Car.java

package ioc;

public class Car {
    private final Engine engine;
    private int fuel;
    public Car(Engine engine) {
        this.engine = engine;
    }

    public void addFuel(int fuel) {
        if(fuel < 0) {
            throw new IllegalArgumentException("Can't add negative fuel");
        }
        this.fuel += fuel;
    }

    public void start() {
        if(fuel <= 0) {
            throw new RuntimeException("Empty tank!");
        }
        engine.start();
    }

    public static class Engine {
        public void start() {
            System.out.println("Engine start");
        }
    }
}

CarTest.java

package ioc;

import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;
import org.mockito.Mockito;

public class CarTest {
    private Car.Engine mockEngine;
    private Car car;

    @Rule
    public ExpectedException expectedException = ExpectedException.none();

    @Before
    public void setup() {
        mockEngine = Mockito.mock(Car.Engine.class);
        car = new Car(mockEngine);
    }

    @Test
    public void addFuel_NotNegative() throws Exception {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Can't add negative fuel");
        car.addFuel(-1);
    }

    @Test
    public void start_NoFuel() throws Exception {
        expectedException.expect(RuntimeException.class);
        expectedException.expectMessage("Empty tank!");
        car.start();
    }

    @Test
    public void start() throws Exception {
        car.addFuel(10);
        car.start();

        Mockito.verify(mockEngine).start();
    }
}