Interface in Java: A Complete Guide

The Java programming language is widely used across industries like the Internet of Things (IoT), Big Data, Blockchain and Artificial Intelligence, as it has always kept up with the market trends. While there are a bunch of things that make this programming language a developer favourite, an Interface in Java is one of its core concepts. It is a behavioural feature that helps describe the behaviour Classes that need to be implemented.

According to Oracle, Java is the number one programming language for today’s tech trends. The concept of Interface is a major contributing factor as it helps achieve multiple inheritances and abstraction. An Interface in Java specifies the Class's behaviour by providing an abstract type. Keep reading to understand more about how Interfaces work in Java.

Table of Contents

1) What is an Interface in Java?

2) What are the benefits of Interfaces?

3) How are Interfaces declared?

4) Nesting Interface in Java

5) What are examples of Interfaces?

6) Interface vs Class

7) Conclusion

What is an Interface in Java?

Object-Oriented Programming (OOP) involves a framework within which the Objects use Methods to establish their Interactions with the external world. These Methods shape the Object’s interaction with the external world. An Interface in Java is a collection of correlated Methods with empty bodies and a tool to achieve Abstraction. The Interface represents a Class in which the Constants are Static, and the Methods are Abstract.

An Interface in Java represents an ‘IS-A’ relationship. Similar to how an Abstract Class cannot be initiated, the Java Interface can also not be initiated. The reason behind an Interface being Abstract is because it only specifies what a Class should do and not how it should do it.

Syntax example of an Interface:
 

As shown in the example above, the keyword ‘Interface’ must be used in order to enable complete Abstraction. As a result, all existing Interface Methods are declared with an empty body, and all fields are Public, Static and Final by Default. Any Class implementing an Interface should also implement all existing Methods declared within the Interface using the keyword “Implements”.

The process of writing an Interface in Java is very similar to that of writing a Class. However, a Class outlines the Object’s behaviours and attributes, whereas an Interface has behaviours which a Class implements.


 

What are the benefits of Interfaces in Java?

Interfaces in Java became capable of implementation in the release of Java 8, with ‘Static’ and ‘Default’ Methods being implemented in the ‘Interface’ definition. These updates in Java 8 were followed by the addition of ‘Private’ and ‘Private Static’ Methods. Therefore, an Interface in Java is capable of containing six various types of Methods.

Furthermore, implementing Interfaces in Java involves benefits such as Polymorphism, Abstraction and multiple Inheritances. Interfaces provide the following benefits:

1) Complete Abstraction: Complete Abstraction is a concept crucial to Object-Oriented Programming Methods. The primary benefits of Abstraction are easy implementation, simpler changes and smoother documentation. Hence, design becomes simpler, along with improved indexing and optimisation. Additionally, hiding a Method’s implementation from the end user eliminates unnecessary confusion.

2) Multiple Inheritance: Java Casses are capable of single Inheritance. This is where Interfaces come to the rescue by implementing multiple Inheritances, where a Class inherits the features of multiple Classes. The inherited Class has Fields and Methods which can be reused through Inheritance.

3) Loose Coupling: As the terms suggest, Loose Coupling in Java is where the Classes are independent of one another. There is lower interdependency and information flow between Classes in Loose Coupling. The Class becomes dependent on the Interface rather than the other Class. Moreover, dependencies between Classes must be passed externally during runtime rather than being hardcoded.

4) Polymorphism: Polymorphism is one of the basic features of any Object-Oriented Programming language. It is a concept where variables of different types can be used at different times. An example to substantiate this concept would be where a ‘Number’ Interface can take multiple forms like ‘RealNumber’, ‘WholeNumber,’ or ‘NaturalNumber’.

5) Code Semantics and Readability: Java Interfaces establish a declaration about the intentions of Methods and keep the concepts of Interface and Implementation separate.

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How are Interfaces declared?

Interfaces in Java are declared using the keyword ‘Interface’, which enables complete Abstraction. The definition of an Interface in Java has two parts: the Interface's body and the Interface's declaration. The many attributes of the Interface, like its name and information about whether it extends other interfaces, are all contained within the declaration. Furthermore, the constant and method declaration for that particular Interface is contained within the interface body.