How do you create a functional interface in Java?

 Tale of Contents

Introduction

A functional interface in Java is an interface that contains exactly one abstract method. This concept is central to functional programming in Java, especially with the introduction of lambda expressions in Java 8. Functional interfaces can be used to represent single behavior or functionality, making them a powerful tool for writing concise and expressive code.

Steps to Create a Functional Interface

1. Define the Interface

To create a functional interface, simply declare an interface with one abstract method. You can include any number of default or static methods, but only one abstract method is allowed.

Example:

2. Use the @FunctionalInterface Annotation

Although it's not mandatory, it's a good practice to use the @FunctionalInterface annotation. This annotation indicates that the interface is intended to be a functional interface. The compiler will generate an error if the interface does not meet the requirements.

Example:

3. Implement the Functional Interface with Lambda Expressions

Once you have defined a functional interface, you can implement it using lambda expressions or method references.

Example using Lambda Expression:

4. Using Method References

You can also implement functional interfaces using method references, which provide a shorthand way to call methods.

Example:

Practical Example: Custom Functional Interface

Defining a Functional Interface

Let's create a functional interface that takes two integers and returns their sum.

Implementing the Functional Interface

Now, we can use a lambda expression to implement this interface.

Conclusion

Creating a functional interface in Java is straightforward. By defining an interface with a single abstract method and optionally using the @FunctionalInterface annotation, you can leverage lambda expressions and method references to implement it efficiently. This approach enables you to write more concise, readable, and maintainable code, aligning with functional programming principles in Java.