mockito Getting started with mockito Simple unit test using Mockito

Example

The class we are going to test is:

public class Service {

    private Collaborator collaborator;

    public Service(Collaborator collaborator) {
        this.collaborator = collaborator;
    }
    
    public String performService(String input) {
        return collaborator.transformString(input);
    }
}

Its collaborator is:

public class Collaborator {

    public String transformString(String input) {
        return doStuff();
    }

    private String doStuff() {
        // This method may be full of bugs
        . . .
        return someString;
    }

}

In our test, we want to break the dependency from Collaborator and its bugs, so we are going to mock Collaborator:

import static org.junit.Assert.*;
import static org.mockito.Mockito.*;

import org.junit.Test;

public class ServiceTest {
    @Test
    public void testPerformService() throws Exception {
        // Configure mock
        Collaborator collaboratorMock = mock(Collaborator.class);
        doReturn("output").when(collaboratorMock).transformString("input");

        // Perform the test
        Service service = new Service(collaboratorMock);
        String actual = service.performService("input");
        
        // Junit asserts
        String expected = "output";
        assertEquals(expected, actual);
    }  
}