Concepts of Programming Languages

9.9 Generic Subprograms 425

float flt_list[100];

...
generic_sort(flt_list, 100);

The templated functions of C++ are a kind of poor cousin to a subprogram

in which the types of the formal parameters are dynamically bound to the types

of the actual parameters in a call. In this case, only a single copy of the code

is needed, whereas with the C++ approach, a copy must be created at compile

time for each different type that is required and the binding of subprogram

calls to subprograms is static.

9.9.2 Generic Methods in Java 5.0

Support for generic types and methods was added to Java in Java 5.0. The name

of a generic class in Java 5.0 is specified by a name followed by one or more

type variables delimited by pointed brackets. For example,

generic_class<T>

where T is the type variable. Generic types are discussed in more detail in

Chapter 11.

Java’s generic methods differ from the generic subprograms of C++ in

several important ways. First, generic parameters must be classes—they can-

not be primitive types. This requirement disallows a generic method that

mimics our example in C++, in which the component types of arrays are

generic and can be primitives. In Java, the components of arrays (as opposed

to containers) cannot be generic. Second, although Java generic methods can

be instantiated any number of times, only one copy of the code is built. The

internal version of a generic method, which is called a raw method, operates

on Object class objects. At the point where the generic value of a generic

method is returned, the compiler inserts a cast to the proper type. Third, in

Java, restrictions can be specified on the range of classes that can be passed

to the generic method as generic parameters. Such restrictions are called

bounds.

As an example of a generic Java 5.0 method, consider the following skeletal

method definition:

public static <T> T doIt(T[] list) {

...
}

This defines a method named doIt that takes an array of elements of a generic

type. The name of the generic type is T and it must be an array. Following is

an example call to doIt:

doIt<String>(myList);