# 📚 Java Programming Language – Part 3/10: Methods & Functions
#Java #Programming #Methods #Functions #OOP

Welcome to Part 3 of our Java series! Today we'll dive into methods - the building blocks of Java programs.

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## 🔹 What are Methods in Java?
Methods are blocks of code that:
✔️ Perform specific tasks
✔️ Can be reused multiple times
✔️ Help organize code logically
✔️ Can return a value or perform actions without returning

// Method structure
[access-modifier] [static] return-type methodName(parameters) {
    // method body
    return value; // if not void
}

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## 🔹 Method Components
### 1. Simple Method Example

public class Calculator {
    
    // Method without return (void)
    public static void greet() {
        System.out.println("Welcome to Calculator!");
    }
    
    // Method with return
    public static int add(int a, int b) {
        return a + b;
    }
    
    public static void main(String[] args) {
        greet();  // Calling void method
        int sum = add(5, 3);  // Calling return method
        System.out.println("Sum: " + sum);
    }
}

### 2. Method Parameters

public static void printUserInfo(String name, int age) {
    System.out.println("Name: " + name);
    System.out.println("Age: " + age);
}

### 3. Return Values

public static boolean isAdult(int age) {
    return age >= 18;
}

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## 🔹 Method Overloading
Multiple methods with same name but different parameters.

public class MathOperations {
    
    // Version 1: Add two integers
    public static int add(int a, int b) {
        return a + b;
    }
    
    // Version 2: Add three integers
    public static int add(int a, int b, int c) {
        return a + b + c;
    }
    
    // Version 3: Add two doubles
    public static double add(double a, double b) {
        return a + b;
    }
    
    public static void main(String[] args) {
        System.out.println(add(2, 3));       // 5
        System.out.println(add(2, 3, 4));    // 9
        System.out.println(add(2.5, 3.7));  // 6.2
    }
}

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## 🔹 Recursion
A method that calls itself.

### 1. Factorial Example

public static int factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

### 2. Fibonacci Sequence

public static int fibonacci(int n) {
    if (n <= 1) {
        return n;
    }
    return fibonacci(n-1) + fibonacci(n-2);
}

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## 🔹 Variable Scope
Variables have different scope depending on where they're declared.

public class ScopeExample {
    static int classVar = 10;  // Class-level variable
    
    public static void methodExample() {
        int methodVar = 20;  // Method-level variable
        System.out.println(classVar);  // Accessible
        System.out.println(methodVar); // Accessible
    }
    
    public static void main(String[] args) {
        int mainVar = 30;  // Block-level variable
        System.out.println(classVar);  // Accessible
        // System.out.println(methodVar); // ERROR - not accessible
        System.out.println(mainVar);   // Accessible
    }
}

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## 🔹 Practical Example: Temperature Converter

public class TemperatureConverter {
    
    public static double celsiusToFahrenheit(double celsius) {
        return (celsius * 9/5) + 32;
    }
    
    public static double fahrenheitToCelsius(double fahrenheit) {
        return (fahrenheit - 32) * 5/9;
    }
    
    public static void main(String[] args) {
        System.out.println("20°C to Fahrenheit: " + celsiusToFahrenheit(20));
        System.out.println("68°F to Celsius: " + fahrenheitToCelsius(68));
    }
}

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## 🔹 Best Practices for Methods
1. Single Responsibility Principle - Each method should do one thing
2. Descriptive Names - Use verbs (calculateTotal, validateInput)
3. Limit Parameters - Ideally 3-4 parameters max
4. Proper Indentation - Keep code readable
5. Documentation - Use JavaDoc comments