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

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Khushi Singh · Answer

An Interface acts as a binding document that lists a collection of methods through their signature while abstaining from delivering implementation details for them. Implementing the interface compels any class to maintain a specific structure. The abstract class functions as a base class containing abstract methods with no implementation and concrete methods implementing code along with the structure.

The main distinction occurs because interfaces cannot house instance variables or constructors, but abstract classes maintain access to fields and constructors and implemented methods. The capability to inherit base classes remains different from interface implementation since classes can implement various interfaces but inherit from just one abstract class. Interfaces should be chosen whenever various unrelated classes need to execute the same set of operations, yet abstract classes function better at establishing base functionality among related classes.

Java code illustrates the distinction between an interface and an abstract class as shown below:

// Defining an interface
interface Animal {
   void makeSound(); // Method without implementation
}
// Implementing the interface in a class
class Dog implements Animal {
   public void makeSound() {
       System.out.println("Bark!");
   }
}
// Abstract class example
abstract class Mammal {
   abstract void sleep(); // Abstract method
   void breathe() { // Concrete method
       System.out.println("Breathing...");
   }
}
// Subclass extending abstract class
class Human extends Mammal {
   void sleep() {
       System.out.println("Sleeping...");
   }
}

The Animal interface compels Dog to implement makeSound(), yet the Mammal abstract class allows Dog subclasses, such as Human, to define sleep(). Interfaces enhance system flexibility, but abstract classes combine both specification requirements and common methods for subclasses.

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

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