Mockito tutorial - https://www.tutorialspoint.com/mockito/index.htm
An example using Mockito for unit tests
An example of Chapter Mocking of the book
Test-Driven Development, Build Automation, Continuous Integration (with Java, Eclipse and friends) by Lorenzo Bettini.
See https://github.com/LorenzoBettini/tdd-buildautomation-ci-book-examples for the list of all the examples.