Java™ How To Program (Early Objects), Tenth Edition

Implementing Method sum With a Wildcard Type Argument in Its Parameter

Recall that the purpose of method sum in Fig. 20.13 was to total any type of Numbers stored in an ArrayList. We created an ArrayList of Numbers that contained both Integer and Double objects. The output of Fig. 20.13 demonstrates that method sum worked properly. Given that method sum can total the elements of an ArrayList of Numbers, you might expect that the method would also work for ArrayLists that contain elements of only one numeric type, such as ArrayList<Integer>. So we modified class TotalNumbers to create an ArrayList of Integers and pass it to method sum. When we compile the program, the compiler issues the following error message:

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