What is the difference between Mockito's when().thenReturn() and @Spy? Mockito के when().thenReturn() और @Spy में क्या अंतर है?

when(mock.method()).thenReturn(value) stubs a method on a full mock object, where every method returns a default value (null, 0, false) unless explicitly stubbed — the mock has no real logic behind it at all.

@Spy wraps a real object instance, so unstubbed methods call through to the actual implementation while specific methods can still be selectively stubbed with doReturn().when(). Spies are useful when you want to test real behavior for most of a class but override just one method (e.g. an external call), though they should be used sparingly since partially-real objects can make tests harder to reason about.

@Mock private PaymentGateway paymentGateway;
when(paymentGateway.charge(100)).thenReturn(true);

@Spy private OrderCalculator calculator = new OrderCalculator();
doReturn(50.0).when(calculator).applyDiscount(anyDouble());