Java for Beginners – Functions & Object-Oriented Basics

 

1. What are Functions (Methods) in Java?

A function (in Java we call it a method) is a block of code that performs a specific task.
Instead of repeating code, you write it once inside a method and call it whenever you need it.

Example:

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

2. Why Use Methods?

✅ Reusability – Write once, use many times
✅ Cleaner code – Easier to read & maintain
✅ Better structure – Breaks big programs into smaller parts

3. Syntax of a Method

accessModifier returnType methodName(parameters) {
    // method body
    return value; // only if returnType is not void
}

Example:

public static int add(int a, int b) {
    return a + b;
}

4. Calling a Method

public class Example {
    public static void greet() {
        System.out.println("Hello!");
    }

    public static void main(String[] args) {
        greet(); // method call
    }
}

5. Methods with Parameters and Return Values

public class Calculator {
    public static int multiply(int x, int y) {
        return x * y;
    }

    public static void main(String[] args) {
        int result = multiply(4, 5);
        System.out.println("Result: " + result);
    }
}

6. Method Overloading

Two or more methods can have the same name but different parameter lists.

public class MathOps {
    public static int add(int a, int b) {
        return a + b;
    }

    public static double add(double a, double b) {
        return a + b;
    }

    public static void main(String[] args) {
        System.out.println(add(5, 3));       // Calls int version
        System.out.println(add(2.5, 3.7));   // Calls double version
    }
}

7. Introduction to Object-Oriented Programming (OOP)

Java is an object-oriented language, meaning everything revolves around classes and objects.

Key OOP Concepts:

  • Class → Blueprint or template for creating objects

  • Object → Instance of a class

  • Attributes → Variables inside a class (properties of the object)

  • Methods → Actions/behaviors inside a class

8. Creating a Class and Object

public class Car {
    String brand;
    int year;

    public void start() {
        System.out.println(brand + " is starting...");
    }

    public static void main(String[] args) {
        Car myCar = new Car();  // create object
        myCar.brand = "Toyota";
        myCar.year = 2022;

        System.out.println("Brand: " + myCar.brand);
        myCar.start();
    }
}

Output:

Brand: Toyota
Toyota is starting...

9. Constructors in Java

A constructor is a special method that runs automatically when an object is created.

public class Student {
    String name;
    int age;

    // Constructor
    Student(String n, int a) {
        name = n;
        age = a;
    }

    public void display() {
        System.out.println(name + " - " + age);
    }

    public static void main(String[] args) {
        Student s1 = new Student("Poonam", 20);
        s1.display();
    }
}

10. Mini Practice Exercises

Task 1: Create a class Book with attributes title and author, and a method displayDetails(). Create objects for two books and display their details.

Task 2: Write a method isEven(int number) that returns true if the number is even, otherwise false.

Task 3: Create a Calculator class with overloaded methods for addition (int, double).

11. What’s Next?

In Blog 5, we’ll dive deeper into OOP concepts:

  • Access Modifiers

  • Inheritance

  • Method Overriding

  • Polymorphism

  • Encapsulation

💡 Tip: Think of a class like a "blueprint" and an object like the "real thing" built from that blueprint. Practice by modeling real-world things as Java classes.

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