Difference Between Object and Class

A mould for making clay diyas isn't a diya; you can't light it. But from one mould a potter can make a thousand diyas, each a little different in color. Class and object have exactly this relationship, and "what's the difference between a class and an object?" is asked in almost every fresher interview. This lesson puts them side by side, including what happens in memory.

Side by Side

In Memory

public class Accounts {
    static class BankAccount {
        String holder;
        long balance;
        static int totalAccounts = 0;
 
        BankAccount(String holder, long balance) {
            this.holder = holder;
            this.balance = balance;
            totalAccounts++;
        }
    }
 
    public static void main(String[] args) {
        BankAccount a1 = new BankAccount("Navika", 5000);
        BankAccount a2 = new BankAccount("Srihitha", 12000);
        BankAccount a3 = new BankAccount("Advitha", 800);
        System.out.println("Accounts: " + BankAccount.totalAccounts);
        System.out.println(a1.holder + " " + a2.holder + " " + a3.holder);
        System.out.println(a1.getClass() == a3.getClass());
    }
}

Accounts: 3
Navika Srihitha Advitha
true

How it works

1. One class, BankAccount, is loaded once. Its static field totalAccounts lives with the class, so there's only one counter.
2. Three new calls create three objects, each with its own holder and balance.
3. a1.getClass() == a3.getClass() is true: different objects, but the same class.

Try it yourself: print a1 == a3. Same class, but are they the same object?

Every Object Knows Its Class

Every object can tell you its class with getClass(), and instanceof checks whether an object belongs to a class:

Common Mistakes

Saying a class "has a value". A class defines which fields exist; each object has its own values. Only static fields have a value at class level.

Thinking each object gets a copy of the methods. Method code lives once, with the class; objects only hold field values.

Comparing objects of the same class with == to check their contents. == checks whether they're the same object. Content comparison uses equals (see "Object Equality and Hash Codes in Java").

Interview Corner

Practice

1. Easy: For a Movie class in a ticket app, give three examples of objects and say which values differ between them.
2. Harder: Add a static count to a class Student, create four objects, and print the count using the class name, not an object.

Key Takeaways