Simple mockito example

Car.java

public class Car {
    private Engine engine;
    private FuelTank fuelTank;

    public Car(Engine engine, FuelTank fuelTank) {
        this.engine = engine;
        this.fuelTank = fuelTank;
    }

    public void start() {
        if(engine.isRunning()) {
            throw new IllegalStateException("Engine already running");
        }
        if(fuelTank.getFuel() == 0) {
            throw new IllegalStateException("Can't start: no fuel");
        }

        engine.start();

        if(!engine.isRunning()) {
            throw new IllegalStateException("Started engine but isn't running");
        }
    }

    public boolean isRunning() {
        return engine.isRunning();
    }
}

CarTest.java

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

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.when;

public class CarTest {
    private Car car;
    private Engine engine;
    private FuelTank fuelTank;

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

    @Before
    public void setup() {
        engine = Mockito.mock(Engine.class);
        fuelTank = Mockito.mock(FuelTank.class);

        car = new Car(engine, fuelTank);
    }

    @Test
    public void isRunning() {
        when(engine.isRunning()).thenReturn(true);
        assertEquals(true, car.isRunning());

        when(engine.isRunning()).thenReturn(false);
        assertEquals(false, car.isRunning());
    }

    @Test
    public void start() {
        when(engine.isRunning()).thenReturn(false, true);
        when(fuelTank.getFuel()).thenReturn(100);
        car.start();
    }

    @Test
    public void start_NoFuel() {
        expectedException.expect(IllegalStateException.class);
        expectedException.expectMessage("no fuel");
        when(engine.isRunning()).thenReturn(false);
        when(fuelTank.getFuel()).thenReturn(0);
        car.start();
    }

    @Test
    public void start_IsRunning() {
        expectedException.expect(IllegalStateException.class);
        expectedException.expectMessage("already running");
        when(fuelTank.getFuel()).thenReturn(100);
        when(engine.isRunning()).thenReturn(true);
        car.start();
    }

    @Test
    public void start_DidNotStart() {
        expectedException.expect(IllegalStateException.class);
        expectedException.expectMessage("Started engine but isn't running");
        when(engine.isRunning()).thenReturn(false, false);
        when(fuelTank.getFuel()).thenReturn(100);
        car.start();
    }
}

Engine.java

public class Engine {
    private boolean running;
    public boolean isRunning() {
        return running;
    }

    public void start() {
        running = true;
    }

    public void stop() {
        running = false;
    }
}

FuelTank.java

public class FuelTank {
    private int fuel;
    public int getFuel() {
        return fuel;
    }

    public void setFuel(int fuel) {
        this.fuel = fuel;
    }
}