Mockito is a mocking framework, JAVA-based library that is used for effective unit testing of JAVA applications. Mockito is used to mock interfaces so that a dummy functionality can be added to a mock interface that can be used in unit testing. This tutorial should help you learn how to create unit tests with Mockito as well as how to use its APIs in a simple and intuitive way.
https://www.tutorialspoint.com/mockito/
https://caster.io/lessons/mockito-introduction-and-integration/