In addition, method sum will receive a parameter of type ArrayList and total its elements. For this reason, we'll declare the ArrayList with the type argument Number, which is the superclass of both Integer and Double. We'd like to be able to total all the numbers in the ArrayList regardless of their type. For example, any int value would be autoboxed as an Integer object, and any double value would be autoboxed as a Double object. So the numbers would be autoboxed as objects of the type-wrapper classes. As you know, generic classes can be used only with class or interface types. You would begin by inserting the numbers in the collection. Suppose that you would like to implement a generic method sum that totals the numbers in a collection, such as an ArrayList.
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As part of this discussion, you will learn how to create an ArrayList, add elements to it and traverse those elements using an enhanced for statement.īefore we introduce wildcards, let's consider an example that helps us motivate their use. Perhaps the simplest of these data structures is class ArrayLista dynamically resizable, array-like data structure. Chapter 19, Collections, discusses the Java Collections Framework, which provides many generic data structures and algorithms that manipulate the elements of those data structures. For this purpose, we will also introduce a new data structure from package java.util. In this section, we introduce a powerful generics concept known as wildcards.
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