Difference Between Interface and Abstract Class - OOPS

Basic Difference

Interface	Abstract Class
An Interface defines a contract that a class must follow. 1. It contains only method signatures and property types. 2. No Implementation. 3. Used for defining shape of objects.	An abstract class is a class that: 1. Cannot be instantiated directly. 2. Can have both abstract methods and concrete methods. 3. Can contain state (properties with values) and access modifiers.

Examples

Interface:

interface NotificationProvider {
  send(to: string, message: string): Promise<void>;
}

class EmailProvider implements NotificationProvider {}
class SMSProvider implements NotificationProvider {}

Abstract Class:

abstract class PaymentService {
  protected apiKey: string;

  constructor(apiKey: string) {
    this.apiKey = apiKey;
  }

  abstract processPayment(amount: number): Promise<boolean>;

  logTransaction(amount: number) {
    console.log(`Transaction logged: ${amount}`);
  }
}

Use Cases

Interface	Abstract Class
1. Define a contract for multiple implementations. 2. Create pluggable architectures.	1. When basic structure + default behaviour is needed.

Details

Feature	Interface	Abstract Class
Implementation allowed?	❌	✅
Can have constructor?	❌	✅
Can have access modifiers?	❌	✅
Multiple inheritance?	✅	❌
Used for	Structure	Base behavior + structure
Runtime existence?	❌ Removed after compile	✅ Exists in JS

Using both together.

interface Payment {
  process(): void;
}

abstract class BasePayment implements Payment {
  abstract process(): void;

  log() {
    console.log("Logging...");
  }
}