Mocking File.listFiles using Mockito3.3.3 and Junit 5.6 - mockito

Question

Method for testing
public int check() {
File f =new File("as.txt");
File[] arr = f.listFiles();
return arr.length;
}
Test class
#ExtendWith(MockitoExtension.class)
public class TestDataService {
#InjectMocks
private DataService dataService;
#Test
void chett() {
File[] arr = {new File("we.txt")};
File ff =Mockito.mock(File.class);
when(ff.listFiles()).thenReturn(arr);
assertEquals(1, dataService.check());
}
}
I am getting null pointer exception at File[] arr = f.listFiles();

Answer 1

You need to change the implementation of your check() method
public int check() {
File f = new File("C:\\");
File[] arr = f.listFiles();
return arr.length;
}
As per listFiles() documentations it should contain the directory, otherwise it will return you null.
If this abstract pathname does not denote a directory, then this method returns null. Otherwise, an array of File objects is returned, one for each file or directory in the directory. Pathnamesdenoting the directory itself and the directory's parent directory are not included in the result.
Then you can create your test class like this
#ExtendWith(MockitoExtension.class)
public class TestDataService {
#InjectMocks
private DataService dataService;
#Mock
private File f;
#Test
void chett() {
File[] arr = { new File("C://") };
when(f.listFiles()).thenReturn(arr);
assertEquals(1, dataService.check());
}
}
OR this
#ExtendWith(MockitoExtension.class)
public class TestDataService {
#InjectMocks
private DataService dataService;
#Mock
private File f;
#Test
void chett() {
File[] arr = { new File("C://") };
File ff = Mockito.mock(File.class);
when(ff.listFiles()).thenReturn(arr);
assertEquals(1, dataService.check());
}
}