A class is a blueprint that bundles state (fields) and behaviour (methods); an object is one instance of it living on the heap.

The four pillars of OOP

Declaring a class and creating objects

Java

public class Main {
    public static void main(String[] args) {
        BankAccount a = new BankAccount("Asha", 500);
        BankAccount b = new BankAccount("Ravi", 100);
        a.deposit(250);
        b.withdraw(30);
        System.out.println(a.describe());
        System.out.println(b.describe());
        System.out.println("accounts opened: " + BankAccount.count);
    }
}

class BankAccount {
    String owner;          // instance field: one copy per object
    double balance;
    static int count = 0;  // static field: one copy per class

    BankAccount(String owner, double balance) {   // constructor
        this.owner = owner;
        this.balance = balance;
        count++;
    }

    void deposit(double amount) { balance += amount; }
    void withdraw(double amount) { balance -= amount; }
    String describe() { return owner + " has " + balance; }
}

Output

Asha has 750.0
Ravi has 70.0
accounts opened: 2

What `new` does

Load the class
If BankAccount isn't loaded yet, the class loader loads, links and initialises it (static fields/blocks run once).
Allocate
Memory for the object is reserved on the heap; every field gets its default value (0, false, null).
Initialise
Field initialisers and instance initializer blocks run, then the constructor body.
Return a reference
new evaluates to a reference (think: a safe pointer) that is stored in the variable.

References, not objects, live in variables

Java

public class Main {
    public static void main(String[] args) {
        Point p1 = new Point();
        p1.x = 1;
        Point p2 = p1;          // copies the reference, not the object
        p2.x = 99;
        System.out.println(p1.x);        // same object
        System.out.println(p1 == p2);    // same reference?

        Point p3 = new Point();
        p3.x = 99;
        System.out.println(p1 == p3);    // different objects

        Point nothing = null;
        System.out.println(nothing == null);
    }
}

class Point { int x, y; }

Output

99
true
false
true

Default values of fields

Field type	Default
`byte`, `short`, `int`, `long`	`0`
`float`, `double`	`0.0`
`char`	`'\u0000'`
`boolean`	`false`
Any reference (`String`, arrays, objects)	`null`

Instance vs class members

	Instance member	Static (class) member
Belongs to	each object	the class itself
Copies	one per object	exactly one
Accessed via	`obj.field`	`ClassName.field`
Can use `this`?	Yes	No

What is the difference between a class and an object?

Is p2 = p1 a copy of the object?

Where do objects and references live?