Which of the following keywords is used to declare an upper bounded wildcard? a. bound b. extends c. implement d. super

To determine the correct answer for the keyword used to declare an upper bounded wildcard in Java, let’s analyze the options provided:

1. Break Down the Problem
   The question asks about the keyword that allows the declaration of an upper bounded wildcard, which restricts the unknown type to be a specific type or its subtypes.

2. Relevant Concepts
   In Java, wildcards are used in generics. An upper bounded wildcard is declared using the extends keyword, which allows you to define a wildcard that can represent a type that extends a specified class or implements a specific interface.

3. Analysis and Detail

   • a. bound: Not a valid keyword in Java for declaring wildcards.
   • b. extends: Correct keyword for upper bounded wildcards (e.g., <? extends Number>).
   • c. implement: Not a valid keyword in the context of wildcards. "Implements" is used with interfaces in class definitions.
   • d. super: This is used for declaring lower bounded wildcards (e.g., <? super Integer>).

4. Verify and Summarize
   The proper keyword for declaring upper bounded wildcards in Java is clearly extends, as it defines a bound that restricts the type to a specific subclass or class itself.

Final Answer

b. extends