Mock method to return a value only when passed any *non-null* object of a given class

Question

I'm using Mockito to mock a method to return a Date when given a Date.

when(supplier.calculateDeliveryDate(any(Date.class)))
    .thenReturn(supplierDeliveryDate);

However, I only want it to return the supplierDeliveryDate when it is passed a non-null Date object. When passed null, it should return null.

Is this possible? How can I do it?

Answer 1

You could use an anonymous inner class:

// unit test
public OrderServiceTest {

    // instance of class-under-test
    private OrderService instance;
    // stub value
    private Date supplierDeliveryDate = new Date();

    // mock as an anonymous inner class
    private Supplier supplier = new Supplier() {
        public Date calculateDeliveryDate(Date input) {
            if (input == null) {
                return null;
            }
            else {
                return supplierDeliveryDate;
            } 
        }
    };

    @Before
    public void setUp() {
        instance = new OrderService();

        // dependency injection
        instance.setSupplier(supplier);
    }

    @Test
    public void testOrderHappy() {
        // SETUP
        Date orderDate = new Date();            

        // CALL
        Date result = instance.order(orderDate);

        // VERIFY
        assertTrue(supplierDeliveryDate == result);
    }

    @Test
    public void testOrderNull() {
        // SETUP
        Date orderDate = null;

        // CALL
        Date result = instance.order(orderDate);

        // VERIFY
        assertNull(result);
    }
}

But you should really wonder why you need this kind of behavior.

If you write a well defined test case then you should know exactly how often, and with which arguments, your mock is called. If so, then you can just stub the expected calls instead of wiring your mock with conditional behavior.

Note that is is useful if your test is as 'sharp' as possible. If a different number of calls hits your mock than expected, or with different arguments, then the test should fail.