Functional Interfaces In Java

 In Java, a functional interface is an interface that has only one abstract method. These interfaces are used to represent functions, and they can be implemented using lambda expressions or method references.

Functional interfaces are useful because they allow you to use anonymous inner classes to implement them, which can make your code more concise and easier to read. To create a functional interface, you simply define an interface with a single abstract method and annotate it with the @FunctionalInterface annotation. For example:

@FunctionalInterface

public interface MyFunctionalInterface {

  void doSomething();

}

You can then implement this interface using a lambda expression like this:

MyFunctionalInterface myFunction = () -> {
    // code goes here
};

Or using a method reference like this: 

MyFunctionalInterface myFunction = MyClass::myMethod;

Functional interfaces are a key part of the Java Streams API, which allows you to perform functional-style operations on streams of data. They are also used in the JavaFX event handling system, among other things.

It's important to note that while a functional interface can only have one abstract method, it can have multiple default methods (methods with a default implementation). These default methods are not considered abstract, so they do not count towards the single abstract method limit. For example:

@FunctionalInterface
public interface MyFunctionalInterface {
    void doSomething();
    default void doSomethingElse() {
        // default implementation
    }
}

Functional interfaces are a useful feature of Java 8 and later versions that allow you to use lambda expressions and method references to implement interfaces with a single abstract method in a more concise and readable way.