// File: Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.print("No newline");
        System.out.printf("Name: %s, Age: %d%n", "Alice", 25);
    }
}

// Comments
// Single line comment
/* Multi-line
   comment */
/** Javadoc comment */

int age = 25;
double price = 9.99;
boolean active = true;
char grade = 'A';
String name = "Alice";

// Constants
final double PI = 3.14159;
final int MAX = 100;

// var (Java 10+)
var list = new ArrayList<String>();
var message = "Hello";

// Type casting
int x = (int) 3.7;   // 3
double d = (double) 5; // 5.0

byte   b = 127;          // 8-bit  -128 to 127
short  s = 32000;        // 16-bit
int    i = 2_000_000;    // 32-bit (default int)
long   l = 9_000_000_000L; // 64-bit (needs L)
float  f = 3.14f;        // 32-bit (needs f)
double d = 3.14159265;   // 64-bit (default)
boolean flag = true;     // true/false
char   c = 'A';          // unicode char (U+0041)

// Wrapper classes (for collections)
Integer, Double, Boolean, Character, Long

+ - * / %    // arithmetic
++x  x++     // pre/post increment
--x  x--     // pre/post decrement
x += 5; x -= 2; x *= 3; x /= 2;

== != > >= < <=   // comparison
&&  ||  !         // logical
&   |   ^   ~     // bitwise
<<  >>  >>>       // shift

int x = (condition) ? 1 : 0; // ternary
x instanceof String           // type check

// if-else
if (score >= 90) {
    System.out.println("A");
} else if (score >= 70) {
    System.out.println("B");
} else {
    System.out.println("F");
}

// switch
switch (day) {
    case "Mon": System.out.println("Monday"); break;
    default: System.out.println("Other");
}

// for
for (int i = 0; i < 5; i++) { System.out.println(i); }

// while
int i = 0;
while (i < 5) { System.out.println(i++); }

// for-each
for (String item : list) { System.out.println(item); }

// do-while
do { System.out.println(i++); } while (i < 5);

// Declare & initialize
int[] nums = {1, 2, 3, 4, 5};
String[] names = new String[3];
names[0] = "Alice";

int len = nums.length;   // 5
nums[0]                  // 1

// 2D array
int[][] matrix = {{1,2,3},{4,5,6}};
matrix[1][2]             // 6

// Sorting
import java.util.Arrays;
Arrays.sort(nums);
Arrays.fill(nums, 0);
int[] copy = Arrays.copyOf(nums, nums.length);
System.out.println(Arrays.toString(nums));

String s = "Hello World";

s.length()                // 11
s.toUpperCase()           // "HELLO WORLD"
s.toLowerCase()           // "hello world"
s.trim()                  // remove whitespace
s.charAt(0)               // 'H'
s.substring(0, 5)         // "Hello"
s.indexOf("World")        // 6
s.contains("World")       // true
s.startsWith("He")        // true
s.endsWith("ld")          // true
s.replace("World","Java") // "Hello Java"
s.split(" ")              // ["Hello","World"]
s.equals("Hello World")   // true (use this, not ==)
s.equalsIgnoreCase("HELLO") // true
String.format("Hi %s, age %d", name, age);

// StringBuilder (mutable)
StringBuilder sb = new StringBuilder();
sb.append("Hello").append(" ").append("World");
sb.toString();  // "Hello World"

public class Animal {
    private String name;      // private field
    protected int age;        // accessible by subclasses

    // Constructor
    public Animal(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Getter
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public String speak() { return name + " makes a sound."; }

    @Override
    public String toString() { return "Animal(" + name + ")"; }
}

// Inheritance
public class Dog extends Animal {
    public Dog(String name, int age) {
        super(name, age);  // call parent constructor
    }

    @Override
    public String speak() { return getName() + " barks!"; }
}

// Interface
public interface Swimmable {
    void swim(); // abstract by default
    default void float_() { System.out.println("Floating"); }
}

public class Duck extends Animal implements Swimmable {
    public void swim() { System.out.println("Duck swimming"); }
}

import java.util.*;

// ArrayList
ArrayList<String> list = new ArrayList<>();
list.add("Alice");
list.add(0, "Bob");      // add at index
list.get(0);             // "Bob"
list.set(0, "Charlie");  // update
list.remove(0);          // by index
list.remove("Alice");    // by value
list.size();
list.contains("Alice");
Collections.sort(list);

// HashMap
HashMap<String,Integer> map = new HashMap<>();
map.put("age", 25);
map.get("age");          // 25
map.getOrDefault("x",0); // 0
map.containsKey("age");
map.remove("age");
for (var entry : map.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

// HashSet (no duplicates)
HashSet<Integer> set = new HashSet<>();
set.add(1); set.contains(1); set.remove(1);

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
} catch (Exception e) {
    e.printStackTrace();
} finally {
    System.out.println("Always runs");
}

// Throw exception
throw new IllegalArgumentException("Invalid input");

// Custom exception
public class MyException extends RuntimeException {
    public MyException(String message) {
        super(message);
    }
}

// throws declaration
public void readFile() throws IOException {
    // ...
}