Consider the following inheritance hierarchy:

If we run the following code, what is printed to the screen?

Question

Consider the following inheritance hierarchy:

If we run the following code, what is printed to the screen?

BlackTom AI · Accepted Answer

First, I identify the class hierarchy and what each class does. There is an abstract class Drink with a protected name and price, and an abstract method getDiscount. The concrete subclasses Coffee and Beer override getDiscount to provide their own discount logic: Coffee multiplies the price by 0.5, while Beer subtracts 1 from the price. The constructors initialize name and price via super(name, price).

Next, I examine the main method setup. An ArrayList of Drink named drinks is created and four items are added in order: a Beer with name "VB" and price 6.0, another Beer with name "Crownie" (likely a misspelling of Crownie) and price 8.0, a Coffee with name "Flat white" and price 6.0, and a Coffee with name "Mocha" and price 4.0.

Then, a double s is initialized to 0. A for-each loop iterates over each Drink d in drinks. Inside the loop, there is a check: if (d instanceof Beer) { s += d.getDiscount(); }. This means only the Beer objects contribute to s, and for those Beers, their specific getDiscount implementation is used (dynamic dispatch).

Now, I compute the contributions from Beer objects. For the first Beer (price 6.0), getDiscount() in Beer returns price - 1, which is 6.0 - 1 = 5.0. For the second Beer (price 8.0), getDiscount() also returns price - 1, which is 8.0 - 1 = 7.0. Since the Coffee instances are not Beers, their getDiscount() results are not used in the sum.

Finally, s is the sum of the two Beer contributions: 5.0 + 7.0 = 12.0. The code then prints s with System.out.println(s). Therefore, the program outputs the numeric value 12.0.

If you consider potential edge cases: the instanceof check ensures only Beer objects contribute; if there were other subclasses of Drink, they would be ignored in the sum unless the code were changed to include them. The use of overridden getDiscount guarantees the correct discount logic for each specific Beer instance, demonstrating polymorphism in action.

Consider the following inheritance hierarchy: If we run the following code, what is printed to the screen?

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