# 📚 Java Programming Language – Part 6/10: Interfaces & Abstract Classes
#Java #OOP #Interfaces #AbstractClasses #Programming

Welcome to Part 6 of our Java series! Today we'll explore two crucial concepts for achieving abstraction in Java: Interfaces and Abstract Classes.

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## 🔹 Interfaces in Java
Interfaces define contracts that classes must implement (100% abstraction).

### 1. Interface Declaration (Pre-Java 8)

interface Drawable {
    // Constant fields (implicitly public static final)
    String COLOR = "Black";
    
    // Abstract methods (implicitly public abstract)
    void draw();
    double calculateArea();
}

### 2. Implementing Interfaces

class Circle implements Drawable {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
    
    @Override
    public double calculateArea() {
        return Math.PI * radius * radius;
    }
}

### 3. Modern Interfaces (Java 8+) Features

interface Vehicle {
    // Traditional abstract method
    void start();
    
    // Default method (with implementation)
    default void honk() {
        System.out.println("Beep beep!");
    }
    
    // Static method
    static void printType() {
        System.out.println("I'm a vehicle");
    }
}

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## 🔹 Abstract Classes
Classes that can't be instantiated and may contain abstract methods.

### 1. Abstract Class Example

abstract class Animal {
    // Concrete method
    public void breathe() {
        System.out.println("Breathing...");
    }
    
    // Abstract method (no implementation)
    public abstract void makeSound();
}

### 2. Extending Abstract Classes

class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Woof woof!");
    }
}

// Usage:
Animal myPet = new Dog();
myPet.breathe();    // Inherited concrete method
myPet.makeSound();  // Implemented abstract method

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## 🔹 Key Differences

| Feature | Interface | Abstract Class |
|------------------------|-----------------------------------|------------------------------------|
| Instantiation | Cannot be instantiated | Cannot be instantiated |
| Methods | All abstract (pre-Java 8) | Can have both abstract & concrete |
| Variables | Only constants | Any variables |
| Multiple Inheritance | Class can implement many interfaces | Class extends only one abstract class |
| Default Methods | Supported (Java 8+) | Not applicable |
| Constructor | No constructors | Can have constructors |
| When to Use | Define contracts/APIs | Share code among related classes |

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