Wildcards and Generic Methods in Java - Wildcard Type Relationships
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Before we leave our wild discussion of wildcard types, let's return one more time to the notion of wildcard type instantiations as types in the Java type system. Earlier in this chapter, we described how regular concrete instantiations of generic types are related by virtue of their "base" generic type inheritance, only with the proviso that their type parameters are exactly the same. Later, we tried to instill the idea that wildcard instantiations add an inheritance relationship to the type parameters, the other half of the generic instantiation. Now, we'll bring the two together. Things can get arcane pretty quickly, but the simple cases are easy to swallow.
The question is, If we have two different wildcard instantiations of a type or related types, how, if at all, are they related? For example, since an unbounded wildcard instantiation can hold any instantiation, can it be assigned a value with a more restrictive bound?
List< ? extends Date > dateLists = ... ;
List< ? > anylists;
anyLists = dateLists; // Ok!
The answer is yes. For purposes of assignability, wildcard instantiations can be considered as types with possible supertype or subtype relationships determined by their bounds. Let's spell out the unbounded wildcard instantiation as it really is, an instantiation with an upper bound of Object:
List< ? extends Date > dateLists = ... ;
List< ? extends Object > objectLists;
objectLists = dateLists; // Ok!
The rule is that if the "base" generic, raw type is assignable and the bounds of the wildcard instantiation are also assignable, the overall types are assignable. Let's look at another example:
List< ? extends Integer > intLists = ...;
Collection< ? extends Number > numCollections;
numCollections = intLists; // Ok!
What this effectively says is that some List of Integer types can be treated as some Collection of Number types through the wildcard instantiation. If you think about it, you'll see that there is no conflict here. A List is certainly a Collection. And all we're doing is widening the type by which we can read the elements from Integer to Number. In neither case could we have written to the collection via the wildcard instantiation anyway.
What all this ultimately means is that with the introduction of wildcard instantiations, the type relationships of Java generic classes becomes two-dimensional. There is the raw type relationship to consider and then the wildcard parameter relationship. In fact, if you consider that generic classes may have more than one type parameter, the relationships can get even more complicated (N-dimensional). Fortunately, none of this comes up very often in the real world.
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 This article was excerpted from chapter eight of the book Learning Java, third edition, written by Patrick Niemeyer and Jonathan Knudsen (O'Reilly; ISBN: 0596008732). Check it out today at your favorite bookstore. Buy this book now.
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