A mock object is a dummy implementation for an interface or class in which you specify the output of certain method calls. In other words, it’s a fake object used for testing. For example, say you’re testing a class that relies on another class to perform some action. Rather than using the real implementation of the second class, you would instead use a mock object that simulates the behavior of the second class. This allows you to test the first class in isolation without having to worry about the behavior of the second class. Mock objects are also often used to mimic the behavior of expensive or difficult-to-obtain objects, such as databases or network connections. By using a mock object in place of the real object, you can avoid the overhead of setting up and maintaining a database or establishing a network connection. Instead, you can focus on testing the functionality of your code that uses these objects. Moreover, Mockito is a popular Java library for creating mock objects.