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Exploring Interface in Java with Examples

Are you ready to embark on an exhilarating journey into the dynamic realm of Interface-Based Programming in Java? If you’re a Java enthusiast seeking to elevate your coding skills and harness the full potential of this language, then you’re in for a treat. In this enlightening blog, we’ll delve into the fascinating world of Java interfaces, unveiling their key role in achieving abstraction, promoting code flexibility, and propelling your applications to new heights.


What are interfaces in Java?

In Java, an interface is a powerful construct that defines a a set of methods or contracts that a class must implement. It is like a blueprint that lays down the rules and requirements for classes that want to adhere to it. Interfaces allow you to achieve abstraction, separating the “what” from the “how” in your code.


Syntax and declaration of interfaces in Java

Defining an interface in Java is straightforward and follows this syntax:

access_modifier interface InterfaceName {
    // Constant declarations (optional)
    // Method signatures (without implementation)

An example of a simple interface is:

public interface Animal {
    void makeSound();
    void eat();

Role of Java interfaces in achieving abstraction

One of the key roles of java interfaces is to facilitate abstraction, which is the process of hiding the implementation details from the user and exposing only the essential features. By defining a java interface, you ensure that classes that implement it will provide specific functionality, but you don’t need to know how each class achieves that functionality internally. This allows for a more modular and maintainable codebase.


Advantages of using Java interfaces

Utilizing interfaces in Java comes with several benefits that contribute to better code design and flexibility:

  • Multiple Inheritance: Unlike classes, which support single inheritance, a class can implement multiple interfaces. This feature allows you to group related behaviors from various java interfaces into a single class.
  • Code Reusability: Java Interfaces promote code reuse by enforcing a standardized structure across classes. Different classes can implement the same interface, ensuring consistent behavior across the application.
  • Flexibility: Java Interfaces enable loose coupling, which means that classes can interact with each other without knowing each other’s intricate details. This enhances flexibility and makes it easier to extend and modify the codebase.
  • Support for Callbacks: Java Interfaces are instrumental in implementing callbacks, where a class can invoke methods defined in another class without having a direct reference to it.
  • Mocking and Testing: Java Interfaces are invaluable for testing and mocking. You can create mock implementations of interfaces to simulate behavior during testing.

Learn about Hierarchical Inheritance in Java


How to declare an interface in Java? 

Declaring an interface in Java is a breeze. Simply follow these steps:

  • Step 1: Define the interface using the interface keyword, followed by the interface name.
  • Step 2: Declare the methods without providing an implementation (no curly braces).
  • Step 3: Optionally, you can define constants, which are implicitly public, static, and final.

Let’s take a look at a more detailed example:

public interface Shape {
    double PI = 3.14159; // Constant
    double calculateArea(); // Method 1 signature
    double calculatePerimeter(); // Method 2 signature

In this example, we define the Shape interface in java with two method signatures (calculateArea and calculatePerimeter) and a constant PI.


Methods of Implementing a Java Interface

In Java, there are two primary ways to implement an interface in Java:

  • Class Implements Interface: A class can implement an interface by using the implements keyword in its class declaration. The class must provide concrete implementations for all the methods defined in the interface.
  • Anonymous Class Implementation: If a class needs to implement an interface for a one-time use or in a limited scope, it can be done using an anonymous class. This involves creating an instance of the interface and providing the method implementations within curly braces.

Check out: Java Fundamentals – classes and objects in java


Avoiding common pitfalls with interfaces in Java

While Java interfaces are a powerful tool, developers may encounter some common mistakes and misconceptions. Here are some tips to avoid these pitfalls:

  • Excessive Use: Don’t create interfaces for every class just for the sake of it. Interfaces in java should be meaningful and have a clear application design purpose.
  • Large Interfaces: Avoid defining interfaces with too many methods. Large interfaces defeat the purpose of abstraction and might lead to bloated implementations.
  • Changing Existing Interfaces: Be cautious when modifying existing interfaces, as it may break the implemented classes. Instead, consider creating new interfaces to accommodate changes.
  • Choosing Descriptive Names: Make sure to choose meaningful names for your interfaces that clearly reflect their purpose and functionality.
  • Method Overloads: Be cautious when using method overloads with interfaces, as it may lead to confusion and unexpected behaviors.

Check out: Java Interview questions


Enhancing code flexibility with interfaces in Java

One of the most exciting aspects of interfaces is their ability to improve code flexibility and extensibility. Here are a few ways to enhance code flexibility:

  • Dependency Injection: Interfaces in java play a crucial role in dependency injection, allowing you to decouple classes and their dependencies. This makes it easier to change implementations without affecting the rest of the codebase.
  • Plug-and-Play Architecture: With java interfaces, you can build a plug-and-play architecture, where different implementations can be easily swapped in and out without modifying existing code.
  • Versioning and Upgrades: When you need to introduce new features or make changes to existing functionality, interfaces in java help manage versioning and upgrades more effectively.
  • Third-Party Integration: When integrating with third-party libraries or APIs, interfaces in java provide a standardized way to interact with them, shielding your code from changes in the external components.


Difference Between Class and Interface in Java

Below are the differences between class and interface in java:

Aspect Classes in Java Interface in Java
Instantiation Objects can be instantiated from classes Interfaces cannot be instantiated
Inheritance Supports single inheritance Supports multiple inheritance
Abstract Methods Can have abstract and concrete methods Can only have abstract method signatures
Variables Can have instance variables and methods Can only have constants (final variables)
Role in Abstraction Provides concrete implementation Defines contract for classes to follow


Best practices for using Java interfaces effectively

To make the most out of interfaces in your Java projects, consider these best practices:

  • Interface Segregation: Aim to keep interfaces focused on a specific set of related methods rather than creating monolithic interfaces with numerous unrelated methods.
  • Naming Conventions: Use clear and descriptive names for interfaces that indicate their purpose.
  • Small and Cohesive: Keep your interfaces in java small and cohesive, with a clear and well-defined purpose.
  • Choose Interfaces over Abstract Classes: In general, prefer interfaces over abstract classes when defining contracts that multiple unrelated classes can implement.
  • Documentation: Always provide documentation for your interfaces in java, explaining their intended usage and behavior.



Congratulations! You’ve explored the wonderful world of Interface-Based Programming in Java. We’ve covered the syntax and declaration of interfaces, their role in achieving abstraction, and their advantages, such as code reusability and flexibility. We also provided best practices and real-world examples to demonstrate how interfaces can improve the design of your Java applications.

Check out Hero Vired’s data-science-and-analytics-course-certification course an master your career in java.





No, interfaces can only have constants (final variables) but not instance variables.
Yes, Java supports multiple inheritance through interfaces. A class can implement multiple interfaces to inherit behaviors from each one of them.
Abstract classes can have both abstract and concrete methods, as well as instance variables. On the other hand, interfaces can only have abstract method signatures and constants (final variables). Classes can inherit from only one abstract class but can implement multiple interfaces.
While not mandatory, interfaces are a powerful tool in Java that promotes code reusability, flexibility, and abstraction. Using interfaces can greatly enhance the design and maintainability of your Java applications.

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