In Java, a functional interface is an interface that has only one abstract method. It is a key concept in functional programming and serves as the foundation for working with lambda expressions and method references. Java 8 introduced functional interfaces to support the new features of the Java language, such as streams and lambdas. Functional interfaces are annotated with the `@FunctionalInterface` annotation, which is optional but recommended. This annotation ensures that the interface has only one abstract method and generates a compile-time error if multiple abstract methods are declared. Although a functional interface can have default methods and static methods, it must have only one abstract method. The single abstract method in a functional interface represents the behavior that can be implemented by lambda expressions or method references. Lambda expressions allow you to define anonymous functions concisely. They provide a way to pass behavior as an argument to methods, ...