How do you verify the behavior of a mock object in Mockito?

Question

Accepted Answer

verify() is Mockito's mechanism for behavior verification - confirming that specific methods were called on a mock, how many times, and with what arguments.

Key Points:
• Basic usage: verify(mock).someMethod(args) checks the call happened exactly once with those arguments.
• Call count can be tightened with verify(mock, times(n)), never(), atLeastOnce(), or atMost(n).
• ArgumentMatchers like any() or eq() can loosen or refine which arguments are expected.
• verifyNoInteractions() and verifyNoMoreInteractions() confirm a mock was never touched, or that nothing beyond what you verified occurred.
• Verification order across multiple mocks can be checked using InOrder.

Example:
For a NotificationService that should email a user exactly once after checkout, you'd write verify(mailSender, times(1)).send(eq("user@example.com"), any(String.class)).

Code Example:
```
@Test
void sendsWelcomeEmailOnce() {
    MailSender mailSender = mock(MailSender.class);
    UserService service = new UserService(mailSender);

service.register("user@example.com");

verify(mailSender, times(1)).send(eq("user@example.com"), anyString());
}
```

Interview Tip:
A concise interview answer is:

"I use verify() to assert that a mock was interacted with the way I expect - a specific method, call count, and arguments - using helpers like times(), never(), and argument matchers. It's how I confirm side-effecting behavior, like sending an email, actually happened without a return value to check."