Java Reference
In-Depth Information
the list
Object item;
// The item for this ListItem
The type parameter following the class name in the original generic type definition has been removed,
and all occurrences of the type variable Point within the class definition have been replaced by type
Object . The bold lines are the one affected. Type Object is chosen by the compiler to replace the type
variable because type Object is the ultimate superclass class from which type Point is derived. The type
that the compiler selects to replace a type variable is the leftmostbound of the type variable. Type Object
is the default leftmost bound that applies to any class type because all classes have type Object as their
ultimate superclass. You see later in the chapter how you can specify a different leftmost bound for a type
parameter and what the reasons are for doing this.
Of course, this class doesn't exist as a separate entity. The preceding code represents a description of how
the generic type behaves when you supply a type argument as type Point . Looking at the class that now
works with references of type Object , you may wonder what the advantage of being able to specify the
type parameter is; after all, you can supply a reference to an object of any type for a parameter of type
Object . The answer is that the type variable you supply is used by the compiler to ensure compile-time
type safety. When you use an object of type LinkedList<Point> in your code, the compiler checks that
you use it only to store objects of type Point and flags any attempt to store objects of other types as
an error. When you call methods for an object of type LinkedList<Point> , the compiler ensures that
you supply references only of type Point where the original method parameter was specified as the type
You use the methods in the parameterized type in the same way as those in the original LinkedList class.
Everything is very straightforward, and you now have a PolyLine implementation that uses a type safe
linked list. I think you'll agree that the essentials of defining and using a generic type could be described
as a piece of cake.
Using Primitive Type Wrapper Class Types as Arguments
On occasion you will want to store values of a primitive type in a collection such as a linked list. In this situ-
ation you use the generic type with one of the wrapper classes for primitive types as the type argument —
these are the classes Integer , Short , Double , and so on that are defined in the java.lang package. Here's
how you could use the LinkedList<T> generic type to hold values of type double :
LinkedList<Double> temperatures = new LinkedList<>();
Here you have created a linked list that stores objects of type Double , and the autoboxing facility that you
met in Chapter 5 enables you to use the LinkedList<Double> object directly with values of type double .
For example, to add a value of type double to the linked list you have just created, you could write the fol-
Because the parameter type for this method for the LinkedList<Double> object is of type Double , the
compiler automatically inserts a boxing conversion to convert the double value 10.5 to an object of type
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