Classes and Objects

Why This File Exists

Object-Oriented Programming starts with classes and objects.

Many developers use them daily but don’t clearly understand:

• what a class really represents
• how objects are created in memory
• why OOP was introduced in the first place

This file builds the mental model required for all OOP concepts.

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Why Object-Oriented Programming Exists

Before OOP:

• programs were written as large procedural code
• data and behavior were loosely connected
• maintenance was difficult as systems grew

OOP was introduced to:

• model real-world entities
• group data with behavior
• improve readability and maintainability

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What Is a Class

A class is a blueprint or template.

It defines:

• properties (fields / variables)
• behaviors (methods)

Example:

class User {
    String name;
    int age;

    void login() {
        System.out.println("User logged in");
    }
}

A class does not occupy memory by itself.

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What Is an Object

An object is a real instance of a class.

• Created at runtime
• Occupies memory
• Has its own state

Example:

User u1 = new User();
User u2 = new User();

Here:

• u1 and u2 are separate objects
• both follow the same class blueprint

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Class vs Object (Clear Difference)

Aspect	Class	Object
Nature	Blueprint	Instance
Memory	No	Yes
Created	At compile-time	At runtime
Count	One definition	Many instances

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Object Creation Process (Important)

User u = new User();

Steps:

1. Memory allocated on heap
2. Instance variables initialized
3. Constructor executed
4. Reference returned

Understanding this helps with:

• constructors
• memory management
• object lifecycle

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Reference Variables

User u = new User();

• u is a reference variable
• stores address of object
• lives on stack
• object lives on heap

Multiple references can point to same object.

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Object State and Behavior

• State → values of fields
• Behavior → methods

Example:

u.name = "Alex";   // state
u.login();         // behavior

OOP binds state and behavior together.

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Anonymous Objects

new User().login();

• Object without reference
• Used once
• Eligible for GC after use

Use sparingly for clarity.

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Why This Matters

Understanding classes and objects helps you:

• design better systems
• reason about memory
• understand inheritance and polymorphism
• debug reference-related issues

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Common Mistakes

• Thinking class occupies memory
• Confusing reference with object
• Creating unnecessary objects
• Overusing anonymous objects

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Best Practices

• Design classes around responsibility
• Keep classes cohesive
• Avoid large “God classes”
• Create objects only when needed

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Interview Notes

• Difference between class and object
• How objects are created in Java
• Where objects and references are stored
• Why OOP was introduced

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Summary

• Class defines structure and behavior
• Object represents real instance
• OOP starts with understanding this relationship

All other OOP concepts build on this.