A. Abstraction

1. What is an Abstract Class?

An abstract class in Java is a class that cannot be instantiated on its own and is meant to be extended by other classes.

It may include:

Abstract methods (methods without a body)
Non-abstract methods (with implementation)
Fields and constructors
Static and final methods

It provides a base for subclasses to build upon.

2. Declaring an Abstract Class

abstract class Animal {
    abstract void speak(); // abstract method

    void breathe() {
        System.out.println("Breathing...");
    }
}

Any class with at least one abstract method must be declared abstract.

3. Creating a Subclass

class Dog extends Animal {
    @Override
    void speak() {
        System.out.println("Bark");
    }
}

Subclasses must implement all abstract methods, unless they are also declared abstract.

4. Key Features

Feature	Abstract Class
Can be instantiated	No
Can have constructors	Yes
Can contain fields	Yes
Can have method bodies	Yes (both abstract and concrete methods)
Supports inheritance	Single class inheritance

5. Use Case

Use an abstract class when:

You want to provide common functionality to subclasses
You want to partially define a class and force subclasses to complete it

6. Example: Template Pattern

abstract class Payment {
    void process() {
        authenticate();
        pay();
        receipt();
    }

    void authenticate() {
        System.out.println("Authenticating...");
    }

    abstract void pay();

    void receipt() {
        System.out.println("Transaction complete.");
    }
}

class CreditCardPayment extends Payment {
    void pay() {
        System.out.println("Paid by credit card");
    }
}

public class Main {
    public static void main(String[] args) {
        Payment payment = new CreditCardPayment();
        payment.process();
    }
}

This is an example of the Template Method Pattern.

7. Abstract Class vs Interface

Feature	Abstract Class	Interface
Multiple inheritance	No	Yes
Method types	Both abstract and concrete	Abstract, default, static
Fields	Variables allowed	Only constants (`public static final`)
Constructors	Yes	No
Use case	Shared base with some logic	Common capability contract

8. Abstract vs Concrete Class

Type	Can instantiate?	Has abstract methods?	Has method bodies?
Abstract class	No	Optional	Yes
Concrete class	Yes	No	Yes

B. Interface

1. What is an Interface?

An interface in Java is a reference type, similar to a class, that can contain:

Abstract method signatures (without implementation)
Default methods (since Java 8)
Static methods
Constants (implicitly public, static, and final)

Interfaces are used to define a contract that implementing classes must follow.

2. Why Use Interfaces?

To achieve abstraction and multiple inheritance
To define a standard set of behaviors across unrelated classes
To support loose coupling and flexible architecture

3. Declaring an Interface

public interface Animal {
    void speak(); // abstract method
}

4. Implementing an Interface

public class Dog implements Animal {
    public void speak() {
        System.out.println("Bark");
    }
}

A class must implement all methods declared in the interface, unless the class is abstract.

5. Multiple Interfaces

A class can implement multiple interfaces, unlike class inheritance.

interface Flyable {
    void fly();
}

interface Swimmable {
    void swim();
}

class Duck implements Flyable, Swimmable {
    public void fly() {
        System.out.println("Duck flying");
    }

    public void swim() {
        System.out.println("Duck swimming");
    }
}

6. Interface Features by Java Version

Feature	Available Since	Description
Abstract methods	Java 1	Method signatures without body
Default methods	Java 8	Methods with body; allow backward compatibility
Static methods	Java 8	Utility methods within interfaces
Private methods	Java 9	Helper methods inside default/static methods

7. Default Methods

interface Logger {
    default void log(String msg) {
        System.out.println("LOG: " + msg);
    }
}

A class implementing Logger can use or override log().

8. Constants in Interfaces

All fields in interfaces are implicitly:

public
static
final

interface Config {
    int MAX_USERS = 100;
}

9. Interface vs Abstract Class

Feature	Interface	Abstract Class
Inheritance	Multiple (via `implements`)	Single (via `extends`)
Methods	Abstract, default, static	Abstract and concrete
Fields	Constants only	Variables and constants
Constructor	No	Yes
Use Case	Capability/Contract	Common base class with logic

10. Real-world Example

interface Payment {
    void pay(double amount);
}

class CreditCardPayment implements Payment {
    public void pay(double amount) {
        System.out.println("Paid $" + amount + " by credit card");
    }
}

class PayPalPayment implements Payment {
    public void pay(double amount) {
        System.out.println("Paid $" + amount + " using PayPal");
    }
}

class Checkout {
    void completePayment(Payment method, double amount) {
        method.pay(amount); // Polymorphism via interface
    }
}