What is a functional interface in Java? | #SpringBoot #JakartaEE #JDK #JVM #softwaredevelopment

A functional interface in Java is a type of interface that contains exactly one abstract method. This single abstract method is what makes the interface "functional," as it defines the contract that can be implemented with a Lambda expression or method reference.

Features of a Functional Interface:
Single Abstract Method: The defining feature of a functional interface is that it has only one abstract method. This method is what a Lambda expression will implement.

Default and Static Methods: A functional interface can have multiple default and static methods. These methods provide additional functionality but don't affect the "functional" aspect of the interface since they aren't abstract.

Inheritance from Object Class: Functional interfaces can inherit methods from the Object class, such as toString(), equals(), or hashCode(). However, these don't count as the abstract method for the interface.

@FunctionalInterface Annotation: Java provides the @FunctionalInterface annotation that can be applied to an interface. This is optional but highly recommended, as it instructs the compiler to ensure the interface adheres to the functional interface rules—specifically, that it has only one abstract method.

How They Work with Lambda Expressions:
Lambda expressions in Java are essentially a shorthand way of implementing the abstract method of a functional interface. When you use a Lambda expression, it gets mapped to the single abstract method of the functional interface. This makes your code more concise and easier to read compared to traditional anonymous class implementations.

For example, if you have a functional interface that represents a task to be performed, you can use a Lambda expression to define that task's behavior in a clear and succinct manner, without needing to write a full-fledged class or method implementation.

A functional interface has one abstract method that defines the contract for a Lambda expression.
It can include default and static methods, and inherit from the Object class.
The @FunctionalInterface annotation is a useful tool to ensure your interface is correctly defined.
Lambda expressions in Java are tied to functional interfaces and provide a simple way to implement their abstract methods.
This combination of features makes functional interfaces and Lambdas a powerful and flexible tool in Java, particularly for simplifying code and making it more readable.

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[dudepty]

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