DateTimeProvider.java

import java.util.Date;

public interface DateTimeProvider {

    Date getCurrentDate();
}

DefaultProvider.java

import java.util.Date;

public class DefaultProvider implements DateTimeProvider {

    public Date getCurrentDate() {
        return new Date();
    }
}

FakeTimeProvider.java

import java.util.Date;

public class FakeTimeProvider implements DateTimeProvider {

    public Date getCurrentDate() {
        return new Date(2016, 1, 1);
    }
}

Logger.java

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

public class Logger {

    private DateTimeProvider dateTimeProvider;
    private List<Message> messages;

    public Logger(DateTimeProvider dateTimeProvider) {
        this.dateTimeProvider = dateTimeProvider;
        this.messages = new ArrayList<Message>();
    }

    public void log(String message) {
        Date currentTime = dateTimeProvider.getCurrentDate();
        messages.add(new Message(currentTime, message));
    }

    public List<Message> getMessages() {
        return messages;
    }
}

LoggerTest.java

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

import java.util.Date;

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

public class LoggerTest {

    @Test
    public void shouldLogMessageWithCurrentTime_1() throws Exception {
        DateTimeProvider dateTimeProvider = mock(DateTimeProvider.class);

        // nez aktualni je lepsi pouzit nejaky fake cas,
        // aby kazde spusteni testu pracovalo se stejnym casem
        // takto:
        // Date currentDate = new Date(2016, 1, 1);

        // pro ukazku tu mam ale opravdu aktualni cas:
        Date currentDate = new Date();
        when(dateTimeProvider.getCurrentDate()).thenReturn(currentDate);

        Logger logger = new Logger(dateTimeProvider);
        logger.log("Hello message");

        Message actualMessage = logger.getMessages().get(0);
        Assert.assertEquals(currentDate, actualMessage.getDate());
    }

    @Test
    public void shouldLogMessageWithCurrentTime_2() throws Exception {
        DateTimeProvider dateTimeProvider = new FakeTimeProvider();
        Logger logger = new Logger(dateTimeProvider);
        logger.log("Hello message");

        Message actualMessage = logger.getMessages().get(0);
        Assert.assertEquals(new Date(2016, 1, 1), actualMessage.getDate());
    }
}

Message.java

import java.util.Date;

public class Message {

    private Date date;
    private String message;

    public Message(Date date, String message) {
        this.date = date;
        this.message = message;
    }

    public Date getDate() {
        return date;
    }

    public String getMessage() {
        return message;
    }
}