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ArgumentCaptor is a class provided by Mockito and used to verify the arguments passed to a mock method. It should be used for verification and not for stubbing.

Normally with matchers you can only verify that a method has been called with a particular object type or a specific object. ArgumentCaptor is more flexible and allows you to check each specific member of the object passed.

Here's a short example:

    ArgumentCaptor<Student> arg = ArgumentCaptor.forClass(Student.class);
    verify(mock).execute(arg.capture());
    assertEquals("Ale", arg.getValue().getName());

Let's now analize a booking system:

	import java.util.Date;

	/**
	 * Contains all the information of an appointment
	 */
	public class Appointment {

		private Date date;
		private String event;

		public Appointment(Date date, String event) {
			this.date = date;
			this.event = event;
		}
	
		public Date getDate() {
			return date;
		}
	
		public String getEvent() {
			return event;
		}
	}

	import java.util.ArrayList;
	import java.util.List;

	/**
	 * Calendar used to set appointments
	 */
	public class Calendar {

		private List<Appointment> appointments = new ArrayList<>();
	
		public void addAppointment(Appointment appointment) {
			appointments.add(appointment);
		}
	
		// Other methods
	}

	import java.util.Date;

	/**
	 * Manage appointments on a calendar
	 */
	public class BookingSystem {

		private Calendar calendar;
	
		public BookingSystem(Calendar calendar) {
			this.calendar = calendar;
		}
	
		public void book(Date date, String event) {

			Appointment appointment = new Appointment(date, event);
		
			calendar.addAppointment(appointment);
		}
	}

Here's the test, using ArgumentCaptor:

	import org.junit.Test;
	import org.junit.runner.RunWith;
	import org.mockito.ArgumentCaptor;
	import org.mockito.runners.MockitoJUnitRunner;
	import static org.mockito.Mockito.mock;
	import static org.mockito.Mockito.verify;
	import java.util.Date;
	import static org.junit.Assert.assertEquals;

	/**
	 * Test to demonstrate the use of ArgumentCaptor
	 */
	@RunWith(MockitoJUnitRunner.class)
	public class BookingSystemTest {

		@Test
		public void myTest() {
	
			Calendar calendar = mock(Calendar.class);
			BookingSystem bookingSystem = new BookingSystem(calendar);
		
			// Initialize the ArgumentCaptor
			ArgumentCaptor<Appointment> argCaptor = ArgumentCaptor.forClass(Appointment.class);
		
			// Initialize date and event
			Date currentDate = new Date();
			String event = "Do this test!";
		
			// Add the appointment to the system
			bookingSystem.book(currentDate, event);
		
			// Verify that the appointment has been added
			verify(calendar).addAppointment(argCaptor.capture());

			// Verify that it has been added with the correct values		
			assertEquals(currentDate, argCaptor.getValue().getDate());		
			assertEquals(event, argCaptor.getValue().getEvent());
		}
	}

As you can see, I can now make sure that date and event are as expected.