Day 2: Functions & Methods – A Simple Understanding

So today, I learned about functions and methods in Java. Basically, instead of writing the same code again and again like a copy-paste champion, I can just write a function once and call it whenever needed.

It’s like ordering pizza—instead of making one from scratch every time, you just call the pizza shop, and they handle it. Functions work the same way! Write once, use forever (or at least until Java decides to throw an error).

---

What is a Function? 🤔

A function is a block of code that does something specific. Instead of writing the same logic multiple times, you just write it once inside a function and call it whenever needed.

Think of it like this:

• Without functions: You’re making tea from scratch every time—boiling water, adding tea leaves, sugar, milk... exhausting!

• With functions: You create a "tea-making machine" (a function). Just press a button (call the function), and it makes tea for you!

---

How Do We Write a Function?

Java, being Java, has some rules. Here’s the basic structure of a function:

returnType functionName(parameters) {
    // function body (this is where the logic goes)
    return something; 
}

Looks a bit scary? Don't worry, let’s break it down.

Example 1: A Simple Function That Adds Two Numbers

public class Main {
    // Function definition
    public static int addNumbers(int a, int b) { 
        int sum = a + b; // Adds the numbers
        return sum; // Returns the result
    }

    public static void main(String[] args) {
        int result = addNumbers(5, 10); // Calling the function
        System.out.println("Sum: " + result); // Printing the result
    }
}

---

Understanding the Function 👀

Step 1: Function Definition

public static int addNumbers(int a, int b) {

• public static → Just some Java keywords. Ignore them for now.

• int → The return type. It means this function will return an integer.

• addNumbers → The function name (you can name it whatever makes sense).

• (int a, int b) → These are parameters (input values for the function).

Step 2: Function Logic

int sum = a + b;

• Takes a and b, adds them, and stores the result in sum.

Step 3: Returning the Result

return sum;

• The function sends back the result to wherever it was called.

Step 4: Calling the Function

int result = addNumbers(5, 10);

• We call the function addNumbers(5, 10), giving it two numbers.

• The function does its magic and returns 15, which gets stored in result.

Output:

Sum: 15

---

Functions with No Return Type (void)

Sometimes, we just want a function to do something without returning a value.

Example 2: A Function That Prints a Message

public class Main {
    public static void greetUser() { 
        System.out.println("Hello! Welcome to Java!");
    }

    public static void main(String[] args) {
        greetUser(); // Calling the function
    }
}

Explanation:

• No return statement because void means "returns nothing."

• The function just prints a message when called.

• Calling greetUser(); in main() runs the function.

Output:

Hello! Welcome to Java!

---

Problem: Reverse a Number Using Functions

Now, let’s use a function to reverse a number (because why do it manually?).

Example 3: Function to Reverse a Number

public class Main {
    public static int reverseNumber(int n) {
        int rev = 0;
        while (n > 0) {
            int lastDigit = n % 10; // Extract last digit
            rev = rev * 10 + lastDigit; // Add it to the reversed number
            n = n / 10; // Remove last digit from n
        }
        return rev;
    }

    public static void main(String[] args) {
        int reversed = reverseNumber(12345);
        System.out.println("Reversed Number: " + reversed);
    }
}

How This Works:

Let’s take 12345 and go step by step:

Output:

Reversed Number: 54321

---

What I Learned Today

✔ Functions let me reuse code instead of writing the same thing repeatedly.
✔ Methods are basically functions inside a class (and everything in Java is inside a class).
✔ Java forces me to return something if the function is not void.
✔ Functions make problem-solving easier—just break problems into small steps.

---

What’s Next?

Tomorrow, I’ll tackle arrays (because one variable is never enough). Wish me luck—Java still has more surprises for me. 😆

---