Mockito How to mock repository findById thenReturn() Optional?

Try to mock a repository findById method, but no idea use thenReturn() to return an object, as it accepts an Optional?

P.S Tested in Spring Boot 2 environment

import static org.mockito.Mockito.*;
import org.springframework.boot.test.mock.mockito.MockBean;
	@MockBean
	private BookRepository mockRepository;
	@Before
    public void init() {
        Book book = new Book(1L, "A Book");
		//error, can't resolve method thenReturn(book)?
		when(mockRepository.findById(1L)).thenReturn(book); 

Solution

Try Optional.of()

	@MockBean
	private BookRepository mockRepository;
	 @Before
    public void init() {
        Book book = new Book(1L, "A Book");
		when(mockRepository.findById(1L)).thenReturn(Optional.of(book));