Exemplo de Mocks - usando Mockito

Book.java

public class Book {
  private String titulo;
  public Book(String titulo) {
    this.titulo = titulo;
  }
  public String getTitulo() {
    return titulo;  
  }
}

BookSearch.java

import org.json.JSONObject;

public class BookSearch {
	
  BookService rbs;
   
  public BookSearch(BookService rbs) {
    this.rbs = rbs;
  }
   
  public Book getBook(int isbn) {
    String json = rbs.search(isbn);
    JSONObject obj = new JSONObject(json);
    String titulo = (String) obj.get("titulo");
    return new Book(titulo);
   }
}

BookSearchTest.java

import static org.junit.Assert.*;
import org.junit.*;
import org.mockito.Mockito;
import static org.mockito.Mockito.when;
import static org.mockito.Matchers.anyInt;

class BookConst {
  public static final String ESM = "{ \"titulo\": \"Eng Soft Moderna\" }";	
  public static final String NULLBOOK = "NULL";
}

public class BookSearchTest {
    
  BookService service;
	
  @Before
  public void init() {
    service = Mockito.mock(BookService.class);
    when(service.search(anyInt())).thenReturn(BookConst.NULLBOOK);
    when(service.search(1234)).thenReturn(BookConst.ESM);
  }
 
  @Test
  public void testGetBook() {
    BookSearch bs = new BookSearch(service);
    String titulo = bs.getBook(1234).getTitulo();
    assertEquals("Eng Soft Moderna", titulo);
  }
}

BookService.java

public interface BookService {
  String search(int isbn);
}