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Java Abstraction

What is Abstraction?

Abstraction is one of the key principles of object-oriented programming (OOP). It is a concept that allows you to model real-world entities by simplifying their complexities. Abstraction involves focusing on the essential features of an object while ignoring the non-essential details.

In Java, abstraction is achieved through abstract classes and interfaces.

1. Abstract Classes:

  • An abstract class is a class that cannot be instantiated on its own.
  • It may have both abstract (unimplemented) and concrete (implemented) methods.
  • Abstract methods are declared with the abstract keyword and must be implemented by any concrete subclass.
     

Syntax

abstract class Shape {
    abstract void draw(); // Abstract method
    void display() {
       // code;
    }
}

 

Example

// Abstract class
abstract class Shape {

    abstract void draw();

    void display() {
        System.out.println("Displaying shape");
    }
}

class Circle extends Shape {
    // Implementing the abstract method
    @Override
    void draw() {
        System.out.println("Drawing a circle");
    }
}

class Rectangle extends Shape {
    // Implementing the abstract method
    @Override
    void draw() {
        System.out.println("Drawing a rectangle");
    }
}
public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle();
        circle.draw();    // Calls the overridden draw method in Circle
        circle.display(); // Calls the inherited display method from Shape

        Rectangle rectangle = new Rectangle();
        rectangle.draw();    // Calls the overridden draw method in Rectangle
        rectangle.display(); // Calls the inherited display method from Shape
    }
}

 

2. Interfaces:

  • An interface is a collection of abstract methods.
  • All methods declared in an interface are implicitly public and abstract.
  • Classes implement interfaces using the implements keyword.
  • Interfaces support multiple inheritance.
     

Syntax

interface Shape {
    void draw(); // Abstract method
    void resize(); // Another abstract method
}

 

Example

// Interface
interface Shape {
    // Abstract method (to be implemented by classes)
    void draw();
}


class Circle implements Shape {
    // Implementing the draw method from the Shape interface
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

// Concrete class implementing the Shape interface
class Rectangle implements Shape {
    // Implementing the draw method from the Shape interface
    @Override
    public void draw() {
        System.out.println("Drawing a rectangle");
    }
}

// Main class to demonstrate the interface and its implementations
public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle();
        circle.draw();    // Calls the overridden draw method in Circle
        
        Rectangle rectangle = new Rectangle();
        rectangle.draw();    // Calls the overridden draw method in Rectangle
    }
}

 

What are Abstract Methods?

  • Abstract methods are methods declared without a body (no implementation).
  • They are meant to be implemented by concrete subclasses.
  • Abstract methods in abstract classes and interfaces contribute to abstraction.

Example

abstract class Shape {
    abstract void draw(); // Abstract method
}

 

Abstract Class vs. Interface:

  • Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods (prior to Java 8).
  • A class can implement multiple interfaces, but it can extend only one class (abstract or not).
  • With Java 8 and later versions, interfaces can have default and static methods with implementations, blurring the distinction between abstract classes and interfaces.
     

Example

interface Shape {
    void draw(); // Abstract method
}

abstract class AbstractShape implements Shape {
    void display() {
        System.out.println("Displaying shape");
    }
}

class Circle extends AbstractShape {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

class Main {
    public static void main(String[] args) {
        Circle circle = new Circle();
        circle.draw();
        circle.display();
    }
}

Note:- In above example, ‘Circle’ implements the abstract method ‘draw’ from the ‘Shape’ interface and inherits the display method from the abstract class 'AbstractShape'

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