? Java Abstract Class

? Quick Overview

An abstract class in Java is a class that cannot be instantiated and is designed to be inherited. It may contain abstract methods (without implementation) as well as concrete methods (with implementation).

? Key Concepts

Declared using the abstract keyword
Cannot create objects directly
Can contain abstract and non-abstract methods
Supports constructors and instance variables
Used to achieve partial abstraction

? Syntax / Theory

An abstract method does not have a body and must be implemented by child classes. If a class contains at least one abstract method, the class itself must be declared abstract.

? View Code Example

// Abstract class defining a common structure
abstract class Animal {
abstract void sound();
void sleep() {
System.out.println("Animal is sleeping");
}
}

? Code Example(s)

? View Code Example

// Concrete class implementing abstract methods
class Dog extends Animal {
void sound() {
System.out.println("Dog barks");
}
}

? View Code Example

// Main class to test abstract class behavior
public class Main {
public static void main(String[] args) {
Animal a = new Dog();
a.sound();
a.sleep();
}
}

? Live Output / Explanation

Output

Dog barks
Animal is sleeping

The abstract reference Animal points to a Dog object. The implemented sound() method of Dog is executed at runtime.

? Interactive Demo: Abstraction in Action

See how the Shape abstract class works with different implementations. Change the inputs below and run the Java code.

Choose Implementation:

// Click Run to see output...

✅ Tips & Best Practices

Use abstract classes when classes share common behavior
Prefer abstract classes over interfaces when method implementation is required
Do not overuse abstraction; keep designs simple
Use abstract methods to enforce implementation rules

? Try It Yourself

Create an abstract class Shape with method area()
Implement it using Circle and Rectangle classes
Test polymorphism using abstract references