Mastering Java: The Ultimate Quiz for 'Thinking in Java'

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Dive into the depths of Java with our quiz based on "Thinking in Java, Fourth Edition" by Bruce Eckel. Test your knowledge, solidify concepts, and prepare for certification with challenging questions and insightful feedback.

Each practice test/flash card set has 50 randomly selected questions from a bank of over 500. You'll get a new set of questions each time!

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Why can't you upcast a List<Apple> to a List<Fruit>?

Because the List types are invariant
Because a Fruit is not an Apple
Because of runtime type information
Because generics do not support covariance by default

The correct answer is: Because generics do not support covariance by default

Upcasting is the process of converting an object of a subtype to its supertype. It is usually allowed when working with inheritance, but it is not possible with generic types by default. This is because generics are invariant in Java, meaning that subtypes cannot be used in place of their supertype. So, even though an Apple is a subtype of Fruit, a List<Apple> cannot be used as a List<Fruit>. This is because it would violate type safety as the List<Fruit> could end up containing other types of Fruit that are not Apples. Therefore, options A, B, and C are incorrect because they do not address the specific reason why upcasting a List<Apple> to a List<Fruit> is not allowed in Java.