Question

How is an interface different from an abstract class, or any class?

Short answer

Answer: The major differences between interfaces and abstract classes are: 1. Interfaces are a collection of abstract methods with no implementation, while abstract classes can contain both abstract and non-abstract methods. 2. A class implementing an interface must provide implementations for all its methods, while a class extending an abstract class can choose which abstract methods to override. 3. A class can implement multiple interfaces but can only inherit from one abstract class, allowing multiple inheritance with interfaces and not with abstract classes. 4. Interfaces can only contain constant variables, while abstract classes can have both constant and non-constant variables. 5. Interfaces do not have constructors, while abstract classes can have constructors used by derived classes to initialize their state.

Step-by-step solution

Definition of Interface and Abstract Class

An interface is a collection of abstract methods (methods with no implementation) that any class can implement. It acts as a contract between the class and the methods it implements to provide the specified functionality. An abstract class, on the other hand, is a class that can contain both abstract methods and non-abstract methods (methods with implementation), and it cannot be directly instantiated but can be inherited.

Differences in Implementation

A class implementing an interface must provide an implementation for all the methods declared in the interface, while a class extending an abstract class can choose to provide an implementation for some, all, or none of the abstract methods, and it can inherit the non-abstract methods.

Multiple Inheritance

In most programming languages, a class can implement multiple interfaces but can only inherit from one abstract class. This means that interfaces allow multiple inheritance, while abstract classes do not.

Variables

Interfaces can only contain constant variables (in some languages called final variables), which means their value cannot be changed once they are assigned. Abstract classes can contain non-constant variables, in addition to constant variables.

Constructors

Interfaces cannot have constructors, as they cannot be instantiated. Abstract classes can have constructors, which can be used by derived classes to initialize their state. Here are some examples to better illustrate the differences:

Example of Interface

```java interface Drawable { void draw(); } class Circle implements Drawable { @Override public void draw() { //Implementation of draw method for Circle } } ```

Example of Abstract Class

```java abstract class Shape { abstract void draw(); void display() { //Implementation of display method } } class Rectangle extends Shape { @Override void draw() { //Implementation of draw method for Rectangle } } ``` These examples show the fundamental differences between interfaces and abstract classes, and how they are used in object-oriented programming.