Question

Discuss the differences between interfaces and abstract classes in Java. What are their respective use cases? How do they impact code design and flexibility?

Interface

An interface defines a contract that implementing classes must fulfill, allowing for multiple inheritance and promoting loose coupling. It can only contain method signatures and static final variables.

interface Animal {
    void makeSound();
}

Abstract Class

An abstract class can provide some method implementations while declaring others as abstract. It allows for shared state and behavior among subclasses, but a class can only inherit from one abstract class.

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

Multiple Inheritance

Interfaces support multiple inheritance, allowing a class to implement multiple interfaces. Abstract classes do not, enforcing a single inheritance hierarchy.

class Dog implements Animal, Pet {}

Use Cases

Use interfaces when you need to define a contract for disparate classes. Use abstract classes when you have a common base with shared code or state.

interface Comparable { int compareTo(Object o); }