15.10.2. Upper Bounded Wildcards

Try these first

1. 1.

   What does List<? extends Number> signify?

   Hint

   Think about what types can be included in the Number class hierarchy.

2. 2.

   Why are upper bounded wildcards used?

   Hint

   Recall how they restrict types to a specific upper limit.

3. 3.

   What does <? extends T> mean in Java?

   • Allows writing to any object of type T.
   • Allows reading any object of type T or its subclasses.
   • Blocks the use of any subclass of T.

   Hint

   Think about how generics enforce constraints.

4. 4.

   True or False: A List of type List<? extends Animal> can accept a List of Dogs.

   • True
   • False

   Hint

   Consider the class hierarchy.

5. 5.

   Implement a generic method that calculates the sum of a list containing any number subclasses. Ensure type safety through upper bounded wildcards.

   Hint

   Student should focus on using `doubleValue()` on the Number class.

6. 6.

   Create a class that manages a collection of objects from various subclasses of a parent class, ensuring it utilizes upper bounded wildcards when retrieving items.

   Hint

   Pay attention to the class definition and method parameter signatures.