Example : Buying a Drink

Setup

We create Matt to test the Patron class.
PatronHelper is a child of the Patron class which helps reveal unaccesable properties for our tests.
We also create a mocked Cafe object. It emulates the class without knowing about the implementation.

@Before is called before every test.

    @Before
    public void SetUp(){
        mMatt = new PatronHelper("Matt", 100);
        mCafe = mock(Cafe.class);
    }

Creating a mocked Drink

    DrinkInterface getMockedDrink(String targetDrinkName, float cost) {
        //Creates the mocked object based on the class
        DrinkInterface nukaColaMock = mock(DrinkInterface.class);

        //when getCost is called on the object, return 
        when(nukaColaMock.getCost()).thenReturn(cost);
        when(nukaColaMock.getName()).thenReturn(targetDrinkName);
        return nukaColaMock;
    }

Creates a mocked or dummy DrinkInterface object without copying any of the functionality.

    DrinkInterface nukaColaMock = mock(DrinkInterface.class);

When getCost is called, it will return the value of cost.
When getName is called, it will return the value of targetDrinkName.

    when(nukaColaMock.getCost()).thenReturn(cost);
    when(nukaColaMock.getName()).thenReturn(targetDrinkName);

All Together Now

This creates a Nuka Cola mocked drink.
Makes sure the mocked cafe returns Nuka Cola.
Check if Matt has a Nuka Cola soft drink.

    @Test
    public void BuyDrinkWithEnoughMoney() throws Patron.NotEnoughMoneyException {
        //create mocked drink
        String targetDrinkName = "Nuka Cola";
        DrinkInterface nukaColaMock = getMockedDrink(targetDrinkName, 4);
        
        when(mCafe.sellDrink(targetDrinkName)).thenReturn(nukaColaMock);

        mMatt.enterCafe(mCafe);
        mMatt.purchaseDrinkFromCafe(targetDrinkName);

        assertFalse(mMatt.mDrinksInPossession.isEmpty());
        assert(mMatt.mDrinksInPossession.get(0) == nukaColaMock);
    }

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Checkout more examples in the codebase