OOP

16. Class vs Object

A class is a blueprint; an object is an instance of that blueprint created at runtime. Reference variables hold memory addresses (references), not the objects themselves.

Java 21 (core language)

Explanation

A class defines structure (fields) and behaviour (methods). An object (instance) is a specific, live chunk of heap memory created from that blueprint by new. Multiple objects of the same class share the class definition but have independent field values.

Reference types in Java: all class instances, arrays, and interfaces are reference types. A reference variable holds the memory address of the object, not the object itself. When you write Dog d = new Dog(), d is a reference (like a pointer) stored on the stack (or in a field), while the Dog object lives on the heap.

Null is the absence of a reference — a reference variable not pointing at any object. Attempting to dereference null (null.method()) throws a NullPointerException. Fields of reference type default to null; primitive fields default to 0/false.

Example

public class ClassVsObject {
    // Class blueprint
    static class Dog {
        String name;  // reference field — default null
        int age;      // primitive field — default 0

        Dog(String name, int age) {
            this.name = name;
            this.age  = age;
        }

        void bark() { System.out.println(name + ": Woof!"); }
    }

    public static void main(String[] args) {
        // Two distinct objects from the same class
        Dog rex  = new Dog("Rex",  3);
        Dog bella = new Dog("Bella", 5);

        rex.bark();   // Rex: Woof!
        bella.bark(); // Bella: Woof!

        // Reference assignment: both point to the SAME object
        Dog alias = rex;
        alias.name = "Max";
        System.out.println(rex.name); // "Max" — same object mutated

        // null reference
        Dog nobody = null;
        System.out.println(nobody); // null
        try {
            nobody.bark(); // NullPointerException!
        } catch (NullPointerException e) {
            System.out.println("NPE: cannot call method on null reference");
        }

        // instanceof check (good practice before casting)
        Object obj = rex;
        if (obj instanceof Dog d) {
            System.out.println("It's a dog named " + d.name);
        }
    }
}

Sources