An example of stub, verify, and captor with Mockito (on TestNG)

Email.java

package com.pillartechnology.mail;

import java.util.Date;

public class Email {

	private String domain;
	private String user;	
	private String body;
	private Date timestamp;

	public String getDomain() {
		return domain;
	}
	public void setDomain(String domain) {
		this.domain = domain;
	}
	public String getUser() {
		return user;
	}
	public void setUser(String user) {
		this.user = user;
	}
	public String getBody() {
		return body;
	}
	public void setBody(String body) {
		this.body = body;
	}
	public Date getTimestamp() {
		return timestamp;
	}
	public void setTimestamp(Date timestamp) {
		this.timestamp = timestamp;
	}	
	
}

ExternalMailSystem.java

package com.pillartechnology.mail;

public interface ExternalMailSystem {

	public void send(String domain, String user, String body);
	
	public void send(Email email);
}

MailDeliverer.java

package com.pillartechnology.mail;

public class MailDeliverer {

	private ExternalMailSystem externalMailSystem;
	private Timestamper timestamper;
	
	public void deliver(String address, String body) {
		Email email = new Email();
		email.setBody(body);
		applyUserAndDomainUsingProvidedAddress(address, email);
		email.setTimestamp(timestamper.stamp());
		externalMailSystem.send(email);
	}

	private void applyUserAndDomainUsingProvidedAddress(String address, Email email) {
		//TODO - refactor this into the Email class (constructor? smart accessor?)
		String[] addressComponents = address.split("@");
		email.setUser(addressComponents[0]);
		email.setDomain(addressComponents[1]);
	}
}

MailDelivererTest.java

package com.pillartechnology.mail;

import static org.hamcrest.MatcherAssert.*;
import static org.hamcrest.Matchers.*;
import static org.mockito.Mockito.*;

import java.util.Date;

import org.mockito.ArgumentCaptor;
import org.mockito.Captor;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;
import org.testng.annotations.BeforeMethod;
import org.testng.annotations.Test;

public class MailDelivererTest {

	@InjectMocks private MailDeliverer subject = new MailDeliverer();
	
	@Mock private ExternalMailSystem externalMailSystem;
	@Mock private Timestamper timestamper;
	
	@Captor private ArgumentCaptor<Email> emailCaptor;
	
	@BeforeMethod(alwaysRun=true)  
	public void injectDoubles() {
		MockitoAnnotations.initMocks(this);
	}
	
	@Test
	public void sendsEmailByConstructingEmailObject() {
		String expectedUser = "tim";
		String expectedDomain = "wingfield.com";
		String expectedBody = "Hi Tim!";
		
		subject.deliver(expectedUser+"@"+expectedDomain,expectedBody);
		
		verify(externalMailSystem).send(emailCaptor.capture());
		Email email = emailCaptor.getValue();
		assertThat(email.getUser(),is(expectedUser));
		assertThat(email.getDomain(),is(expectedDomain));
		assertThat(email.getBody(),is(expectedBody));
	}

	@Test
	public void setsTheTimestampOnTheEmail() {
		Date expected = new Date(8932l);
		when(timestamper.stamp()).thenReturn(expected);
		
		subject.deliver("a@b", null);
		
		verify(externalMailSystem).send(emailCaptor.capture());
		Email email = emailCaptor.getValue();
		assertThat(email.getTimestamp(),is(expected));
	}
	
}

Timestamper.java

package com.pillartechnology.mail;

import java.util.Date;

public class Timestamper {

	public Date stamp() {
		return null; //TODO - not yet implemented
	}

}