📘 Java API: Abstract Classes and Interfaces

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🔹 1. Definition

✅ Abstract Class

An abstract class in Java is a class that cannot be instantiated (object cannot be created) and is used as a base class for other classes.

• It may contain:

  • Abstract methods (without body)
  • Concrete methods (with body)

• Declared using the keyword abstract

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✅ Interface

An interface is a completely abstract blueprint of a class that contains only abstract methods (by default) and constants.

• Used to achieve full abstraction and multiple inheritance
• Declared using the keyword interface

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🔹 2. Key Concepts

✨ Abstract Class Features

• Cannot create objects directly

• Can have:

  • Constructors
  • Instance variables
  • Both abstract and non-abstract methods

• Supports single inheritance

• Subclass must implement all abstract methods

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✨ Interface Features

• All methods are public and abstract (by default)
• Variables are public, static, and final
• No constructors
• Supports multiple inheritance
• A class implements an interface using implements

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🔹 3. Syntax and Examples

🧩 Abstract Class Example

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

    void sleep() {           // concrete method
        System.out.println("Sleeping...");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Barking");
    }
}

public class Test {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();
        d.sleep();
    }
}

✔ Explanation (Step-by-step)

1. Animal is an abstract class.

2. It has:

   • sound() → abstract
   • sleep() → concrete

3. Dog inherits Animal using extends.

4. Dog must override sound().

5. Object of Dog is created (not Animal).

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🧩 Interface Example

interface Vehicle {
    void start();
}

class Car implements Vehicle {
    public void start() {
        System.out.println("Car starts");
    }
}

public class Test {
    public static void main(String[] args) {
        Car c = new Car();
        c.start();
    }
}

✔ Explanation

1. Vehicle is an interface.
2. Car implements it using implements.
3. Method start() must be defined in Car.

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🔹 4. Abstract Class vs Interface

Feature	Abstract Class	Interface
Methods	Abstract + Concrete	Mostly Abstract
Variables	Normal variables allowed	Only constants
Inheritance	Single	Multiple
Constructors	Allowed	Not allowed
Keyword	extends	implements

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🔹 5. Important Rules

📌 Abstract Class Rules

• Must use abstract keyword
• Cannot create object directly
• Can have constructor
• If a class contains abstract method → must be abstract

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📌 Interface Rules

• Methods are public abstract by default
• Variables are public static final
• A class can implement multiple interfaces
• Use implements keyword

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🔹 6. When to Use What?

✔ Use Abstract Class when:

• You want common base functionality
• Classes share some behavior

✔ Use Interface when:

• You want only method declarations
• You need multiple inheritance

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🔹 7. Multiple Inheritance Using Interface

interface A {
    void show();
}

interface B {
    void display();
}

class Test implements A, B {
    public void show() {
        System.out.println("Show method");
    }

    public void display() {
        System.out.println("Display method");
    }
}

✔ Java allows multiple inheritance using interfaces (not classes)

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🔹 8. Diagram Description (for exams)

📊 Abstract Class Diagram (Description)

• A base class Animal
• Derived class Dog
• Arrow showing inheritance (Dog → Animal)
• Abstract method in base, implemented in child

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📊 Interface Diagram (Description)

• Interface Vehicle
• Class Car
• Dotted arrow showing implementation (Car → Vehicle)
• Method defined in class

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🔹 9. Important Points to Remember

• Abstract class = partial abstraction
• Interface = full abstraction
• Abstract class uses extends
• Interface uses implements
• Interface supports multiple inheritance

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📝 Likely Exam Questions

1. Define an abstract class with example.
2. What is an interface in Java? Explain with syntax.
3. Difference between abstract class and interface.
4. Can we create object of abstract class? Why?
5. Explain multiple inheritance using interface.
6. Write a Java program using abstract class.
7. Write a program implementing an interface.
8. What are rules of interfaces in Java?
9. When should you use abstract class instead of interface?

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📌 Quick Revision Summary

• Abstract Class

  • Cannot instantiate
  • Has abstract + concrete methods
  • Supports single inheritance

• Interface

  • Only method declarations
  • Supports multiple inheritance
  • Implemented using implements

• Key Difference

  • Abstract class = partial abstraction
  • Interface = full abstraction

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