args) {

        // ⑥ LOCAL VARIABLE DECLARATION
        int age = 20;

        // ⑦ STATEMENTS (instructions — each ends with ;)
        System.out.println("Hello from MyProgram!");
        System.out.println("Age: " + age);

    } // end main

    // ⑧ OTHER METHODS (optional user-defined methods)
    static void greet() {
        System.out.println("Hello!");
    }

} // end class MyProgram

// This entire line is a comment — compiler ignores it
int age = 20;    // inline comment: this variable stores the student's age

// Comments can temporarily disable code (useful for debugging):
// System.out.println("This line is commented out — won't run");
System.out.println("This line runs");   // active code

/*
 * This is a multi-line comment.
 * Useful for longer explanations,
 * algorithm descriptions, or section headers.
 * The * at the start of each line is just convention — not required.
 */
int result = 0;

/* You can also write it without the leading stars:
   This is still a valid multi-line comment.
   It ends here: */
System.out.println(result);

/**
 * Calculates the area of a rectangle.
 * This method multiplies length by width to compute the area.
 *
 * @param length  The length of the rectangle (must be positive)
 * @param width   The width of the rectangle (must be positive)
 * @return        The area of the rectangle as a double
 * @author        Ahmed Ali
 * @version       1.0
 */
public static double rectangleArea(double length, double width) {
    return length * width;
}

System.out.println("Hello");        // prints: Hello↵
System.out.println("World");        // prints: World↵

System.out.println(42);             // int: 42
System.out.println(3.14);           // double: 3.14
System.out.println(true);           // boolean: true
System.out.println('A');            // char: A
System.out.println(2 + 3);         // expression: 5
System.out.println();              // empty println → blank line
System.out.println("Age: " + 20); // concatenation: Age: 20

// Using print — all on same line:
System.out.print("A");
System.out.print("B");
System.out.print("C");
// Output: ABC  (one line, cursor stays after C)

System.out.println();  // manually add newline

// Mixing print and println:
System.out.print("First ");
System.out.print("Second ");
System.out.println("Third");   // adds newline
System.out.println("Fourth");  // starts on new line
// Output:
// First Second Third
// Fourth

// Syntax: System.out.printf("format string", arg1, arg2, ...);
// Each %specifier in the format string is replaced by the next argument

System.out.printf("Name: %s%n", "Ahmed");
// Output: Name: Ahmed

System.out.printf("Age: %d%n", 20);
// Output: Age: 20

System.out.printf("GPA: %.2f%n", 3.75689);
// Output: GPA: 3.76  (rounds to 2 decimal places)

System.out.printf("%-10s %5d %8.2f%n", "Ahmed", 20, 3.75);
// Output: Ahmed          20     3.75
//         ↑ left-aligned  ↑ right-aligned in 5 chars

// Multiple values in one printf:
String name = "Sara";
int age = 19;
double gpa = 3.9;
System.out.printf("Student: %s | Age: %d | GPA: %.1f%n", name, age, gpa);
// Output: Student: Sara | Age: 19 | GPA: 3.9

// \n — newline within a single string
System.out.println("Line 1\nLine 2\nLine 3");
// Output:
// Line 1
// Line 2
// Line 3

// \t — tab alignment
System.out.println("Name\tAge\tCity");
System.out.println("Ahmed\t20\tDubai");
System.out.println("Sara\t19\tAbu Dhabi");
// Output:
// Name    Age     City
// Ahmed   20      Dubai
// Sara    19      Abu Dhabi

// \" — printing double quotes
System.out.println("She said \"Java is powerful!\"");
// Output: She said "Java is powerful!"

// \\ — printing backslash (Windows file path)
System.out.println("Path: C:\\Users\\Ahmed\\Desktop");
// Output: Path: C:\Users\Ahmed\Desktop

// \u — Unicode character
System.out.println("\u0048\u0065\u006C\u006C\u006F");
// Output: Hello  (each \uXXXX is one Unicode character)

// Combining multiple escapes:
System.out.println("Results:\n\tMath:\t95\n\tScience:\t88");
// Output:
// Results:
//     Math:   95
//     Science:    88

// %10d → right-align integer in 10-character-wide field
System.out.printf("%10d%n", 42);
// Output: "        42"  (8 spaces + 42)

// %-10d → left-align in 10-character-wide field
System.out.printf("%-10d|%n", 42);
// Output: "42        |"

// Building a formatted table:
System.out.printf("%-15s %5s %8s%n", "Name", "Age", "GPA");
System.out.printf("%-15s %5d %8.2f%n", "Ahmed Ali", 20, 3.75);
System.out.printf("%-15s %5d %8.2f%n", "Sara Khan", 19, 3.90);
System.out.printf("%-15s %5d %8.2f%n", "Omar Salem", 21, 3.55);
// Output:
// Name              Age      GPA
// Ahmed Ali          20     3.75
// Sara Khan          19     3.90
// Omar Salem         21     3.55

// Keywords are lowercase — uppercase versions don't exist:
public  ✓    Public  ✗    PUBLIC  ✗
class   ✓    Class   ✗    CLASS   ✗
static  ✓    Static  ✗    STATIC  ✗
void    ✓    Void    ✗    VOID    ✗
int     ✓    Int     ✗    INT     ✗

// Class names — MUST match filename exactly:
HelloWorld  ✓  (file: HelloWorld.java)
helloworld  ✗  (would need file: helloworld.java)

// System.out.println — all three parts case-sensitive:
System.out.println()   ✓
system.out.println()   ✗  (system must be capital S)
System.Out.println()   ✗  (out must be lowercase)
System.out.PrintLn()   ✗  (method name must be println)

// Variable names — case creates completely different variables:
int age = 20;
int Age = 25;   // different variable! 'age' ≠ 'Age'
int AGE = 30;   // another different variable!