Unit test of a function calling 2 tested functions inside the same class

I have the following skeleton of code in a JAVA class called "TestClass.Java":

public String functionA ()
{
    If function B() == true)
    {
    String testVariable = function C();
    String test2 = testVariable +"Here a test";
 }
 else
 {
  ...
 }
}

I need to apply unit tests for this function functionA() where tests had been applied on functionB() and functionC(): I did below:

private TestClass mockTestClass ;
@Test
    public void testFunctionA() 
{
        mockTestClass = Mockito.mock(TestClass.class);
        private MockComponentWorker mockito;
  Mockito.when(mockTestClass.functionB()).thenReturn(true);//already test is done;
  Mockito.when(mockTestClass.functionC()).thenReturn("test"); //already test is done;
  mockito = mockitoContainer.getMockWorker();                   
        mockito.addMock(TestClass.class,mockTestClass);
        mockito.init();
assertEquals("PAssed!", "test Here a test", mockTestClass.functionA());
    }

When I ran my test, I got: NULL in mockTestClass.functionA(). Can you please help? How to test this function?

thanks,

Rosy