Before moving on, we need to learn about a feature in Java called Lambda. Many of you may already know about it. However, since the feature was only introduced in version 8, it is better to get familiar with it if you aren't already. It lets you pass a block of code, called a lambda expression, as an argument to another function. To talk about lambda, we must first see what a functional interface is.
A functional interface is an interface that has only one unimplemented method, that is to say, a class that implements it needs to implement exactly one method. The functional interface may have more than one method declared or inherited, but as long as we can implement it by implementing exactly one method, it is a functional interface. The following example shows one such interface:
@FunctionalInterface
public interface SampleFunctionalInterface {
int modify(int x);
}Note that we also marked it as a functional interface with an annotation, but...