In this recipe, you will learn what a functional interface is and why it was added to Java, along with 43 ready-to-use functional interfaces of the standard Java library that comes with JDK 8 in the java.util.function package.
Without functional interfaces, the only way to pass a functionality into a method would be through writing a class, creating its object, and then passing it as a parameter. But even the least involved style—using an anonymous class—requires writing too much code. Using functional interfaces helps to avoid all of that.
