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Interface in Java: A Complete Guide
Sienna Roberts 22 March 2023Explore the intricacies of Interfaces in Java through our comprehensive guide. From fundamental concepts to advanced techniques, this blog will cover everything you need to know to effectively use Interface in Java. Whether you're a beginner or an experienced developer, dive into this blog to elevate your Java programming skills!
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Table of Contents
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:
| Interface { // declare constant fields // declare methods that abstract // by default. } |
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.
The syntax to declare an Interface is:
|
public interface // declare constant fields // declare methods that abstract // by default. } |
In the example above, the keyword ‘Public’ makes the Interface Public. This access specifier shows that the Interface is usable by another Class in any package. However, if the access specifier is not declared, the Interface access will be limited to only the Classes defined within the same package as the Interface.
The fields declared within the Interface body above are all Public, Static and Final by Default. Additionally, the Class must implement all the Methods declared in the Interface in the interaction.
Nesting Interface in Java
The reason for applying the Nesting Interface method is the fact that the namespace conflicts are resolved by grouping all used interfaces or maybe by keeping related interfaces along with one class.
In the following illustration, interface embedding is the approach adopted. In this example, we print the prime numbers list using interface nesting.
|
//Interface Nesting //Find First 10 Prime numbers package InnerInterface; public interface InterfaceOuter { void display(); interface InterfaceInner { void InnerMethod(); } } //class package InnerInterface; import InnerInterface.InterfaceOuter.InterfaceInner; public class NestedInterface implements InterfaceInner { public void InnerMethod() { int iteration = 0, num = 0, x = 1, y = 1; while (num < 10) { y = 1; iteration = 0; while (y <= x) { if (x % y == 0) iteration++; y++; } if (iteration == 2) { System.out.printf("%d ", x); num++; } x++; } } public static void main(String args[]) { NestedInterface obj = new NestedInterface(); obj.InnerMethod(); } } Expected Output: 2 3 5 7 11 13 17 19 23 29 |
What are examples of Interfaces in Java?
Let’s take a look at some examples of Interfaces in Java:
|
Interface printable{ void print(); } class A6 implements printable{ public void print(){System.out.println("Hello");} public static void main (String args[]){ A6 obj = new A6(); obj.print(); } } |
In the Interface example code above, the Interface name is ‘printable’, followed by the Interface Implementation by Class A6. The program is designed to print the word ‘Hello’. It is to be noted that a Class can extend a Class and implement an Interface, and an Interface can extend another Interface.
Example #2 of an Interface in Java:
|
Interface A{ |
The above example declares two Interfaces, A and B, which are implemented by Class C, which contains Abstract members of both A and B. This is the use-case where multiple Interfaces are implemented by one Class using the ‘Implements’ keyword.
Example #3 of an Interface in Java:
|
Interface Number{ // members of Interface } Interface Sum extends Number { // members of Sum Interfac // members of Number Interface }
|
The above example shows the Interface ‘Sum’ extending the Interface ‘Number’. This is the use-case where one Interface extends another Interface through the ‘Extends’ keyword.
Example #4 of an Interface in Java:
|
Interface A{ // ….. } Interface B{ // ….. } Interface C extends A, B{ //...... } |
The above example shows how multiple Interfaces can be extended by one Interface, similar to how a Class can extend another Class. Although unlike a Class, an Interface can extend any number of other Interfaces. The declaration of this extension would simply include a list separated by commas of every Interface being extended.
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Interface vs Class
Even though a class and an Interface are structurally alike, they are fundamentally distinct. The following are some of the significant differences between them:
|
Feature |
Interface |
Class |
|
Inheritance |
Can extend multiple interfaces |
Can extend only one class |
|
Implementation |
Cannot contain method implementations |
Can contain method implementations |
|
Access modifiers |
All methods are implicitly public |
Methods can have various access modifiers |
|
Constructors |
Cannot have constructors |
Can have constructors |
|
Instantiation |
Cannot be instantiated directly |
Can be instantiated |
|
Object type |
Reference type for objects |
Reference type for objects |
|
Multiple inheritance |
Supports multiple inheritance through interfaces |
Does not support multiple inheritance |
|
Extending |
Extended using the 'extends' keyword |
Extended using the 'extends' keyword |
|
Keyword |
Uses the 'interface' keyword |
Uses the 'class' keyword |
|
Usage |
Used to define contracts for classes |
Used to define blueprints for objects |
|
Interface methods |
All methods are implicitly abstract |
Methods can be abstract or concrete |
Conclusion
Interface in Java is an essential tool to enable complete Abstraction in the code. They come into play by expanding on code capabilities by allowing Classes to be independent of one another. As a result, this decreases the inter-dependencies between the Classes too. The prime advantages of having Interfaces are to achieve multiple Inheritance and Polymorphism. Since the launch of Java 9, Static, Private, and Private Static Methods can also be contained within Interfaces.
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Frequently Asked Questions
What are benefits of Java?
Java offers platform independence, robustness, security, and scalability. With automatic memory management, rich standard library, and strong OOP support, it enables efficient development of diverse applications. Its active community and backward compatibility further enhance its appeal for developers across various domains.
What are four pillars of Java?
Platform independence, Object-Oriented Programming, Robustness and Security.
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