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Generics and Limitations in Java

Last week we began discussing generics and relationships in Java. This week, we'll learn about parameter type limitations, bounds, wildcards, and more. This article, the third in a series, was excerpted from chapter eight of the book Learning Java, third edition, written by Patrick Niemeyer and Jonathan Knudsen (O'Reilly; ISBN: 0596008732). Copyright 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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May 24, 2007
  1. · Generics and Limitations in Java
  2. · Bounds
  3. · Erasure and Bounds (Working with Legacy Code)
  4. · Wildcards
  5. · Thinking Outside the Container

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Generics and Limitations in Java - Thinking Outside the Container
(Page 5 of 5 )

Let's be clear about what the wildcard means in the context of a container type such as List. The unbounded wildcard instantiation may be assigned any type instantiation, but it does ultimately refer to some particular type instantiation. A wildcard instantiation serves as the type of a variable, and that variable eventually holds some actual concrete instantiation of the generic type:

  List<?> someInstantiationOfList;
  someInstantiationOfList = new ArrayList<Date>();
  someInstantiationOfList = new ArrayList<String>();

In this example, our List<?> variable is either a List<String> or a List<Date>. It is not some new kind of List that can hold either String or Date elements.

In the same way, a wildcard with bounds ultimately holds one of the concrete instantiations assignable to its bounds. Imagine for a moment that we have a private class Foo with only one subclass Bar and no others. The expression Collection<? extends Foo> in this case means the set of two possibilities: either Collection<Foo> or Collection<Bar>. That is, either a Collection of elements with a common supertype of Foo or a collection of elements with a common supertype of Bar. Again, the wildcard instantiation matches either of those generic type instantiations. It does not create a new type of collection that can contain either Foos or Bars. (That is actually the job of Collection<Foo>, which can contain both Foo and Bar elements.)

For this reason, wildcard type instantiations are valid types for referencing an object, but they cannot be used as the type to create an instance of an object. In general, you cannot use a wildcard type with the new keyword to allocate an object instance because the wildcard denotes one or a possible set of objects. It doesn't make sense.

Please check back next week for the continuation of this article.

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