Notes

its a paradigm based on concept of objects which combines data (attributes) and behavior (methods).

class

it is a blueprint or template for creation of an object. it defines - attributes - data stored in objects (variables). - methods - behaviors or functions that object can perform

• a class encapsulates data and behavior, supporting oop principles like encapsulation, inheritance and polymorphism.
• contains fields, methods, constructors and optionally other classes and interfaces.

class Car {
    // Fields
    String model;
    int year;

    // Constructor
    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    // Method
    void displayInfo() {
        System.out.println("Model: " + model + ", Year: " + year);
    }
}

creating objects

• an object is an instance of a class, created from class blueprint.
• objects are created using new keyword which allocated memory to the heap.
• a constructor is called during object creation to initlialize fields.
• each object has its own copy of instance variables but shares the class’s methods.

public class Main {
    public static void main(String[] args) {
        // Creating objects
        Car car1 = new Car("Toyota", 2020);
        Car car2 = new Car("Honda", 2022);

        // Accessing object methods
        car1.displayInfo(); // Outputs: Model: Toyota, Year: 2020
        car2.displayInfo(); // Outputs: Model: Honda, Year: 2022
    }
}

assigning object reference variables

• variables of a class type are references to object.
• a reference variable holds the memory address of an object on the heap.
• assigning one variable to another variables make both point to the same object
• objects are not duplicated during assignment. only references are copied
• setting a references to null removes its association with any object, making the object eligible for garbage collection.

public class Main {
    public static void main(String[] args) {
        // Create objects
        Car car1 = new Car("Toyota", 2020);
        Car car2 = new Car("Honda", 2022);

        // Assigning object reference
        Car ref = car1; // ref now points to the same object as car1
        ref.displayInfo(); // Outputs: Model: Toyota, Year: 2020

        // Modify object via ref
        ref.model = "Nissan";
        car1.displayInfo(); // Outputs: Model: Nissan, Year: 2020 (car1 reflects change)

        // Set car1 to null
        car1 = null;
        ref.displayInfo(); // Still works: Model: Nissan, Year: 2020 (ref still points to object)
        // car1.displayInfo(); // Would throw NullPointerException
    }
}