Thursday, October 28, 2010

Generic classes in java vs templates in C++

Here is a small excerpt from the definitions of the interfaces List and Iterator in package :
public interface List<E> {
  void add(E x);
  Iterator<E> iterator();
public interface Iterator<E> {
  E next();
  boolean hasNext();

This should all be familiar, except for the stuff in angle brackets. Those are the
declarations of the formal type parameters of the interfaces List and Iterator.
Type parameters can be used throughout the generic declaration, pretty much where
you would use ordinary types (though there are some important restrictions.

In the introduction, we saw invocations of the generic type declaration List, such
as List<Integer>. In the invocation (usually called a parameterized type), all occurrences of the formal type parameter (E in this case) are replaced by the actual type argument (in this case, Integer).

You might imagine that List<Integer> stands for a version of List where E has been uniformly replaced by Integer:
public interface IntegerList {
  void add(Integer x);
  Iterator<Integer> iterator();

This intuition can be helpful, but it’s also misleading.
It is helpful, because the parameterized type List<Integer> does indeed have methods that look just like this expansion.
It is misleading, because the declaration of a generic is never actually expanded in this way. There aren’t multiple copies of the code: not in source, not in binary, not on disk and not in memory. If you are a C++ programmer, you’ll understand that this is very different than a C++ template.

A generic type declaration is compiled once and for all, and turned into a single class file, just like an ordinary class or interface declaration. Type parameters are analogous to the ordinary parameters used in methods or constructors. Much like a method has formal value parameters that describe the kinds of values it operates on, a generic declaration has formal type parameters. When a method is invoked, actual arguments are substituted for the formal parameters, and the method body is evaluated.
When a generic declaration is invoked, the actual type arguments are substituted for the formal type parameters.

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