What is the Difference Between an Interface and an Abstract Class in Java?

What is the difference between an Interface and an Abstract class in Java, and in what scenarios should I choose one over the other?

// Example of an Interface
interface Animal {
    void makeSound();
}

// Example of an Abstract Class
abstract class Vehicle {
    abstract void start();

    void stop() {
        System.out.println("Vehicle stopped.");
    }
}

In Java, both interfaces and abstract classes play crucial roles in designing an application’s architecture by achieving abstraction and polymorphism. However, there are significant differences in their use cases, capabilities, and constraints.

// Example of implementing an interface
class Dog implements Animal {
    public void makeSound() {
        System.out.println("Woof");
    }
}

// Example of extending an abstract class
class Car extends Vehicle {
    void start() {
        System.out.println("Car started");
    }
}

Causes

• Interfaces are used when multiple classes share common behaviors but do not share implementation.
• Abstract classes are suited for situations where classes share certain behaviors and require a common base implementation.

Solutions

• Use interfaces to define a contract that various classes must follow, promoting loose coupling.
• Use abstract classes when you need to provide a shared base implementation for related classes.

Mistake: Confusing interfaces with abstract classes in terms of functionality and use cases.

Solution: Understand that interfaces are purely for defining methods without behavior, while abstract classes allow both abstract and concrete methods.

Mistake: Claiming that classes can implement multiple abstract classes.

Solution: Remember that a class can only extend one abstract class but can implement multiple interfaces.