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Supertype Wildcards with Generics

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by , 11-30-2011 at 12:12 AM (2228 Views)
In my last tip, I showed how you can use a wildcard with generics. It is also possible to use a supertype wildcards. In this case the wildcard is bounded by any base class of a particular class, by specifying <? super MyClass> or even using a type parameter: <? super T> (although you cannot give a generic parameter a supertype bound; that is, you cannot say <T super MyClass>). This allows you to safely pass a typed object into a generic type. Thus, with supertype wildcards you can write into a Collection:

Java Code:
import java.util.*;
public class SuperTypeWildcards { 
   static void writeTo(List<? super Rectangle> rectangles) {
      rectangles.add(new Rectangle()); 
      rectangles.add(new SubOfRectangle()); 
      rectangles.add(new Shape()); 
The argument rectangles is a List of some type that is the base type of Rectangle; thus you know that it is safe to add an Rectangle or a subtype of Rectangle. Since the lower bound is Rectangle, however, you donít know that it is safe to add Shape to such a List, because that would allow the List to be opened up to the addition of non-Rectangle types, which would violate static type safety.
You can thus begin to think of subtype and supertype bounds in terms of how you can "write" (pass into a method) to a generic type, and "read" (return from a method) from a generic type.

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