Suppose we have a class that we want to mock.

class Apple {
 Color color
 bool isFresh
}

We mock it with Mock. We execute the code we need to test. Mocked Apple object gets its Color color updated. However, the field stays null.

// test class
Apple apple = Mock(Apple.class)

myService.myMethod(apple)

assert apple.color == Color.RED //FAIL because apple.color == null 

// tested code of myMethod(Apple apple) 
apple.color = Color.RED