Working with functional interfaces and lambda expressions

We had to declare an interface and two classes to make it possible for a method to receive an instance of Testable and execute the test method implemented by each class. Luckily, Java 8 introduced functional interfaces and Java 9 makes it easy for us to supply a compatible lambda expression whenever the code requires a functional interface. In a nutshell, we can write less code to achieve the same goal.

Note

A functional interface is an interface that meets the following condition: it has a single abstract method or a single method requirement. We can create instances of functional interfaces with lambda expressions, method references, or constructor references. We will work with different examples that will allow us to understand lambda expressions, method references, and constructor references and we will see them in action.

The IntPredicate functional interface represents a function with one argument of the int type that returns...