Wildcards, Arrays, and Generics in Java - Case Study: The sort() Method
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Poking around in the java.util.Collections class we find all kinds of static utility methods for working with collections. Among them is this goody--the static generic method sort() :
<T extends Comparable<? super T>> void sort( List<T> list ) { ... }
Another nut for us to crack. Let's focus on the last part of the bound:
Comparable<? super T>
This is a wildcard instantiation of the Comparable interface, so we can read the extends as implements, if it helps. Comparable holds a compareTo() method for some parameter type. A Comparable<String> means that the compareTo() method takes type String. Therefore, Comparable<? super T> is the set of instantiations of Comparable on T and all of its superclasses. A Comparable<T> suffices and, at the other end, so does a Comparable<Object>. What this means in English is that the elements must be comparable to their own type or some supertype of their own type. This is sufficient to ensure that the elements can all be compared to one another, but not as restrictive as saying that they must all implement the compareTo() method themselves. Some of the elements may inherit the Comparable interface from a parent class that knows how to compare only to a supertype of T and that is exactly what is allowed here.
Conclusion
Java generics are a very powerful and useful addition to the language. Although some of the details we delved into later in this chapter may seem daunting, the common usage is very simple and compelling: generics make collections better. As you begin to write more code using generics you will find that your code becomes more readable and more understandable. Generics make explicit what previously had to be inferred from usage. They complete the promise of type safety in the Java language and make Java a better language, despite their idiosyncrasies.
* That is, unless you want to use a generic type in a nongeneric way. We’ll talk about “raw” types later in thischapter.* For those of you who might like some context for the title of this section, here is where it comes from:
Boy: Do not try and bend the spoon. That’s impossible. Instead only try to realize the truth.
Neo: What truth?
Boy: There is no spoon.
Neo: There is no spoon?
Boy: Then you’ll see that it is not the spoon that bends, it is only yourself.
—Wachowski, Andy and Larry. The Matrix. 1 videocassette (136 minutes). Warner Brothers, 1999.
* In real life, Java doesn’t let us extend the Enum type; that’s reserved for the enum keyword and the compiler. But the structure is as shown.
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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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