What is the difference between an abstract class and an interface?

Abstract class — is a class that can contain both implemented methods and abstract methods (without implementation). It serves as a base for inheritance and can contain state (fields).

Interface — is a contract that defines a set of methods without implementation (in the traditional sense). A class implementing an interface must implement all its methods.

Main differences:

• An abstract class can contain implementation and state, an interface — only method signatures (in some languages, interfaces can contain default methods).
• A class can inherit from only one abstract class but can implement multiple interfaces.
• An abstract class provides partial implementation, an interface — only the contract.

Example in Java:

abstract class Animal {
    void eat() { System.out.println("Eating"); }
    abstract void sound();
}

interface Flyable {
    void fly();
}

class Bird extends Animal implements Flyable {
    void sound() { System.out.println("Chirp"); }
    public void fly() { System.out.println("Flying"); }
}