Java How to Program (early objects), 9/e

21.8 Wildcards in Methods That Accept Type Parameters

In this section, we introduce a powerful generics concept known as wildcards. For this purpose, we’ll also introduce a new data structure from package java.util. In Chapter 20, we discussed the Java Collections Framework, which provides many generic data structures and algorithms that manipulate the elements of those data structures. Perhaps the simplest of these data structures is class ArrayList—a dynamically resizable, arraylike data structure. As part of this discussion, you’ll learn how to create an ArrayList, add elements to it and traverse those elements using an enhanced for statement.

Let’s consider an example that motivates wildcards. Suppose that you’d like to implement a generic ...