📚 1. Class

What is a Class?

A class is a blueprint or template for creating objects. It defines: ✅ Attributes (fields or properties — data that the object will have)
✅ Methods (behavior — what the object can do)

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Example

class Car {
    String brand;       // attribute
    int speed;          // attribute

    void accelerate() { // method
        speed += 10;
    }

    void brake() {      // method
        speed -= 10;
    }
}

Key Point

A class itself is not a real-world thing, it’s just a design.

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🚗 2. Object

What is an Object?

An object is a real-world instance of a class. When you create an object from a class, it gets actual values and can perform actions using the methods of the class.

Car myCar = new Car();
myCar.brand = "Toyota";
myCar.speed = 0;

myCar.accelerate(); // myCar's speed = 10

Key Point

Objects are the real entities created using the class blueprint.

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🔌 3. Interface

What is an Interface?

An interface is a special type of class that: ✅ Only defines method signatures (what should be done — not how)
✅ Has no implementation (no method body) — the actual logic is provided by classes that implement the interface

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Example

interface Payment {
    void processPayment(double amount);
}

Classes like CreditCardPayment and PayPalPayment can implement this interface and provide their own versions of processPayment():

class CreditCardPayment implements Payment {
    @Override
    public void processPayment(double amount) {
        System.out.println("Processing credit card payment of $" + amount);
    }
}

class PayPalPayment implements Payment {
    @Override
    public void processPayment(double amount) {
        System.out.println("Processing PayPal payment of $" + amount);
    }
}

Why Use Interfaces?

✅ To enforce a common contract across different classes (they must implement the methods).
✅ Supports polymorphism — you can write flexible code that works with any Payment object (credit card, PayPal, etc.), without caring about the specific type.

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Key Point

An interface defines what must be done, but leaves the how to the classes that implement it.

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🔗 Quick Summary Table

Concept	What is it?	Key Role
Class	Blueprint/template	Defines structure (attributes + methods)
Object	Instance of a class	Real thing, actual data + behavior
Interface	Contract	Defines behavior that multiple classes must follow