Class vs Interface: Difference and Comparison

Both Class and Interface define what an object looks like and also the behaviour of an object. If we study and understand both structures thoroughly, it will be easier to deal with the programming.

They are the two basic structures in an object-oriented programming language. Also, a better understanding will enhance our coding quality and the developer’s experience.

Key Takeaways

1. Classes define the implementation details of an object, while interfaces only specify the methods an object must implement.
2. A class can inherit from multiple interfaces but only one parent class.
3. Interfaces emphasize code reusability and abstraction, while classes focus on the concrete implementation.

Class vs Interface

Class does not support multiple inheritance but is supported by inheritance. To declare a class, you can use the keyword “Class”, however, in order to declare an interface, a keyword called “interface” is used. Class can be instantiated but interface cant. Class contains instructors, and the interface doesn’t.

Class is a basic structure in a program from which objects are created. The class itself belongs to certain packages that contain an identical group of bundles packed together.

The declaration of a class contains keywords, modifiers, and a class name.
A class can be further classified into two categories:
i. Built-in-classes
ii. User-defined classes

The idea of the interface was introduced for the implementation of multiple classes at a time.

It contains the method declaration of a program. Not only abstract methods but an interface can also contain static methods.

They are always declared as public by default.

Comparison Table

Parameters of Comparison	Class	Interface
Definition	A class basically describes and defines the behavior of an object	Interface basically contains the attributes and behavior defined by a class
Keyword	The keyword “class” is used to declare a class	The keyword “interface” is used to declare an interface
Methods	A class is allowed to have both abstract and concrete methods. It means the method can have a body	An interface can have only abstract methods. It means the method can not have a body
Access specifier	The members in a class can be either public, protected, or default	All the members of an interface, by default, are public
Inheritance	A class does not support multiple inheritances. It uses the keyword extends to inherit another class	An interface does support multiple inheritances. It can however inherit only an interface

What is Class?

A class is a blueprint or a plan from which objects are created. It is a group of objects having similar properties. It is a logical entity and can not be physical. A class name conventionally starts with a capital letter.

It is the basic concept of object-oriented programming. Determines the behaviour of an object. It can have any number of methods and can access its values.

A class can contain the following type of variables:
A local variable, instance variable, and class variable.

The syntax of a class is as follows:

class {
field;
method;
}

To create an object under a class, we need to use the keyword known as ‘new’, along with a constructor. A constructor has the same name as that of a class. Its function is similar to that of a method.

For example, if the class name is Program, the constructor is Program(). To access the members of the class, we need to use the name of the object along with the class.

What is Interface?

The interface is defined as a group of similar methods but with empty bodies.

When we implement an interface, it makes a class more proper. This, in turn, makes the behaviour of the class more formal. It is a contract enforced by the compiler which connects the class to the outside world.

Before a class gets compiled successfully, the methods defined by the interface must appear in the source code. To compile a class, we need to add the public keyword at the beginning. There can be no private variables and methods.

It comprises abstract methods as well as static constants. An interface can not contain any method body. It is used for achieving multiple inheritances in a program.

Default and static methods can be introduced in an interface. Also, we can have private methods in an interface. It is used for achieving abstraction.

Main Differences Between Class and Interface

1. In a class, the keyword named class is used to declare it. In an interface, a keyword named interface is used to declare it.
2. The members of a class don’t need to be public. It can be public, private, as well as protected. In contrast, the members of an interface are public by default.
3. A class can have a method body. It contains concrete methods. The interface can not have a method with bodies. They contain abstract methods.
4. A class is allowed to extend only a single class but as many interfaces are required. An interface can not implement interfaces itself but is allowed to extend any number of classes. This, in turn, is an advantage of multiple-class implementation.
5. Classes in a program might have constructors. They are a block of coding structures in a program, quite similar to methods. The interface does not have any constructors.

References

1. https://ieeexplore.ieee.org/abstract/document/6141290/
2. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0009813

Last Updated : 30 July, 2023