☕ What is Java?

📌 1. About Java

Java is a powerful, object-oriented programming language that is:

Platform Independent: "Write Once, Run Anywhere" (WORA)
Object-Oriented: Everything is an object
Secure: Built-in security features
Robust: Strong memory management
Multi-threaded: Supports concurrent programming

📌 2. Java is Used For

🌐 Web Development: Spring, JSP, Servlets
📱 Mobile Apps: Android development
🏢 Enterprise Applications: Large-scale systems
🖥️ Desktop Apps: Swing, JavaFX
☁️ Cloud Applications: Microservices
🎮 Games: Minecraft was built in Java!

📌 3. Why Learn Java?

💼 High Demand: One of the most popular languages
💰 Great Salary: Well-paying career opportunities
🌍 Large Community: Huge support and resources
📚 Rich Libraries: Extensive built-in functionality
🔄 Regular Updates: Constantly evolving

📌 4. Java vs Other Languages

Feature	Java	Python	C++
Learning Curve	Moderate	Easy	Hard
Performance	Fast	Slower	Very Fast
Platform	Cross-platform	Cross-platform	Platform-specific

📌 5. Java Versions

Java 8 (LTS): Lambda expressions, Stream API
Java 11 (LTS): Local variable type inference
Java 17 (LTS): Records, sealed classes
Java 21 (LTS): Latest long-term support version

💡 LTS = Long Term Support (recommended for production)

📌 6. Java Architecture

Java follows the "Write Once, Run Anywhere" principle:

📝 Source Code (.java) → Written by developers
⚙️ Bytecode (.class) → Compiled by javac
🔄 JVM (Java Virtual Machine) → Executes bytecode
💻 Any Platform → Windows, Linux, macOS

📌 7. Popular Java Frameworks

🌱 Spring Boot: Enterprise applications
🅰️ Android SDK: Mobile app development
🏗️ Hibernate: Database operations
🔧 Apache Maven: Project management
🐘 Apache Kafka: Streaming platform

📌 8. First Java Program Example

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

This simple program prints "Hello, World!" to the console!

Practice Editor

Java Editor

Output

☕ Getting Started with Java

📌 1. What You Need to Begin

A computer (Windows, macOS, or Linux)
Internet connection (for downloading JDK)
A text editor or IDE (like IntelliJ IDEA, Eclipse, or VS Code)
Java Development Kit (JDK)

📌 2. Installing Java Development Kit (JDK)

✅ Official Website:

Download JDK from: 👉 Oracle JDK or Eclipse Temurin

📦 Steps:

Choose the latest LTS version (Java 21 recommended)
Select your operating system
Download and run the installer
Follow the installation wizard

✅ Verify Installation:

Open your terminal (CMD, PowerShell, or Bash):

java --version

and

javac --version

📌 3. Setting up JAVA_HOME (Important!)

🪟 Windows:

Right-click "This PC" → Properties
Advanced System Settings → Environment Variables
New System Variable: JAVA_HOME
Value: Path to JDK (e.g., C:\Program Files\Java\jdk-21)

🐧 Linux/macOS:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk
export PATH=$JAVA_HOME/bin:$PATH

📌 4. Your First Java Program

✅ Create a file named HelloWorld.java:

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

✅ Compile and Run:

# Compile
javac HelloWorld.java

# Run
java HelloWorld

💡 Important: File name must match the class name!

📌 5. Java Development Tools

🔧 IDEs (Integrated Development Environments):

IntelliJ IDEA: Most popular, excellent features
Eclipse: Free, widely used in enterprise
VS Code: Lightweight with Java extensions
NetBeans: Official Oracle IDE

📝 Text Editors:

Sublime Text, Atom, Vim, Emacs

🌐 Online Compilers:

repl.it, codepen.io, jdoodle.com

📌 6. Java File Structure

Java files end with .java
Compiled files end with .class
Each public class must be in its own file
File name must match the public class name
Java is case-sensitive

📌 7. Understanding the Hello World Program

public class HelloWorld {           // Class declaration
    public static void main(String[] args) {  // Main method
        System.out.println("Hello, World!");  // Print statement
    }                                   // End of main method
}                                      // End of class

🔍 Breakdown:

public class HelloWorld - Defines a public class
main - Entry point of the program
String[] args - Command-line arguments
System.out.println() - Prints text to console

☕ Java Syntax: The Basics

📌 1. Java is Case Sensitive

Java is case-sensitive, meaning myVariable and MyVariable are different!

public class CaseSensitive {
    public static void main(String[] args) {
        int myVariable = 10;
        int MyVariable = 20;  // Different variable!
        
        System.out.println(myVariable);  // Prints: 10
        System.out.println(MyVariable);  // Prints: 20
    }
}

📌 2. Comments

Used to explain code. They are ignored by the compiler.

// This is a single-line comment

/*
 * This is a 
 * multi-line comment
 */

/**
 * This is a JavaDoc comment
 * Used for documentation
 */

📌 3. Print Statements

System.out.println("Hello World!");  // With new line
System.out.print("Hello ");           // Without new line
System.out.print("World!");           // Continues on same line

📌 4. Variables Declaration

Java is statically typed - you must declare variable types.

int age = 25;              // Integer
double price = 19.99;      // Double (decimal)
String name = "John";      // String
boolean isValid = true;    // Boolean
char grade = 'A';          // Character

📌 5. String Formatting

String name = "Alice";
int age = 30;

// Concatenation
System.out.println("Hello " + name + ", you are " + age);

// printf style
System.out.printf("Hello %s, you are %d%n", name, age);

// String.format
String message = String.format("Hello %s, you are %d", name, age);

📌 6. Basic Input (Scanner)

import java.util.Scanner;

public class InputExample {
    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 + ", age " + age);
        scanner.close();
    }
}

📌 7. Conditional Statements

int score = 85;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else if (score >= 70) {
    System.out.println("Grade: C");
} else {
    System.out.println("Grade: F");
}

📌 8. Loops

🔹 for Loop

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

🔹 while Loop

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

🔹 Enhanced for Loop (for-each)

int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

📌 9. Switch Statement

int day = 3;
String dayName;

switch (day) {
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    default:
        dayName = "Unknown";
}

System.out.println("Day: " + dayName);

📌 10. Basic Math Operations

int a = 10;
int b = 3;

System.out.println("Addition: " + (a + b));        // 13
System.out.println("Subtraction: " + (a - b));     // 7
System.out.println("Multiplication: " + (a * b));  // 30
System.out.println("Division: " + (a / b));        // 3 (integer)
System.out.println("Modulus: " + (a % b));         // 1

📝 Java Variables

✅ What is a Variable?

A variable is a container that holds data. In Java, you must specify the type of data it will hold.

int age = 25;           // Integer variable
String name = "John";   // String variable
double height = 5.9;    // Double variable
boolean isStudent = true; // Boolean variable

🔹 1. Variable Naming Rules

✅ Valid names:

Must start with a letter, underscore (_), or dollar sign ($)
Can contain letters, digits, underscores, and dollar signs
Cannot be a Java keyword
Case-sensitive

❌ Invalid names:

int 2name = 10;      // ❌ Cannot start with digit
int user-name = 10;  // ❌ Hyphens not allowed
int class = 10;      // ❌ 'class' is a keyword

✅ Valid examples:

int userName = 10;
int _age = 25;
int $price = 100;
int name2 = 5;

🔹 2. Variable Types

🔸 Primitive Types:

byte smallNumber = 127;        // 8-bit integer
short mediumNumber = 32000;    // 16-bit integer
int number = 2000000000;       // 32-bit integer
long bigNumber = 9000000000L;  // 64-bit integer

float decimal = 3.14f;         // 32-bit decimal
double preciseDecimal = 3.14159; // 64-bit decimal

char letter = 'A';             // Single character
boolean flag = true;           // true or false

🔸 Reference Types:

String text = "Hello World";
int[] numbers = {1, 2, 3, 4, 5};
Scanner input = new Scanner(System.in);

🔹 3. Variable Declaration and Initialization

// Declaration only
int age;
String name;

// Declaration with initialization
int score = 95;
String city = "New York";

// Multiple declarations
int x, y, z;
int a = 1, b = 2, c = 3;

🔹 4. Constants (final keyword)

final int MAX_SCORE = 100;
final double PI = 3.14159;
final String COMPANY_NAME = "TechCorp";

// MAX_SCORE = 200; // ❌ Error: cannot reassign

🔒 Constants cannot be changed once assigned. Use UPPER_CASE naming convention.

🔹 5. Type Casting

// Implicit casting (automatic)
int intValue = 100;
double doubleValue = intValue;  // int to double

// Explicit casting (manual)
double bigDecimal = 9.87;
int wholeNumber = (int) bigDecimal;  // 9 (loses decimal part)

// String to primitive
String numberText = "123";
int parsedNumber = Integer.parseInt(numberText);
double parsedDecimal = Double.parseDouble("45.67");

🔹 6. Variable Scope

public class VariableScope {
    static int globalVar = 100;  // Class variable
    
    public static void main(String[] args) {
        int localVar = 50;       // Local variable
        
        if (true) {
            int blockVar = 25;   // Block variable
            System.out.println(globalVar); // ✅ Accessible
            System.out.println(localVar);  // ✅ Accessible
            System.out.println(blockVar);  // ✅ Accessible
        }
        
        // System.out.println(blockVar); // ❌ Error: out of scope
    }
}

🔹 7. Default Values

public class DefaultValues {
    // Instance variables have default values
    int number;        // 0
    double decimal;    // 0.0
    boolean flag;      // false
    String text;       // null
    
    public static void main(String[] args) {
        DefaultValues obj = new DefaultValues();
        System.out.println(obj.number);   // Prints: 0
        System.out.println(obj.flag);     // Prints: false
        
        // Local variables must be initialized before use
        int localVar;
        // System.out.println(localVar); // ❌ Error: not initialized
    }
}

📊 Java Data Types

✅ Java Data Types Overview

Java has two categories of data types:

Primitive Types: Store simple values directly
Reference Types: Store references to objects

🔹 1. Primitive Data Types

🔸 Integer Types

byte smallInt = 127;           // 8-bit: -128 to 127
short mediumInt = 32000;       // 16-bit: -32,768 to 32,767
int regularInt = 2000000000;   // 32-bit: -2^31 to 2^31-1
long bigInt = 9000000000L;     // 64-bit: -2^63 to 2^63-1

System.out.println("byte: " + smallInt);
System.out.println("short: " + mediumInt);
System.out.println("int: " + regularInt);
System.out.println("long: " + bigInt);

🔸 Floating-Point Types

float singlePrecision = 3.14f;      // 32-bit decimal
double doublePrecision = 3.14159;    // 64-bit decimal (more precise)

System.out.println("float: " + singlePrecision);
System.out.println("double: " + doublePrecision);

🔸 Character Type

char letter = 'A';           // Single character
char unicode = '\u0041';     // Unicode for 'A'
char number = '7';           // Character '7', not number 7

System.out.println("char: " + letter);
System.out.println("unicode: " + unicode);

🔸 Boolean Type

boolean isTrue = true;
boolean isFalse = false;
boolean result = (5 > 3);    // true

System.out.println("boolean: " + isTrue);
System.out.println("comparison: " + result);

🔹 2. Reference Data Types

🔸 String

String greeting = "Hello World";
String name = "Java";
String empty = "";
String nullString = null;

System.out.println("greeting: " + greeting);
System.out.println("length: " + greeting.length());
System.out.println("uppercase: " + greeting.toUpperCase());

🔸 Arrays

int[] numbers = {1, 2, 3, 4, 5};
String[] names = {"Alice", "Bob", "Charlie"};
double[] prices = new double[3];  // Creates array of size 3

System.out.println("first number: " + numbers[0]);
System.out.println("array length: " + numbers.length);

🔹 3. Data Type Sizes and Ranges

Type	Size	Range
byte	8 bits	-128 to 127
short	16 bits	-32,768 to 32,767
int	32 bits	-2,147,483,648 to 2,147,483,647
long	64 bits	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
float	32 bits	±3.4e−038 to ±3.4e+038
double	64 bits	±1.7e−308 to ±1.7e+308
char	16 bits	0 to 65,535 (Unicode)
boolean	1 bit	true or false

🔹 4. Type Conversion

🔸 Automatic (Implicit) Conversion

byte b = 10;
short s = b;    // byte → short
int i = s;      // short → int
long l = i;     // int → long
float f = l;    // long → float
double d = f;   // float → double

System.out.println("Automatic conversion: " + d);

🔸 Manual (Explicit) Conversion

double largeNumber = 123.456;
int wholeNumber = (int) largeNumber;  // 123 (loses decimal)

long bigValue = 1000L;
int smallValue = (int) bigValue;      // Possible data loss

System.out.println("Explicit conversion: " + wholeNumber);

🔹 5. Wrapper Classes

Each primitive type has a corresponding wrapper class:

// Primitive → Wrapper (Boxing)
int primitive = 10;
Integer wrapper = Integer.valueOf(primitive);

// Wrapper → Primitive (Unboxing)
Integer wrapperObj = 20;
int primitiveValue = wrapperObj.intValue();

// Autoboxing (automatic conversion)
Integer auto = 30;           // int → Integer
int autoValue = auto;        // Integer → int

System.out.println("Wrapper methods:");
System.out.println("Max int: " + Integer.MAX_VALUE);
System.out.println("Parse string: " + Integer.parseInt("123"));

🔹 6. Common Type Operations

// Math operations
int a = 15, b = 4;
System.out.println("Addition: " + (a + b));        // 19
System.out.println("Division: " + (a / b));        // 3 (integer)
System.out.println("Division: " + (a / 4.0));      // 3.75 (double)
System.out.println("Modulus: " + (a % b));         // 3

// Boolean operations
boolean x = true, y = false;
System.out.println("AND: " + (x && y));            // false
System.out.println("OR: " + (x || y));             // true
System.out.println("NOT: " + (!x));                // false

// Character operations
char ch = 'A';
System.out.println("ASCII value: " + (int) ch);    // 65
System.out.println("Next char: " + (char)(ch + 1)); // B

🔢 Java Operators

✅ What are Operators?

Operators are symbols that perform operations on variables and values. Java has several types of operators:

Arithmetic: +, -, *, /, %
Assignment: =, +=, -=, *=, /=
Comparison: ==, !=, <, >, <=, >=
Logical: &&, ||, !
Increment/Decrement: ++, --

🔹 1. Arithmetic Operators

int a = 15, b = 4;

System.out.println("Addition: " + (a + b));        // 19
System.out.println("Subtraction: " + (a - b));     // 11
System.out.println("Multiplication: " + (a * b));  // 60
System.out.println("Division: " + (a / b));        // 3 (integer division)
System.out.println("Modulus: " + (a % b));         // 3 (remainder)

// With floating point
double x = 15.0, y = 4.0;
System.out.println("Float Division: " + (x / y));  // 3.75

🔹 2. Assignment Operators

int x = 10;

x += 5;    // x = x + 5;  → x becomes 15
x -= 3;    // x = x - 3;  → x becomes 12
x *= 2;    // x = x * 2;  → x becomes 24
x /= 4;    // x = x / 4;  → x becomes 6
x %= 4;    // x = x % 4;  → x becomes 2

System.out.println("Final value: " + x);

🔹 3. Comparison Operators

int a = 10, b = 20;

System.out.println("a == b: " + (a == b));  // false
System.out.println("a != b: " + (a != b));  // true
System.out.println("a < b: " + (a < b));    // true
System.out.println("a > b: " + (a > b));    // false
System.out.println("a <= b: " + (a <= b));  // true
System.out.println("a >= b: " + (a >= b));  // false

// String comparison
String name1 = "Java";
String name2 = "Python";
System.out.println("Strings equal: " + name1.equals(name2));

🔹 4. Logical Operators

boolean x = true, y = false;

System.out.println("x && y: " + (x && y));  // false (AND)
System.out.println("x || y: " + (x || y));  // true (OR)
System.out.println("!x: " + (!x));          // false (NOT)

// Practical example
int age = 25;
boolean hasLicense = true;
boolean canDrive = (age >= 18) && hasLicense;
System.out.println("Can drive: " + canDrive);

🔹 5. Increment/Decrement Operators

int count = 5;

// Pre-increment: increment first, then use
System.out.println("Pre-increment: " + (++count));  // 6

// Post-increment: use first, then increment
System.out.println("Post-increment: " + (count++)); // 6
System.out.println("After post-increment: " + count); // 7

// Pre-decrement
System.out.println("Pre-decrement: " + (--count));  // 6

// Post-decrement
System.out.println("Post-decrement: " + (count--)); // 6
System.out.println("Final value: " + count);        // 5

🔹 6. Ternary Operator (?:)

int age = 20;

// Traditional if-else
String message1;
if (age >= 18) {
    message1 = "Adult";
} else {
    message1 = "Minor";
}

// Ternary operator (shorter)
String message2 = (age >= 18) ? "Adult" : "Minor";

System.out.println("Message: " + message2);

// Nested ternary
int score = 85;
String grade = (score >= 90) ? "A" : 
               (score >= 80) ? "B" : 
               (score >= 70) ? "C" : "F";
System.out.println("Grade: " + grade);

🔹 7. Operator Precedence

// Order of operations matters!
int result1 = 10 + 5 * 2;      // 20 (not 30)
int result2 = (10 + 5) * 2;    // 30

System.out.println("10 + 5 * 2 = " + result1);
System.out.println("(10 + 5) * 2 = " + result2);

// Complex expression
boolean complex = 5 > 3 && 10 < 20 || 2 == 3;
System.out.println("Complex: " + complex); // true

Precedence Order (High to Low):

Parentheses ()
Unary: ++, --, !
Multiplicative: *, /, %
Additive: +, -
Relational: <, >, <=, >=
Equality: ==, !=
Logical AND: &&
Logical OR: ||
Ternary: ?:
Assignment: =, +=, -=, etc.

🔄 Control Flow

✅ What is Control Flow?

Control flow statements determine the order in which code executes. They allow you to make decisions, repeat actions, and branch program execution.

🔹 1. If-Else Statements

int temperature = 25;

// Simple if
if (temperature > 30) {
    System.out.println("It's hot!");
}

// If-else
if (temperature > 30) {
    System.out.println("It's hot!");
} else {
    System.out.println("It's not that hot.");
}

// If-else if-else
if (temperature > 35) {
    System.out.println("Extremely hot!");
} else if (temperature > 25) {
    System.out.println("Warm weather");
} else if (temperature > 15) {
    System.out.println("Mild weather");
} else {
    System.out.println("Cold weather");
}

🔹 2. Switch Statement

int dayOfWeek = 3;
String dayName;

switch (dayOfWeek) {
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
    case 7:
        dayName = "Weekend";
        break;
    default:
        dayName = "Invalid day";
}

System.out.println("Day: " + dayName);

// Switch with Strings (Java 7+)
String grade = "A";
switch (grade) {
    case "A":
        System.out.println("Excellent!");
        break;
    case "B":
        System.out.println("Good job!");
        break;
    case "C":
        System.out.println("Average");
        break;
    default:
        System.out.println("Keep trying!");
}

🔹 3. For Loops

🔸 Basic For Loop

// Print numbers 1 to 5
for (int i = 1; i <= 5; i++) {
    System.out.println("Count: " + i);
}

// Countdown
for (int i = 10; i >= 1; i--) {
    System.out.println("Countdown: " + i);
}

// Step by 2
for (int i = 0; i <= 10; i += 2) {
    System.out.println("Even: " + i);
}

🔸 Enhanced For Loop (For-Each)

// Array iteration
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println("Number: " + num);
}

// String array
String[] names = {"Alice", "Bob", "Charlie"};
for (String name : names) {
    System.out.println("Hello, " + name);
}

// Character iteration
String text = "Java";
for (char c : text.toCharArray()) {
    System.out.println("Character: " + c);
}

🔹 4. While Loops

🔸 While Loop

int count = 1;
while (count <= 5) {
    System.out.println("While count: " + count);
    count++;
}

// Finding first number divisible by both 3 and 7
int number = 1;
while (true) {
    if (number % 3 == 0 && number % 7 == 0) {
        System.out.println("First number divisible by 3 and 7: " + number);
        break;
    }
    number++;
}

🔸 Do-While Loop

int num = 1;
do {
    System.out.println("Do-while: " + num);
    num++;
} while (num <= 3);

// Menu example
Scanner scanner = new Scanner(System.in);
int choice;
do {
    System.out.println("\n1. Option A");
    System.out.println("2. Option B");
    System.out.println("0. Exit");
    System.out.print("Choose: ");
    choice = scanner.nextInt();
    
    switch (choice) {
        case 1:
            System.out.println("You chose A");
            break;
        case 2:
            System.out.println("You chose B");
            break;
        case 0:
            System.out.println("Goodbye!");
            break;
        default:
            System.out.println("Invalid choice");
    }
} while (choice != 0);

🔹 5. Break and Continue

// Break - exits the loop
System.out.println("Break example:");
for (int i = 1; i <= 10; i++) {
    if (i == 6) {
        break;  // Stop when i equals 6
    }
    System.out.print(i + " ");
}
System.out.println(); // Output: 1 2 3 4 5

// Continue - skips current iteration
System.out.println("Continue example:");
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    System.out.print(i + " ");
}
System.out.println(); // Output: 1 3 5 7 9

// Labeled break (for nested loops)
outer: for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (i == 2 && j == 2) {
            break outer;  // Breaks out of both loops
        }
        System.out.println("i=" + i + ", j=" + j);
    }
}

🔹 6. Nested Loops

// Multiplication table
System.out.println("Multiplication Table:");
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 5; j++) {
        System.out.printf("%3d ", i * j);
    }
    System.out.println();
}

// Pattern printing
System.out.println("\nStar Pattern:");
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

// Number pyramid
System.out.println("\nNumber Pyramid:");
for (int i = 1; i <= 4; i++) {
    // Print spaces
    for (int j = 1; j <= 4 - i; j++) {
        System.out.print(" ");
    }
    // Print numbers
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

🔹 7. Common Control Flow Patterns

// Input validation
Scanner scanner = new Scanner(System.in);
int age;
while (true) {
    System.out.print("Enter your age (0-120): ");
    age = scanner.nextInt();
    if (age >= 0 && age <= 120) {
        break;
    }
    System.out.println("Invalid age! Please try again.");
}

// Finding max in array
int[] scores = {85, 92, 78, 96, 88};
int maxScore = scores[0];
for (int score : scores) {
    if (score > maxScore) {
        maxScore = score;
    }
}
System.out.println("Highest score: " + maxScore);

// Counting specific elements
String[] fruits = {"apple", "banana", "apple", "orange", "apple"};
int appleCount = 0;
for (String fruit : fruits) {
    if (fruit.equals("apple")) {
        appleCount++;
    }
}
System.out.println("Apple count: " + appleCount);

⚙️ Methods

✅ What are Methods?

Methods are blocks of code that perform specific tasks. They help organize code, avoid repetition, and make programs more readable and maintainable.

// Method syntax
public static returnType methodName(parameters) {
    // method body
    return value; // if not void
}

🔹 1. Basic Method Declaration

public class MethodExample {
    
    // Method with no parameters and no return value
    public static void greet() {
        System.out.println("Hello, World!");
    }
    
    // Method with parameters but no return value
    public static void greetPerson(String name) {
        System.out.println("Hello, " + name + "!");
    }
    
    // Method with parameters and return value
    public static int add(int a, int b) {
        return a + b;
    }
    
    // Method with multiple parameters
    public static double calculateArea(double length, double width) {
        return length * width;
    }
    
    public static void main(String[] args) {
        greet();                           // Hello, World!
        greetPerson("Alice");              // Hello, Alice!
        int sum = add(5, 3);              // 8
        double area = calculateArea(4.5, 2.0); // 9.0
        
        System.out.println("Sum: " + sum);
        System.out.println("Area: " + area);
    }
}

🔹 2. Method Parameters and Arguments

// Method with different parameter types
public static void displayInfo(String name, int age, boolean isStudent) {
    System.out.println("Name: " + name);
    System.out.println("Age: " + age);
    System.out.println("Student: " + isStudent);
}

// Method with array parameter
public static void printArray(int[] numbers) {
    System.out.print("Array: ");
    for (int num : numbers) {
        System.out.print(num + " ");
    }
    System.out.println();
}

// Variable arguments (varargs)
public static int sum(int... numbers) {
    int total = 0;
    for (int num : numbers) {
        total += num;
    }
    return total;
}

public static void main(String[] args) {
    displayInfo("Bob", 20, true);
    
    int[] scores = {85, 92, 78};
    printArray(scores);
    
    // Varargs can take any number of arguments
    System.out.println("Sum of 2 numbers: " + sum(5, 3));
    System.out.println("Sum of 4 numbers: " + sum(1, 2, 3, 4));
    System.out.println("Sum of 6 numbers: " + sum(10, 20, 30, 40, 50, 60));
}

🔹 3. Return Values and Types

// Different return types
public static boolean isEven(int number) {
    return number % 2 == 0;
}

public static String getGrade(int score) {
    if (score >= 90) return "A";
    else if (score >= 80) return "B";
    else if (score >= 70) return "C";
    else if (score >= 60) return "D";
    else return "F";
}

public static double[] getStatistics(int[] numbers) {
    double sum = 0;
    int min = numbers[0];
    int max = numbers[0];
    
    for (int num : numbers) {
        sum += num;
        if (num < min) min = num;
        if (num > max) max = num;
    }
    
    double average = sum / numbers.length;
    return new double[]{sum, average, min, max};
}

// Early return
public static String checkAge(int age) {
    if (age < 0) {
        return "Invalid age";
    }
    if (age < 18) {
        return "Minor";
    }
    if (age < 65) {
        return "Adult";
    }
    return "Senior";
}

public static void main(String[] args) {
    System.out.println("Is 4 even? " + isEven(4));
    System.out.println("Grade for 85: " + getGrade(85));
    
    int[] scores = {75, 82, 94, 68, 91};
    double[] stats = getStatistics(scores);
    System.out.println("Sum: " + stats[0]);
    System.out.println("Average: " + stats[1]);
    System.out.println("Min: " + stats[2]);
    System.out.println("Max: " + stats[3]);
    
    System.out.println("Age category: " + checkAge(25));
}

🔹 4. Method Overloading

public class Calculator {
    
    // Same method name, different parameters
    public static int multiply(int a, int b) {
        return a * b;
    }
    
    public static double multiply(double a, double b) {
        return a * b;
    }
    
    public static int multiply(int a, int b, int c) {
        return a * b * c;
    }
    
    // Print methods with different parameter types
    public static void print(int value) {
        System.out.println("Integer: " + value);
    }
    
    public static void print(double value) {
        System.out.println("Double: " + value);
    }
    
    public static void print(String value) {
        System.out.println("String: " + value);
    }
    
    public static void print(boolean value) {
        System.out.println("Boolean: " + value);
    }
    
    public static void main(String[] args) {
        // Java automatically chooses the right method
        System.out.println(multiply(5, 3));        // Calls int version
        System.out.println(multiply(2.5, 4.0));    // Calls double version
        System.out.println(multiply(2, 3, 4));     // Calls 3-parameter version
        
        print(42);          // Calls int version
        print(3.14);        // Calls double version
        print("Hello");     // Calls String version
        print(true);        // Calls boolean version
    }
}

🔹 5. Scope and Local Variables

public class ScopeExample {
    static int globalVar = 100; // Class variable (global scope)
    
    public static void methodA() {
        int localVar = 50;  // Local to methodA
        System.out.println("In methodA:");
        System.out.println("Global: " + globalVar);
        System.out.println("Local: " + localVar);
        
        // Can modify global variable
        globalVar = 200;
    }
    
    public static void methodB() {
        int localVar = 75;  // Different local variable
        System.out.println("In methodB:");
        System.out.println("Global: " + globalVar);
        System.out.println("Local: " + localVar);
        
        // System.out.println(localVar from methodA); // ❌ Error: not accessible
    }
    
    public static int processNumber(int input) {
        int result = input * 2;  // Local to this method
        
        if (input > 10) {
            int bonus = 5;       // Local to this if block
            result += bonus;
        }
        // System.out.println(bonus); // ❌ Error: bonus not accessible here
        
        return result;
    }
    
    public static void main(String[] args) {
        System.out.println("Initial global: " + globalVar);
        methodA();
        methodB();
        
        int result = processNumber(15);
        System.out.println("Processed result: " + result);
    }
}

🔹 6. Utility Methods

public class Utilities {
    
    // String utilities
    public static boolean isPalindrome(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", "");
        int left = 0, right = str.length() - 1;
        while (left < right) {
            if (str.charAt(left) != str.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
    
    public static String reverse(String str) {
        StringBuilder sb = new StringBuilder(str);
        return sb.reverse().toString();
    }
    
    // Math utilities
    public static boolean isPrime(int number) {
        if (number <= 1) return false;
        if (number <= 3) return true;
        if (number % 2 == 0 || number % 3 == 0) return false;
        
        for (int i = 5; i * i <= number; i += 6) {
            if (number % i == 0 || number % (i + 2) == 0) {
                return false;
            }
        }
        return true;
    }
    
    public static int factorial(int n) {
        if (n <= 1) return 1;
        int result = 1;
        for (int i = 2; i <= n; i++) {
            result *= i;
        }
        return result;
    }
    
    // Array utilities
    public static void printArray(int[] arr) {
        System.out.print("[");
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i]);
            if (i < arr.length - 1) System.out.print(", ");
        }
        System.out.println("]");
    }
    
    public static int findMax(int[] arr) {
        int max = arr[0];
        for (int num : arr) {
            if (num > max) max = num;
        }
        return max;
    }
    
    public static void main(String[] args) {
        // String utilities
        System.out.println("Is 'radar' palindrome? " + isPalindrome("radar"));
        System.out.println("Reverse of 'hello': " + reverse("hello"));
        
        // Math utilities
        System.out.println("Is 17 prime? " + isPrime(17));
        System.out.println("Factorial of 5: " + factorial(5));
        
        // Array utilities
        int[] numbers = {3, 7, 1, 9, 4};
        printArray(numbers);
        System.out.println("Max value: " + findMax(numbers));
    }
}

🔹 7. Recursive Methods

public class RecursionExample {
    
    // Factorial using recursion
    public static int factorialRecursive(int n) {
        if (n <= 1) {
            return 1; // Base case
        }
        return n * factorialRecursive(n - 1); // Recursive case
    }
    
    // Fibonacci sequence
    public static int fibonacci(int n) {
        if (n <= 1) {
            return n; // Base cases: fib(0)=0, fib(1)=1
        }
        return fibonacci(n - 1) + fibonacci(n - 2);
    }
    
    // Power calculation
    public static double power(double base, int exponent) {
        if (exponent == 0) {
            return 1; // Any number to the power 0 is 1
        }
        if (exponent < 0) {
            return 1 / power(base, -exponent);
        }
        return base * power(base, exponent - 1);
    }
    
    // Sum of digits
    public static int sumOfDigits(int number) {
        if (number == 0) {
            return 0; // Base case
        }
        return (number % 10) + sumOfDigits(number / 10);
    }
    
    public static void main(String[] args) {
        System.out.println("Factorial of 5: " + factorialRecursive(5));
        
        System.out.print("Fibonacci sequence (first 10): ");
        for (int i = 0; i < 10; i++) {
            System.out.print(fibonacci(i) + " ");
        }
        System.out.println();
        
        System.out.println("2^8 = " + power(2, 8));
        System.out.println("Sum of digits in 12345: " + sumOfDigits(12345));
    }
}

🏗️ Classes & Objects

✅ Object-Oriented Programming (OOP)

Java is an object-oriented language. Everything in Java is associated with classes and objects. A class is a template for creating objects, and an object is an instance of a class.

// Class definition
public class Car {
    // Attributes (fields)
    String brand;
    String model;
    int year;
    
    // Method
    public void start() {
        System.out.println("The car is starting...");
    }
}

🔹 1. Creating Classes and Objects

// Student class
public class Student {
    // Instance variables (attributes)
    String name;
    int age;
    String major;
    double gpa;
    
    // Method to display student info
    public void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Major: " + major);
        System.out.println("GPA: " + gpa);
    }
    
    // Method to study
    public void study(String subject) {
        System.out.println(name + " is studying " + subject);
    }
    
    // Method to calculate if honor student
    public boolean isHonorStudent() {
        return gpa >= 3.5;
    }
}

// Using the class
public class StudentTest {
    public static void main(String[] args) {
        // Creating objects (instances)
        Student student1 = new Student();
        Student student2 = new Student();
        
        // Setting values
        student1.name = "Alice";
        student1.age = 20;
        student1.major = "Computer Science";
        student1.gpa = 3.8;
        
        student2.name = "Bob";
        student2.age = 19;
        student2.major = "Mathematics";
        student2.gpa = 3.2;
        
        // Calling methods
        System.out.println("Student 1:");
        student1.displayInfo();
        student1.study("Java Programming");
        System.out.println("Honor student: " + student1.isHonorStudent());
        
        System.out.println("\nStudent 2:");
        student2.displayInfo();
        student2.study("Calculus");
        System.out.println("Honor student: " + student2.isHonorStudent());
    }
}

🔹 2. Constructors

public class Book {
    // Instance variables
    private String title;
    private String author;
    private int pages;
    private double price;
    
    // Default constructor
    public Book() {
        this.title = "Unknown";
        this.author = "Unknown";
        this.pages = 0;
        this.price = 0.0;
    }
    
    // Parameterized constructor
    public Book(String title, String author, int pages, double price) {
        this.title = title;
        this.author = author;
        this.pages = pages;
        this.price = price;
    }
    
    // Constructor overloading
    public Book(String title, String author) {
        this.title = title;
        this.author = author;
        this.pages = 100; // Default pages
        this.price = 9.99; // Default price
    }
    
    // Method to display book info
    public void displayInfo() {
        System.out.println("Title: " + title);
        System.out.println("Author: " + author);
        System.out.println("Pages: " + pages);
        System.out.println("Price: $" + price);
        System.out.println("---");
    }
    
    // Getter methods
    public String getTitle() { return title; }
    public String getAuthor() { return author; }
    public int getPages() { return pages; }
    public double getPrice() { return price; }
    
    // Setter methods
    public void setTitle(String title) { this.title = title; }
    public void setAuthor(String author) { this.author = author; }
    public void setPages(int pages) { this.pages = pages; }
    public void setPrice(double price) { this.price = price; }
}

public class BookTest {
    public static void main(String[] args) {
        // Using different constructors
        Book book1 = new Book(); // Default constructor
        Book book2 = new Book("1984", "George Orwell", 328, 13.99);
        Book book3 = new Book("To Kill a Mockingbird", "Harper Lee");
        
        System.out.println("Book 1 (default):");
        book1.displayInfo();
        
        System.out.println("Book 2 (full constructor):");
        book2.displayInfo();
        
        System.out.println("Book 3 (partial constructor):");
        book3.displayInfo();
        
        // Using setters to modify book1
        book1.setTitle("The Hobbit");
        book1.setAuthor("J.R.R. Tolkien");
        book1.setPages(310);
        book1.setPrice(12.50);
        
        System.out.println("Book 1 (after modification):");
        book1.displayInfo();
    }
}

🔹 3. Encapsulation (Private Variables and Methods)

public class BankAccount {
    // Private variables (encapsulated)
    private String accountNumber;
    private String ownerName;
    private double balance;
    private String accountType;
    
    // Constructor
    public BankAccount(String accountNumber, String ownerName, 
                      double initialBalance, String accountType) {
        this.accountNumber = accountNumber;
        this.ownerName = ownerName;
        this.balance = initialBalance;
        this.accountType = accountType;
    }
    
    // Public methods to access private data
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Deposited $" + amount);
            displayBalance();
        } else {
            System.out.println("Invalid deposit amount");
        }
    }
    
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.println("Withdrew $" + amount);
            displayBalance();
        } else if (amount > balance) {
            System.out.println("Insufficient funds");
        } else {
            System.out.println("Invalid withdrawal amount");
        }
    }
    
    public double getBalance() {
        return balance;
    }
    
    public void displayAccountInfo() {
        System.out.println("Account Number: " + accountNumber);
        System.out.println("Owner: " + ownerName);
        System.out.println("Account Type: " + accountType);
        displayBalance();
        System.out.println("---");
    }
    
    // Private method (can only be used within this class)
    private void displayBalance() {
        System.out.println("Current Balance: $" + String.format("%.2f", balance));
    }
    
    // Getters for read-only access
    public String getAccountNumber() { return accountNumber; }
    public String getOwnerName() { return ownerName; }
    public String getAccountType() { return accountType; }
    
    // No setters for critical data to maintain security
}

public class BankTest {
    public static void main(String[] args) {
        BankAccount account = new BankAccount("ACC001", "John Doe", 1000.0, "Checking");
        
        account.displayAccountInfo();
        
        account.deposit(250.50);
        account.withdraw(100.0);
        account.withdraw(2000.0); // Should fail
        account.deposit(-50.0);   // Should fail
        
        // Direct access to private variables is not allowed
        // account.balance = 5000.0; // ❌ Error: balance is private
        
        // Must use public methods
        System.out.println("Final balance: $" + account.getBalance());
    }
}

🔹 4. Static vs Instance Members

public class Counter {
    // Static variable (shared by all instances)
    private static int totalCount = 0;
    
    // Instance variable (unique to each object)
    private int instanceCount;
    private String name;
    
    // Constructor
    public Counter(String name) {
        this.name = name;
        this.instanceCount = 0;
        totalCount++; // Increment static counter when object is created
    }
    
    // Instance method
    public void increment() {
        instanceCount++;
        totalCount++;
        System.out.println(name + " incremented. Instance: " + instanceCount + 
                          ", Total: " + totalCount);
    }
    
    // Static method (can be called without creating an object)
    public static int getTotalCount() {
        return totalCount;
    }
    
    // Static method to reset total count
    public static void resetTotalCount() {
        totalCount = 0;
        System.out.println("Total count reset to 0");
    }
    
    // Instance method
    public void displayInfo() {
        System.out.println("Counter: " + name + 
                          ", Instance count: " + instanceCount +
                          ", Total count: " + totalCount);
    }
    
    // Static block (executed when class is first loaded)
    static {
        System.out.println("Counter class loaded!");
        totalCount = 0;
    }
}

public class CounterTest {
    public static void main(String[] args) {
        // Using static method without creating object
        System.out.println("Initial total count: " + Counter.getTotalCount());
        
        // Creating objects
        Counter counter1 = new Counter("Counter1");
        Counter counter2 = new Counter("Counter2");
        Counter counter3 = new Counter("Counter3");
        
        System.out.println("After creating 3 counters: " + Counter.getTotalCount());
        
        // Using instance methods
        counter1.increment();
        counter1.increment();
        counter2.increment();
        counter3.increment();
        counter3.increment();
        counter3.increment();
        
        // Display info for each counter
        counter1.displayInfo();
        counter2.displayInfo();
        counter3.displayInfo();
        
        System.out.println("Final total count: " + Counter.getTotalCount());
        
        // Reset using static method
        Counter.resetTotalCount();
        System.out.println("After reset: " + Counter.getTotalCount());
    }
}

🔹 5. Object Interaction

// Teacher class
class Teacher {
    private String name;
    private String subject;
    private int experience;
    
    public Teacher(String name, String subject, int experience) {
        this.name = name;
        this.subject = subject;
        this.experience = experience;
    }
    
    public void teach(Student student) {
        System.out.println(name + " is teaching " + subject + " to " + student.getName());
    }
    
    // Getters
    public String getName() { return name; }
    public String getSubject() { return subject; }
    public int getExperience() { return experience; }
}

// Updated Student class
class Student {
    private String name;
    private int age;
    private String major;
    private double gpa;
    
    public Student(String name, int age, String major, double gpa) {
        this.name = name;
        this.age = age;
        this.major = major;
        this.gpa = gpa;
    }
    
    public void attendClass(Teacher teacher) {
        System.out.println(name + " is attending " + teacher.getSubject() + 
                          " class taught by " + teacher.getName());
    }
    
    public void study(String subject) {
        System.out.println(name + " is studying " + subject);
        // Simulate GPA improvement
        if (gpa < 4.0) {
            gpa += 0.1;
            System.out.println(name + "'s GPA improved to " + String.format("%.2f", gpa));
        }
    }
    
    // Getters
    public String getName() { return name; }
    public int getAge() { return age; }
    public String getMajor() { return major; }
    public double getGpa() { return gpa; }
}

// Classroom class that manages multiple students and teachers
class Classroom {
    private String courseCode;
    private Teacher teacher;
    private Student[] students;
    private int studentCount;
    
    public Classroom(String courseCode, Teacher teacher, int maxStudents) {
        this.courseCode = courseCode;
        this.teacher = teacher;
        this.students = new Student[maxStudents];
        this.studentCount = 0;
    }
    
    public void addStudent(Student student) {
        if (studentCount < students.length) {
            students[studentCount] = student;
            studentCount++;
            System.out.println(student.getName() + " added to " + courseCode + " class");
        } else {
            System.out.println("Classroom is full!");
        }
    }
    
    public void conductClass() {
        System.out.println("\n--- " + courseCode + " Class Session ---");
        System.out.println("Teacher: " + teacher.getName() + " (" + teacher.getSubject() + ")");
        System.out.println("Students present: " + studentCount);
        
        for (int i = 0; i < studentCount; i++) {
            students[i].attendClass(teacher);
            teacher.teach(students[i]);
        }
        
        System.out.println("Class session completed!\n");
    }
    
    public void displayClassInfo() {
        System.out.println("Course: " + courseCode);
        System.out.println("Teacher: " + teacher.getName());
        System.out.println("Students enrolled: " + studentCount);
        for (int i = 0; i < studentCount; i++) {
            System.out.println("- " + students[i].getName() + " (GPA: " + 
                             String.format("%.2f", students[i].getGpa()) + ")");
        }
    }
}

public class SchoolTest {
    public static void main(String[] args) {
        // Create teacher and students
        Teacher javaTeacher = new Teacher("Dr. Smith", "Java Programming", 10);
        
        Student student1 = new Student("Alice", 20, "Computer Science", 3.5);
        Student student2 = new Student("Bob", 19, "Information Technology", 3.2);
        Student student3 = new Student("Carol", 21, "Computer Engineering", 3.8);
        
        // Create classroom
        Classroom javaClass = new Classroom("CS101", javaTeacher, 5);
        
        // Add students to classroom
        javaClass.addStudent(student1);
        javaClass.addStudent(student2);
        javaClass.addStudent(student3);
        
        // Display initial class info
        javaClass.displayClassInfo();
        
        // Conduct a class session
        javaClass.conductClass();
        
        // Students study after class
        student1.study("Object-Oriented Programming");
        student2.study("Data Structures");
        student3.study("Algorithms");
        
        System.out.println("\n--- Updated Class Info ---");
        javaClass.displayClassInfo();
    }
}

🔹 6. toString() Method

public class Person {
    private String firstName;
    private String lastName;
    private int age;
    private String email;
    
    public Person(String firstName, String lastName, int age, String email) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.email = email;
    }
    
    // Override toString() method for better object representation
    @Override
    public String toString() {
        return "Person{" +
               "firstName='" + firstName + '\'' +
               ", lastName='" + lastName + '\'' +
               ", age=" + age +
               ", email='" + email + '\'' +
               '}';
    }
    
    // Alternative toString() for user-friendly display
    public String toDisplayString() {
        return firstName + " " + lastName + " (" + age + " years old) - " + email;
    }
    
    // Getters
    public String getFirstName() { return firstName; }
    public String getLastName() { return lastName; }
    public String getFullName() { return firstName + " " + lastName; }
    public int getAge() { return age; }
    public String getEmail() { return email; }
}

public class PersonTest {
    public static void main(String[] args) {
        Person person1 = new Person("John", "Doe", 30, "john.doe@email.com");
        Person person2 = new Person("Jane", "Smith", 25, "jane.smith@email.com");
        
        // toString() is automatically called when printing objects
        System.out.println("Using toString():");
        System.out.println(person1); // Calls person1.toString()
        System.out.println(person2);
        
        // Using custom display method
        System.out.println("\nUsing custom display:");
        System.out.println(person1.toDisplayString());
        System.out.println(person2.toDisplayString());
        
        // Array of Person objects
        Person[] people = {person1, person2, 
                          new Person("Alice", "Johnson", 28, "alice.j@email.com")};
        
        System.out.println("\nArray of people:");
        for (Person person : people) {
            System.out.println(person.toDisplayString());
        }
    }
}

📋 Arrays

Arrays are containers that hold multiple values of the same type. They're fundamental data structures in Java that allow you to store and manipulate collections of data efficiently.

🔸 Array Declaration and Initialization

Arrays can be declared and initialized in several ways:

Basic Array Declaration

// Declaration syntax
int[] numbers;           // Preferred way
int numbers[];          // Alternative way

// Declaration with initialization
int[] numbers = new int[5];  // Array of 5 integers (default value 0)
String[] names = new String[3];  // Array of 3 strings (default value null)

// Declaration with values
int[] scores = {85, 92, 78, 96, 89};
String[] colors = {"red", "green", "blue", "yellow"};

// Alternative initialization syntax
int[] grades = new int[]{88, 92, 76, 84, 91};

📋 Featured Example: Student Grades System

public class StudentGrades {
    public static void main(String[] args) {
        // Initialize array with student grades
        int[] grades = {85, 92, 78, 96, 89, 83, 91};
        String[] students = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank", "Grace"};
        
        System.out.println("Student Grade Report:");
        System.out.println("=====================");
        
        for (int i = 0; i < students.length; i++) {
            System.out.println(students[i] + ": " + grades[i] + "%");
        }
        
        System.out.println("\nArray Length: " + grades.length);
    }
}

🔸 Array Access and Modification

Elements in arrays are accessed using zero-based indexing:

Accessing Array Elements

int[] numbers = {10, 20, 30, 40, 50};

// Accessing elements (index starts from 0)
int first = numbers[0];    // Gets 10
int third = numbers[2];    // Gets 30
int last = numbers[numbers.length - 1];  // Gets 50

// Modifying elements
numbers[1] = 25;  // Changes second element from 20 to 25
numbers[4] = 55;  // Changes last element from 50 to 55

System.out.println("Modified array:");
for (int num : numbers) {
    System.out.print(num + " ");  // Output: 10 25 30 40 55
}

Array Bounds and Safety

public class ArraySafety {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        
        // Safe array access
        System.out.println("Array length: " + arr.length);
        
        // Check bounds before accessing
        int index = 10;
        if (index >= 0 && index < arr.length) {
            System.out.println("Element at index " + index + ": " + arr[index]);
        } else {
            System.out.println("Index " + index + " is out of bounds!");
        }
        
        // This would cause ArrayIndexOutOfBoundsException:
        // System.out.println(arr[10]);  // DON'T DO THIS
    }
}

🔸 Array Iteration

There are multiple ways to iterate through arrays in Java:

Different Iteration Methods

int[] numbers = {15, 23, 8, 42, 16, 31, 7};

// 1. Traditional for loop
System.out.println("Traditional for loop:");
for (int i = 0; i < numbers.length; i++) {
    System.out.println("Index " + i + ": " + numbers[i]);
}

// 2. Enhanced for loop (for-each)
System.out.println("\nEnhanced for loop:");
for (int num : numbers) {
    System.out.println("Value: " + num);
}

// 3. While loop
System.out.println("\nWhile loop:");
int i = 0;
while (i < numbers.length) {
    System.out.println("numbers[" + i + "] = " + numbers[i]);
    i++;
}

📋 Featured Example: Array Statistics Calculator

public class ArrayStats {
    public static void main(String[] args) {
        double[] scores = {78.5, 92.3, 85.7, 69.2, 88.9, 94.1, 76.8, 82.4};
        
        // Calculate sum and average
        double sum = 0;
        double max = scores[0];
        double min = scores[0];
        
        for (double score : scores) {
            sum += score;
            if (score > max) max = score;
            if (score < min) min = score;
        }
        
        double average = sum / scores.length;
        
        System.out.println("Array Statistics:");
        System.out.println("================");
        System.out.println("Count: " + scores.length);
        System.out.println("Sum: " + sum);
        System.out.println("Average: " + String.format("%.2f", average));
        System.out.println("Maximum: " + max);
        System.out.println("Minimum: " + min);
    }
}

🔸 Array Manipulation and Operations

Common operations performed on arrays include searching, sorting, and copying:

Basic Array Operations

import java.util.Arrays;

public class ArrayOperations {
    public static void main(String[] args) {
        int[] original = {64, 34, 25, 12, 22, 11, 90};
        
        // Display original array
        System.out.println("Original: " + Arrays.toString(original));
        
        // Copy array
        int[] copy = Arrays.copyOf(original, original.length);
        
        // Sort array
        Arrays.sort(copy);
        System.out.println("Sorted: " + Arrays.toString(copy));
        
        // Search in sorted array
        int target = 25;
        int index = Arrays.binarySearch(copy, target);
        System.out.println("Element " + target + " found at index: " + index);
        
        // Fill array with specific value
        int[] filled = new int[5];
        Arrays.fill(filled, 42);
        System.out.println("Filled: " + Arrays.toString(filled));
    }
}

Manual Array Operations

public class ManualArrayOps {
    public static void main(String[] args) {
        int[] numbers = {5, 2, 8, 1, 9, 3};
        
        System.out.println("Original: " + arrayToString(numbers));
        
        // Manual reverse
        reverseArray(numbers);
        System.out.println("Reversed: " + arrayToString(numbers));
        
        // Manual search
        int searchValue = 8;
        int position = linearSearch(numbers, searchValue);
        if (position != -1) {
            System.out.println("Found " + searchValue + " at index " + position);
        } else {
            System.out.println(searchValue + " not found");
        }
    }
    
    public static void reverseArray(int[] arr) {
        int start = 0;
        int end = arr.length - 1;
        
        while (start < end) {
            // Swap elements
            int temp = arr[start];
            arr[start] = arr[end];
            arr[end] = temp;
            
            start++;
            end--;
        }
    }
    
    public static int linearSearch(int[] arr, int target) {
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == target) {
                return i;
            }
        }
        return -1; // Not found
    }
    
    public static String arrayToString(int[] arr) {
        StringBuilder sb = new StringBuilder("[");
        for (int i = 0; i < arr.length; i++) {
            sb.append(arr[i]);
            if (i < arr.length - 1) sb.append(", ");
        }
        sb.append("]");
        return sb.toString();
    }
}

🔸 Multi-dimensional Arrays

Java supports arrays of arrays, commonly used for matrices and tables:

2D Array Basics

// 2D array declaration and initialization
int[][] matrix = new int[3][4];  // 3 rows, 4 columns

// Initialize with values
int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

// Jagged array (rows with different lengths)
int[][] jagged = {
    {1, 2},
    {3, 4, 5, 6},
    {7, 8, 9}
};

System.out.println("Grid dimensions: " + grid.length + "x" + grid[0].length);
System.out.println("Element at [1][2]: " + grid[1][2]);  // Output: 6

📋 Featured Example: Tic-Tac-Toe Board

public class TicTacToe {
    public static void main(String[] args) {
        char[][] board = {
            {'X', 'O', 'X'},
            {'O', 'X', 'O'},
            {'X', 'O', 'X'}
        };
        
        System.out.println("Tic-Tac-Toe Board:");
        printBoard(board);
        
        // Check if X wins in first row
        if (checkRow(board, 0, 'X')) {
            System.out.println("X wins in the first row!");
        }
    }
    
    public static void printBoard(char[][] board) {
        for (int row = 0; row < board.length; row++) {
            for (int col = 0; col < board[row].length; col++) {
                System.out.print(board[row][col]);
                if (col < board[row].length - 1) {
                    System.out.print(" | ");
                }
            }
            System.out.println();
            if (row < board.length - 1) {
                System.out.println("---------");
            }
        }
    }
    
    public static boolean checkRow(char[][] board, int row, char player) {
        for (int col = 0; col < board[row].length; col++) {
            if (board[row][col] != player) {
                return false;
            }
        }
        return true;
    }
}

🔸 Array Utility Methods

Creating reusable methods for common array operations:

Useful Array Utility Methods

import java.util.Arrays;
import java.util.Random;

public class ArrayUtils {
    public static void main(String[] args) {
        int[] numbers = generateRandomArray(10, 1, 100);
        System.out.println("Random array: " + Arrays.toString(numbers));
        
        System.out.println("Sum: " + sum(numbers));
        System.out.println("Average: " + average(numbers));
        System.out.println("Contains 50? " + contains(numbers, 50));
        
        int[] filtered = filterGreaterThan(numbers, 50);
        System.out.println("Numbers > 50: " + Arrays.toString(filtered));
    }
    
    // Generate random array
    public static int[] generateRandomArray(int size, int min, int max) {
        Random random = new Random();
        int[] arr = new int[size];
        for (int i = 0; i < size; i++) {
            arr[i] = random.nextInt(max - min + 1) + min;
        }
        return arr;
    }
    
    // Calculate sum
    public static int sum(int[] arr) {
        int total = 0;
        for (int num : arr) {
            total += num;
        }
        return total;
    }
    
    // Calculate average
    public static double average(int[] arr) {
        return (double) sum(arr) / arr.length;
    }
    
    // Check if array contains value
    public static boolean contains(int[] arr, int value) {
        for (int num : arr) {
            if (num == value) {
                return true;
            }
        }
        return false;
    }
    
    // Filter elements greater than threshold
    public static int[] filterGreaterThan(int[] arr, int threshold) {
        // First count how many elements meet criteria
        int count = 0;
        for (int num : arr) {
            if (num > threshold) count++;
        }
        
        // Create result array and fill it
        int[] result = new int[count];
        int index = 0;
        for (int num : arr) {
            if (num > threshold) {
                result[index++] = num;
            }
        }
        return result;
    }
}

🔸 Common Array Patterns and Best Practices

Frequently used array patterns and programming best practices:

Array Processing Patterns

public class ArrayPatterns {
    public static void main(String[] args) {
        int[] data = {3, 7, 1, 9, 4, 6, 8, 2, 5};
        
        // Pattern 1: Find maximum and minimum
        int[] minMax = findMinMax(data);
        System.out.println("Min: " + minMax[0] + ", Max: " + minMax[1]);
        
        // Pattern 2: Count elements meeting condition
        int evenCount = countEven(data);
        System.out.println("Even numbers count: " + evenCount);
        
        // Pattern 3: Remove duplicates (simplified)
        int[] unique = {1, 2, 2, 3, 3, 3, 4, 5, 5};
        int[] noDuplicates = removeDuplicatesSorted(unique);
        System.out.println("Unique elements: " + Arrays.toString(noDuplicates));
        
        // Pattern 4: Rotate array
        int[] toRotate = {1, 2, 3, 4, 5};
        rotateLeft(toRotate, 2);
        System.out.println("Rotated array: " + Arrays.toString(toRotate));
    }
    
    public static int[] findMinMax(int[] arr) {
        if (arr.length == 0) return new int[]{0, 0};
        
        int min = arr[0];
        int max = arr[0];
        
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] < min) min = arr[i];
            if (arr[i] > max) max = arr[i];
        }
        
        return new int[]{min, max};
    }
    
    public static int countEven(int[] arr) {
        int count = 0;
        for (int num : arr) {
            if (num % 2 == 0) count++;
        }
        return count;
    }
    
    public static int[] removeDuplicatesSorted(int[] arr) {
        if (arr.length == 0) return arr;
        
        int uniqueCount = 1;
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] != arr[i-1]) {
                uniqueCount++;
            }
        }
        
        int[] result = new int[uniqueCount];
        result[0] = arr[0];
        int resultIndex = 1;
        
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] != arr[i-1]) {
                result[resultIndex++] = arr[i];
            }
        }
        
        return result;
    }
    
    public static void rotateLeft(int[] arr, int positions) {
        positions = positions % arr.length; // Handle positions > array length
        reverse(arr, 0, positions - 1);
        reverse(arr, positions, arr.length - 1);
        reverse(arr, 0, arr.length - 1);
    }
    
    private static void reverse(int[] arr, int start, int end) {
        while (start < end) {
            int temp = arr[start];
            arr[start] = arr[end];
            arr[end] = temp;
            start++;
            end--;
        }
    }
}

📋 Featured Example: Grade Book Management System

public class GradeBook {
    private String[] studentNames;
    private double[][] grades; // [student][assignment]
    private String[] assignmentNames;
    
    public GradeBook(String[] names, String[] assignments) {
        this.studentNames = names.clone();
        this.assignmentNames = assignments.clone();
        this.grades = new double[names.length][assignments.length];
    }
    
    public void setGrade(int studentIndex, int assignmentIndex, double grade) {
        if (isValidIndex(studentIndex, assignmentIndex)) {
            grades[studentIndex][assignmentIndex] = grade;
        }
    }
    
    public double getStudentAverage(int studentIndex) {
        if (studentIndex < 0 || studentIndex >= studentNames.length) {
            return 0;
        }
        
        double sum = 0;
        for (int i = 0; i < assignmentNames.length; i++) {
            sum += grades[studentIndex][i];
        }
        return sum / assignmentNames.length;
    }
    
    public double getAssignmentAverage(int assignmentIndex) {
        if (assignmentIndex < 0 || assignmentIndex >= assignmentNames.length) {
            return 0;
        }
        
        double sum = 0;
        for (int i = 0; i < studentNames.length; i++) {
            sum += grades[i][assignmentIndex];
        }
        return sum / studentNames.length;
    }
    
    public void printGradeReport() {
        System.out.println("Grade Book Report");
        System.out.println("=================");
        
        // Print header
        System.out.printf("%-12s", "Student");
        for (String assignment : assignmentNames) {
            System.out.printf("%8s", assignment);
        }
        System.out.printf("%8s%n", "Average");
        
        // Print student grades
        for (int i = 0; i < studentNames.length; i++) {
            System.out.printf("%-12s", studentNames[i]);
            for (int j = 0; j < assignmentNames.length; j++) {
                System.out.printf("%8.1f", grades[i][j]);
            }
            System.out.printf("%8.1f%n", getStudentAverage(i));
        }
        
        // Print assignment averages
        System.out.printf("%-12s", "Average");
        for (int j = 0; j < assignmentNames.length; j++) {
            System.out.printf("%8.1f", getAssignmentAverage(j));
        }
        System.out.println();
    }
    
    private boolean isValidIndex(int studentIndex, int assignmentIndex) {
        return studentIndex >= 0 && studentIndex < studentNames.length &&
               assignmentIndex >= 0 && assignmentIndex < assignmentNames.length;
    }
    
    public static void main(String[] args) {
        String[] students = {"Alice", "Bob", "Charlie", "Diana"};
        String[] assignments = {"Quiz1", "Quiz2", "Midterm", "Final"};
        
        GradeBook book = new GradeBook(students, assignments);
        
        // Add some grades
        book.setGrade(0, 0, 85); book.setGrade(0, 1, 92); book.setGrade(0, 2, 88); book.setGrade(0, 3, 94);
        book.setGrade(1, 0, 78); book.setGrade(1, 1, 84); book.setGrade(1, 2, 82); book.setGrade(1, 3, 89);
        book.setGrade(2, 0, 92); book.setGrade(2, 1, 88); book.setGrade(2, 2, 95); book.setGrade(2, 3, 91);
        book.setGrade(3, 0, 87); book.setGrade(3, 1, 90); book.setGrade(3, 2, 86); book.setGrade(3, 3, 93);
        
        book.printGradeReport();
    }
}

💡 Array Best Practices

Always check bounds: Validate array indices before accessing elements
Use enhanced for loops: When you don't need the index, use for-each loops
Initialize properly: Ensure arrays are properly initialized before use
Consider Arrays class: Use java.util.Arrays for common operations like sorting and searching
Handle empty arrays: Always check for null or empty arrays in methods
Use meaningful names: Choose descriptive variable names for arrays
Avoid hardcoded sizes: Use array.length instead of magic numbers