ChristmasDiscount.java

package tudelft.sqt.mocks;

import java.util.Calendar;

public class ChristmasDiscount {

	private Clock clock;

	public ChristmasDiscount(Clock clock) {
		this.clock = clock;
	}

	public double discount(double value) {

		Calendar today = clock.today();

		boolean isDecember = today.get(Calendar.MONTH) == Calendar.DECEMBER;
		boolean isTwentyFifth = today.get(Calendar.DAY_OF_MONTH) == 25;
		boolean isChristmas = isDecember && isTwentyFifth;

		return isChristmas ? value * 0.10 : 0;

	}
}

ChristmasDiscountTest.java

package tudelft.sqt.mocks;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

import java.util.Calendar;
import java.util.GregorianCalendar;

public class ChristmasDiscountTest {

	@Test
	void isChristmas() {
		Calendar christmas = new GregorianCalendar(2019,Calendar.DECEMBER, 25);

		Clock fakeClock = Mockito.mock(Clock.class);
		Mockito.when(fakeClock.today()).thenReturn(christmas);

		double discountValue = new ChristmasDiscount(fakeClock).discount(100);

		Assertions.assertEquals(10, discountValue, 0.0001);
	}
	@Test
	void notChristmas() {
		Calendar christmas = new GregorianCalendar(2019,Calendar.DECEMBER, 26);

		Clock fakeClock = Mockito.mock(Clock.class);
		Mockito.when(fakeClock.today()).thenReturn(christmas);

		double discountValue = new ChristmasDiscount(fakeClock).discount(100);

		Assertions.assertEquals(0, discountValue, 0.0001);
	}
}

Clock.java

package tudelft.sqt.mocks;

import java.util.Calendar;

public interface Clock {
	Calendar today();
}

DefaultClock.java

package tudelft.sqt.mocks;

import java.util.Calendar;

public class DefaultClock implements Clock{
	@Override
	public Calendar today() {
		return Calendar.getInstance();
	}
}