An example of behavior verification with Mockito (on TestNG)

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;
	
	public void deliver(String expectedAddress, String expectedBody) {
		String[] addressComponents = expectedAddress.split("@");
		externalMailSystem.send(addressComponents[1],addressComponents[0], expectedBody);
	}
}

MailDelivererTest.java

package com.pillartechnology.mail;

import static org.mockito.Mockito.*;

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;
	
	@BeforeMethod(alwaysRun=true)  
	public void injectDoubles() {
		MockitoAnnotations.initMocks(this);
	}
	
	@Test
	public void sendsEmail() {
		String expectedBody = "Hi Tim!";
		
		subject.deliver("[email protected]",expectedBody);
		
		verify(externalMailSystem).send("wingfield.com","tim", expectedBody);
	}
	
}