Method inside "When" actually being called - mockito

Question

When I am testing a class A (TestNG) which uses some methods from some other class Helper Class
I am mocking the helper class(Mockito) for testing class A.
but when(helper.methodUsedByClassA(value)).thenReturn(new HashMap>())
this line of code is actually calling helper.methodUsedByClassA and a null pointer exception is thrown (because of data I am using for testing is not valid)
why is this happening? Why would the method name inside mockito "when" actually be called ?
class ATest{
#Mock
private Helper helper;
private A target;
#BeforeTest
public void setUp() {
MockitoAnnotations.initMocks(this);
this.target = new UpdateDigicatKdpAsinUtil(helper);
}
#Test(dataProvider = "data")
public void testMethod(List<String> value) {
when(helper.methodUsedByClassA(value)).thenReturn(new HashMap<String, List<String>>({{put("test", new ArrayList<>())}}));
}
#DataProvider
public Object[][] data(){
List<String> list = new ArrayList<>();
return new Object[][]
{
{list}
}
}
class Helper{
public Map<String, List<String>> methoUsedByClassA(int value) {
//This method is being executed because it is mentioned inside "when"
}
}

Answer 1

I guess you get an NPE because helper is NULL.
The problem is that the #Mock annotation isn't processed and because of that, the variable helper isn't initialized.
You have to annotate the test class with #RunWith(MockitoJUnitRunner.class). MockitoJUnitRunner will process the #Mock annotation and create the mock.