Mocking a method which returns Page interface

You can use a Mock reponse or an actual response and then use when, e.g.:

Page<Company> companies = Mockito.mock(Page.class);
Mockito.when(companyRepository.findAllByIsActiveTrue(pageable)).thenReturn(companies);

Or, just instantiate the class:

List<Company> companies = new ArrayList<>();
Page<Company> pagedResponse = new PageImpl(companies);
Mockito.when(companyRepository.findAllByIsActiveTrue(pagedResponse)).thenReturn(pagedResponse);