Difference Between Abstract Class and Interface in Java (original) (raw)

Last Updated : 16 May, 2026

Abstract classes and interfaces are used in Java to achieve abstraction and design flexible applications. They define a structure that other classes must follow, helping in code reusability and maintainability. While both serve similar purposes, they differ in implementation and usage.

• Abstract class can have both abstract and normal methods, while interface mainly contains abstract methods.
• A class can extend only one abstract class, but can implement multiple interfaces.
• Abstract class can have constructors and variables, whereas interface contains only constants and no constructors.

Depiction of Abstract class and Interface

Abstract Class

An abstract class is a class that cannot be instantiated and is used as a base for other classes. It can contain both abstract (unimplemented) and concrete (implemented) methods.

• Supports both abstract and non-abstract methods.
• Allows constructors and instance variables. Java `

abstract class Shape {

// Abstract method
abstract double area();

// Concrete method
void display() {
    System.out.println("This is a shape");
}

}

class Circle extends Shape {

int radius = 5;

// Implementing abstract method
@Override
double area() {
    return 3.14 * radius * radius;
}

}

public class Main { public static void main(String[] args) {

    Shape shape = new Circle();  // Upcasting
    shape.display();
    System.out.println("Area of Circle: " + shape.area());
}

}

`

Output

This is a shape Area of Circle: 78.5

**Explanation: Shape is an abstract class with an abstract method area() and a concrete method display(). Circle extends it and implements area(). Using Shape shape = new Circle(); shows upcasting, and calling shape.area() demonstrates runtime polymorphism.

Interface

An interface is a blueprint that defines a set of methods a class must implement. It focuses on behavior rather than implementation.

• All methods are abstract by default (except default/static methods in Java 8+).
• Supports multiple inheritance. Java `

// Interface interface Drawable { void draw(); }

// Implementation class class Rectangle implements Drawable { @Override public void draw() { System.out.println("Drawing Rectangle"); } }

// Main class public class Main { public static void main(String[] args) { // Interface reference pointing to implementation object Drawable d = new Rectangle(); d.draw(); } }

`

**Explanation: Drawable is an interface with method draw(). Rectangle implements it and defines the method. Drawable d = new Rectangle(); shows polymorphism, and d.draw() executes the Rectangle method at runtime.

Abstract Class vs Interface